Rozprawy doktorskie na temat „Vegetation and climate”

The early Eocene to the Cretaceous (48-148 Ma) was a period in the Earth's history where the climate was much warmer than the present day, with no permanent ice sheets and atmospheric CO2 levels higher than the present day. Using the climate model HadCM3L coupled to a dynamic vegetation model, this thesis aims to analyse vegetation-climate interactions during these past greenhouse climates, and how the climate, vegetation and climate sensitivity of these time periods are influenced by changes in palaeogeography and CO2 . The results of these model simulations are also evaluated against climatologically-sensitive geological proxies. Past modelling studies for the early Eocene have struggled to model the shallow equator to pole temperature gradient that data suggests was present during this time. However, most models have neglected vegetation feedbacks and incorporating these may help to reduce the model-data discrepancy. In this thesis, vegetation climate interactions during the early Eocene are modelled and analysed, and the results compared to available proxy data. The model-data discrepancies for temperatures are also reduced when vegetation feedbacks were included (compared to simulations with static vegetation), although there are still differences, particularly at high latitudes. This suggests that the models are still missing important processes or the data is not being interpreted correctly. In addition, twelve consistent simulations are carried out , each representing a different stage of the Cretaceous. Each simulation has the same atmospheric CO2 level, allowing the effect of palaeogeography on climate, climate sensitivity and vegetation to be analysed. It was found that, in general, the temperature trends that occurred in the mid-Cretaceous simulations were consistent with data. However, the data record does not extend to the earliest Cretaceous, and in the late Cretaceous the results deviate from the data. The model results suggest that, in order for the model to be consistent with the data there must have been a decline in CO2 from the early to late Cretaceous, which is supported by the CO2 proxy record. More data from the early Cretaceous needs to be collected in order to carry out a more robust model-data comparison for this time period.

Land use / land cover (LULC) change assessment is getting more consideration by global environmental change studies as land use change is exposing dryland environments for transitions and higher rates of resource depletion. The semiarid regions of northwestern Ethiopia are not different as land use transition is the major problem of the region. However, there is no satisfactory study to quantify the change process of the region up to now. Hence, spatiotemporal change analysis is vital for understanding and identification of major threats and solicit solutions for sustainable management of the ecosystem. LULC change studies focus on understanding the patterns, processes and dynamics of land use transitions and driving forces of change. The change processes in dryland ecosystems can be either seasonal, gradual or abrupt changes of random or systematic change processes that result in a pattern or permanent transition in land use. Identification of these processes of change and their type supports adoption of monitoring options and indicate possible measures to be taken to safeguard this dynamic ecosystem. This study examines the spatiotemporal patterns of LULC change, temporal trends in climate variables and the insights of the communities on change patterns of ecosystems. Landsat imagery, MODIS NDVI, CRU temperature, TAMSAT rainfall and socio-ecological field data were used in order to identify change processes. LULC transformation was monitored using support vector machine (SVM) algorithm. A cross-tabulation matrix assessment was implemented in order to assess the total change of land use categories based on net change and swap change. In addition, the pattern of change was identified based on expected gain and loss under a random process of gain and loss, respectively. Breaks For Additive Seasonal and Trend (BFAST) analysis was employed for determining the time, direction and magnitude of seasonal, abrupt and trend changes within the time series datasets. In addition, Man Kendall test statistic and Sen’s slope estimator were used for assessing long term trends on detrended time series data components. Distributed lag (DL) model was also adopted in order to determine the time lag response of vegetation to the current and past rainfall distribution. Over the study period of 1972- 2014, there is a significant change in LULC as evidenced by a significant increase in size of cropland of about 53% and a net loss of over 61% of woodland area. The period 2000-2014 has shown a sharp increase of cropland and a sharp decline of woodland areas. Proximate causes include agricultural expansion and excessive wood harvesting; and underlying causes of demographic factor, economic factors and policy contributed the most to an overuse of existing natural resources. In both the observed and expected proportion of random process of change and of systematic changes, woodland has shown the highest loss compared to other land use types. The observed transition and expected transition under random process of gain of woodland to cropland is 1.7%, implies that cropland systematically gains to replace woodland. The comparison of the difference between observed and expected loss under random process of loss also showed that when woodland loses cropland systematically replaces it. The assessment of magnitude and time of breakpoints on climate data and NDVI showed different results. Accordingly, NDVI analysis demonstrated the existence of breakpoints that are statistically significant on the seasonal and long term trends. There is a positive trend, but no breakpoints on the long term precipitation data during the study period. The maximum temperature also showed a positive trend with two breakpoints which are not statistically significant. On the other hand, there is no seasonal and trend breakpoints in minimum temperature, though there is an overall positive trend along the study period. The Man-Kendall test statistic for long term average Tmin and Tmax showed significant variation where as there is no significant trend within the long term rainfall distribution. The lag regression between NDVI and precipitation indicated a lag of up to forty days. This proves that the vegetation growth in this area is not primarily determined by the current precipitation rather with the previous forty days rainfall. The combined analysis showed declining vegetation productivity and a loss of vegetation cover that contributed for an easy movement of dust clouds during the dry period of the year. This affects the land condition of the region, resulting in long term degradation of the environment

The interactions between changing agricultural land and climate are multi faceted and only partially understood. This thesis looks at interactions between crops and climate from assumptions about parameterisations that underpin crop changes in models; the unintended consequences of policies which affect land cover; and the impacts of deliberate crop changes (e.g. biogeoengineering). Focusing on the biogeophysical effects (from albedo, evapotranspiration etc.) these effects are compared to the biogeochemical effects (from greenhouse gases). There are considerable local and global biogeophysical effects to climate from land-use change, which do not necessarily scale linearly with the amount of landuse change itself. Changing the parameterisation of contributory factors to biogeophysical changes can affect the climate at least as much as deliberate alterations. Similarly, climate forced land cover change effects can be larger than land use forced changes. Increases in crop yield from deliberately altered albedo are small, but the changes to climate via albedo from different assumptions of yield are significant at a global and regional scale. This work emphasises the importance of including biogeophysical interactions in assessments of crop and land cover change in policy decisions, but also that the effects of land use change should not be overestimated, as the net effects are often smaller than the parameter uncertainty.

Climate change associated with increasing concentrations of the greenhouse gas, carbon dioxide(CO2), is expected to lead to an increase in global mean temperature of between 1 and 3.5 deg C by the end of the 21st century, with regional changes in rainfall and humidity. This thesis is concerned with modelling the effects of a changing climate and atmospheric C02 concentration on global vegetation. The process-based model, DOLY (Dynamic glObal phtogeographY), is used. It is able to operate using three climate variables, two soil variables and an atmospheric CO2 concentration. Its outputs are leaf area index (LAI), and net primary productivity (NPP). The LAI and NPP values predicted by DOLY were used to run a life-form model with a climate change scenario. It was found that warming led to the spread of trees into the tundra region. The DOLY model was also coupled with the Hadley Centre general circulation model to determine the feedbacks of vegetation on climate. With a global warming of 2◦C, the global feedback of vegetation on temperature was a decrease of 0.1 deg C. However at the regional scale the feedback was +/-2 ◦C, of similar magnitude to the driving temperature change. Finally, the DOLY model was run with transient climate data from the Hadley Centre. The boreal forest moved north, and the Gobi desert and the southern steppes in the former Soviet Union shrank in area. The sensitivity of the model to its soil and climate inputs have also been analysed over a range of environments and the model has been validated with reference to satellite data and experimental data. It was found to perform well. This thesis has shown that it is possible to predict current and possible future distributions of vegetation with climate change using a vegetation model.

Early Palaeocene floras from twenty seven sites within the Raton, southern Powder River and south-western Williston Basins of the western interior of North America were collected, and their leaf physiognomy, ecological character and depositional setting compared. Such a spread of samples enabled the study of spatial and temporal vegetational and climatic variations in the region, following the Cretaceous-Tertiary boundary event. Climatic changes are observed across the Cretaceous-Tertiary boundary. Precipitation increased dramatically, and remained relatively high throughout the earliest Palaeocene. Temperatures were somewhat lower, compared to those of the Late Cretaceous, and seasonality in climate increased. Climatic and vegetation zones shifted southwards as latitudinal climatic equability decreased. Palaeotemperature and palaeoprecipitation were determined using CLAMP and leaf margin analysis. Experiments carried out to assess the robustness of CLAMP to loss of foliar physiognomic data revealed that this data loss did not drastically effect palaeoclimatic determinations but that information about leaf size and margin type had the most effect on results. Vegetation was of low diversity directly after the boundary event, but recovered to stable, but still relatively low levels, within a short time. Changes in diversity are difficult to interpret due to masking by taphonomic biases, which are important within the depositional environments analysed in this study. Climatic deterioration and the prevalence of disturbed environments ultimately facilitated expansion of the angiosperms, although their aspect was changed with a general increase in deciduous forms, in relation to increased seasonality and decreased equability. These trends cannot be related merely to the impact of a bolide at the Cretaceous-Tertiary boundary, but reflect the more global and wide-ranging changes of the period, which were punctuated by this brief, deleterious event. Previous work has tended to concentrate on the North American continent but a more global perspective reveals that the Cretaceous-Tertiary boundary event was not a world-wide catastrophe within terrestrial environments.

The high northern latitudes have warmed faster than anywhere else in the globe during the past few decades. Boreal ecosystems are responding to this rapid climatic change in complex ways and some times contrary to expectations, with large implications for the global climate system. This thesis investigates how boreal vegetation has responded to recent climate change, particularly to the lengthening of the growing season and changes in drought severity with warming. The links between the timing of the growing season and the seasonal cycle of atmospheric CO2 are evaluated in detail to infer large-scale ecosystem responses to changing seasonality and extended period of plant growth. The influence of warming on summer drought severity is estimated at a regional scale for the first time using improved data. The results show that ecosystem responses to warming and lengthening of the growing season in autumn are opposite to those in spring. Earlier springs are associated with earlier onset of photosynthetic uptake of atmospheric CO2 by northern vegetation, whereas a delayed autumn, rather than being associated with prolonged photosynthetic uptake, is associated with earlier ecosystem carbon release to the atmosphere. Moreover, the photosynthetic growing season has closely tracked the pace of warming and extension of the potential growing season in spring, but not in autumn. Rapid warming since the late 1980s has increased evapotranspiration demand and consequently summer and autumn drought severity, offsetting the effect of increasing cold-season precipitation. This is consistent with ongoing amplification of the hydrological cycle and with model projections of summer drying at northern latitudes in response to anthropogenic warming. However, changes in snow dynamics (accumulation and melting) appear to be more important than increased evaporative demand in controlling changes in summer soil moisture availability and vegetation photosynthesis across extensive regions of the boreal zone, where vegetation growth is often assumed to be dominantly temperature-limited. Snow-mediated moisture controls of vegetation growth are particularly significant in northwestern North America. In this region, a non-linear growth response of white spruce growth to recent warming at high elevations was observed. Taken together, these results indicate that net observed responses of northern ecosystems to warming involve significant seasonal contrasts, can be non-linear and are mediated by moisture availability in about a third of the boreal zone.

Thesis (Ph. D.)--University of Oregon, 2008.
Typescript. Includes vita and abstract. Includes bibliographical references (leaves 128-156). Also available online in Scholars' Bank; and in ProQuest, free to University of Oregon users.

The aim of the study was to investigate the influence of meteorological factors on the phenological phases of the vegetation in broadleaved forests under varying climatic conditions. To attain the aim, the following objectives were set: 1. Characterize the changes of values of meteorological parameters (temperature, precipitation) in the studied period; 2. Determine solar radiation intensity under the canopies of trees, phenological phases of trees and bushes and their changes; 3. Determine the changes of projection coverage, height and phenological phases of herbaceous plants during the growing period; 4. Ascertain the relationship between meteorological factors and phenological phases of woody and herbaceous plants. Scientific novelty, theoretical and practical significance. Up till now in Lithuania phenological studies mostly of agricultural plants have been conducted. Phenological studies on woody and herbaceous plants in the forests of Lithuania are almost absent. Most of the studies were conducted with indicator species, such as hazel, coltsfoot, etc. During the study, for the first time in Lithuania a complex investigation of forest community was carried out and the influence of meteorological factors on the phenological phases of herbaceous and woody plants in Kamša botanical-zoological reserve was determined. The results of the study allow to better assess the influence of meteorological factors on seasonal development (phenology) of herbaceous plants, trees and... [to full text]
Darbo tikslas – ištirti meteorologinių veiksnių įtaką plačialapių lapuočių miškų augalijos fenologiniams tarpsniams skirtingų klimatinių sąlygų metais. Tyrimo uždaviniai: 1. charakterizuoti meteorologinių rodiklių (temperatūros, kritulių) reikšmių kaitą tiriamuoju laikotarpiu; 2. nustatyti apšvietimą po medžių lajomis, medžių ir krūmų lapojimo fenologinius tarpsnius ir jų pokyčius; 3. nustatyti žolinių augalų projekcinio padengimo, aukščio, fenologinių tarpsnių kaitą vegetacijos metu; 4. nustatyti ryšį tarp meteorologinių veiksnių ir sumedėjusių, žolinių augalų fenologinių tarpsnių. Darbo mokslinis naujumas, teorinė ir praktinė reikšmė. Lietuvoje iki šiol daugiausia atlikta fenologinių tyrimų su žemės ūkio augalais. Sumedėjusių augalų ir miško žolinių augalų detalių fenologinių tyrimų Lietuvoje beveik nėra. Daugiausia atlikta indikatorinių rūšių, kaip paprastasis lazdynas, paprastasis šalpusnis ir kt., tyrimų. Pirmą kartą Lietuvoje kompleksiškai tirta miško bendrija, nustatyta meteorologinių veiksnių įtaka sumedėjusių augalų lapojimo ir žolinių augalų fenologiniams tarpsniams Kamšos botaniniame-zoologiniame draustinyje. Darbo rezultatai leidžia geriau įvertinti meteorologinių veiksnių įtaką miško žolinės augalijos, medžių ir krūmų sezoniniam vystymuisi (fenologijai). Gautos žinios svarbios ne tik teoriniam išsamesniam atskirų rūšių biologijos pažinimui, bet ir praktiniams tikslams: dendrologijoje, fitopatologijoje ir t. t.

Although herbaceous communities are important components of floodplain ecosystems, the factors constraining their restoration and post-restoration dynamics are poorly understood. Over the decade following restoration of a 3.2 km reach of the Merced River and floodplain in California, we tracked herbaceous community composition to distinguish floodplain habitats and utilized perturbations from revegetation treatments and post-restoration flooding to generate community assembly rule hypotheses regarding treatment effectiveness and persistence, with a particular interest in native perennials capable of suppressing non-natives over time if undisturbed. Revegetation treatments comprised combinations of sowing a sterile cover crop, sowing native species, and inoculating mycorrhizae. Most surveyed floodplain areas comprised a low terrace characterized by exceptionally droughty soils, relatively deep groundwater, and occasional flooding lasting into summer. Few species could tolerate both flood and drought to this extent, and the flood year community was generally distinct from that in non-flood years. Both communities were dominated by ruderals capable of avoiding stress and re-establishing following disturbance, including many non-native annual grassland species. Only Artemisia douglasiana responded to the treatments, as most seeded native species failed to establish, including those native perennial grasses expected to suppress non-native annuals, while other seeded native species either established adequately from natural dispersal or failed to persist through moderate flooding. Neither the cover crop nor mycorrhizal inoculation had any meaningful effect. Restoration efforts in naturally ruderal-dominated habitats may be better spent allowing natural regeneration, addressing particularly noxious invasives, and identifying or constructing habitats supporting long-lived native perennials.

Although originally developed for population sizes and population growth rates, modern capture-recapture models can estimate demographic rates in complex situations: multistate models for multiple study sites and stage-structured populations, superpopulation entry probability models for recruitment, and multievent models when state assessments are uncertain. However, combinations of these complications, such as recruitment studies with uncertain state assessments, are common, yet no single model has explicitly incorporated all of these elements. Ultimately, these models estimate the same fundamental population process with the same general approach, and we combine them in a generalized hidden process model based upon a simple discrete state and transition population model with Poisson recruitment that can estimate how recruitment and survivorship rates vary with respect to measured covariates from uncertain state assessments for a stage-structured population at multiple sites. Although closely related to the motivating models, the generalized model relaxes the Markov assumption. While we provide the distributions necessary to implement Bayesian data augmentation methods, we also provide an efficient analytical likelihood with a compact parameter space that is applicable in the absence of density-dependent mortality. As a demonstration, we estimate the influence of several covariates on recruitment and survivorship rates from uncertain observations of Salix gooddingii seedlings at different locations along a riparian gradient, and we use simulations to examine variation in the precision of estimated parameters.

In Mediterranean climates, cottonwoods and willows often exhibit high germination and seedling mortality rates, with recruitment occurring primarily in the occasional year when favorable spring floods improve survivorship. However, along the Robinson Reach of the Merced River, both germination and mortality rates appeared to be atypically low. To understand why these rates were so low along this recently restored flow-regulated, gravel-bedded stream, we surveyed Populus fremontii, Salix exigua, and Salix gooddingii, estimated germination and survivorship rates, and examined their correlations with factors expected to constrain recruitment, namely seed release, seed arrival, moist germination beds, light levels, groundwater depth, groundwater recession rates, and shear stress. Germination/initial establishment rates were low due in part to low seed arrival rates. Only Salix gooddingii was abundant enough to model in detail, and while moist germination surfaces increased germination/initial establishment, rates were low overall. Survivorship rates for Salix gooddingii seedlings and for small individuals were not correlated with any examined covariates. Seedlings tolerated moderate competition, and the absence of major scouring, even during 6 year flows, enabled survival at sites with sufficiently shallow groundwater that seedlings were unaffected by groundwater recession rates.

Coastal wetlands are globally important ecosystems, valued for their provision of habitat, storm mitigation, water quality improvement, and carbon sequestration. Coastal wetlands are also one of the ecosystems most likely to be impacted by projected changes in climate, particularly changes associated with sea level rise, altered rainfall patterns, and changes to storm patterns and severity. Coastal freshwater wetlands (CFWs) are amongst the most understudied group of coastal wetlands and are characterised by freshwater dominated hydrology but can also experience periods of salinity associated with their proximity to the coast (i.e. as the result of storm surge and spring high tides). CFWs commonly occur as the most landward of coastal wetlands and many adjoin urban development, exposing them to anthropogenic impacts (nutrient enrichment, clearing, hydrology alteration). This position in the coastal landscape makes CFWs highly susceptible to salinity stress, particularly climate change induced sea level rise. Research investigating CFWs, their ecology, and responses to climate change threats, is greatly lacking, particularly for areas outside the United States of America (USA). This thesis investigates the resilience of CFWs to climate change and aims to address significant knowledge gaps by investigating: 1) the current knowledge of CFW responses to projected climate change globally; 2) the structure and composition of CFW vegetation in southeast Queensland and exploring drivers of vegetation patterns; 3) the role of soil seed banks in vegetation resilience for CFWs in southeast Queensland through contributions to vegetation dynamics; and 4) the regenerative potential and responses of CFW vegetation communities to altered hydrology and salinity regimes simulating sea level rise. To begin, I synthesised the current knowledge of CFW responses to projected changes in climate globally, through a systematic quantitative literature review, with the aim of identifying key knowledge gaps regarding geographic locations and research areas, with particular focus on four key aspects of climate change: sea level rise, altered rainfall, extreme events, increased temperature, and greenhouse gases. In Chapter 2, I reviewed published research on responses of CFWs in observational, experimental, and modelling studies within those four key aspects of climate change. This review identified that, despite the increasing research interest, knowledge of all aspects of climate change is lacking, particularly outside of the USA. Within the USA, while there is a significant body of research exploring the response of CFWs to impacts associated with rising sea levels, changes to rainfall patterns, and extreme events, the impacts of temperature and greenhouse gases remain unknown globally. Importantly, research investigating the response of CFWs to multiple climate drivers was identified as a significant knowledge gap.The research then focused on field and greenhouse studies of CFW vegetation communities in southeast Queensland, Australia to expand the knowledge of CFWs outside of the USA. Chapters 3 and 4 explored patterns in standing and soil seed bank vegetation assemblages and provides a baseline understanding of the structure and composition of these vegetation communities. In addition, I assessed local and regional environmental drivers (i.e. local hydrology, soil salinity, local land uses, projected sea level rise extent) of vegetation patterns and discussed potential changes to these drivers with climate change. To explore the effects of sea level rise on ground in CFWs, I conducted an in-situ hydrology and salinity manipulation experiment at an abandoned sugarcane farm which has a regenerating CFW vegetation community (Chapter 5). CFWs in southeast Queensland are important for their roles in nutrient cycling and habitat provision for endangered fauna species. My assessment of these vegetation communities (Chapter 3) also highlights that CFWs are home to a diverse vegetation assemblage including at least two flora species of national significance, flagging the biodiversity importance of these isolated wetland patches within a developed coastal landscape. Species composition was distinct between vegetation patches and a large proportion of variation between sites was associated with differences in local hydrology and salinity influence. The importance of hydrology and salinity as drivers of vegetation patterns suggests that climate change could dramatically impact the structure and composition of CFWs. The ability of CFWs to be maintained in the landscape under a changing climate is influenced by their regenerative capacity from soil seed banks or other propagule banks. In Chapter 4, I assessed the composition of CFW soil seed banks and explored potential drivers of vegetation patterns. Through this study, I again found that hydrology and salinity were strong drivers of patterns in soil seed bank composition, as well as, local land use which was associated with the proportion of exotic species. Similarity of soil seed banks to standing vegetation was low, suggesting that soil seed banks have a minimal role in maintaining standing vegetation communities in southeast Queensland CFWs. Rather, soil seed banks could provide a mechanism for vegetation change along four possible trajectories depending on the soil seed bank composition and abiotic conditions. Hydrology and salinity are important drivers of CFW vegetation composition identified in this thesis. In Chapter 5, I explored the impacts of altered hydrology and salinity regimes on CFW vegetation communities regenerating on abandoned agricultural land. Change in vegetation composition and structure was assessed in-situ at Yandina Creek Wetlands (YCW) where vegetation has naturally regenerated into communities typical of CFWs in southeast Queensland during 15 years since abandoment of sugarcane production. This regeneration potential is important given the projected constriction of CFWs between migrating seaward Changes were varied in each habitat surveyed, however, reductions in vegetation cover and species richness were observed over time in response to altered conditions in freshwater habitats. Conversely, vegetation cover increased in the saltmarsh, suggesting that this community is tolerant of the altered conditions and may expand within YCW. The changes observed at YCW suggest that widespread change is likely for many CFWs with increasing sea levels, but careful management could aid in maintaining these ecosystems in the coastal landscape. Overall, this thesis significantly furthers the understanding of CFWs and their vegetation patterns outside of the USA, and explores the future of these systems with climate change in southeast Queensland, Australia. The findings of this thesis highlight the importance of hydrology and salinity as drivers of vegetation patterns and indicates that dramatic and potentially rapid change in vegetation structure and composition will occur as a result of climate change. CFWs in southeast Queensland are important and highly variable vegetation communities, where loss of even single wetland patches could result in local species extirpation. It is unlikely that CFWs will remain in the landscape in their current state and continue to provide benefits from ecosystem services without significant management action. Even with such action, widespread loss or modification is likely and continued research is required to understand the full scope of climate change impacts.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment and Sc
Science, Environment, Engineering and Technology
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This research presents a new multivariate statistical approach to downscale hydroclimatic variables associated with global climate signals, from low-resolution Global Climate Models (GCMs) to high-resolution grids that are appropriate for regional and local hydrologic analysis. The approach uses Principal Component Analysis (PCA) and Multichannel Singular Spectrum Analysis (MSSA) to: 1) evaluate significant variation modes among global climate signals and spatially distributed hydroclimatic variables within certain spatial domain; 2) downscale the GCMs' projections of the hydroclimatic variables using these significant modes of variation and 3) extend the results to other correlated variables in the space domain. The approach is applied to the Colorado River Basin to determine common oscillations among observed precipitation and temperature patterns in the basin and the global climate signals El Nino Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO). These common oscillations serve as a basis to perform the downscaling of ENSO-related precipitation and temperature projections from GCMs, using a new gap-filling algorithm based on MSSA. The analysis of spatial and temporal correlations between observed precipitation, temperature and vegetation activity (represented by the Normalized Difference Vegetation Index, NDVI) is used to extend the downscaling of precipitation to vegetation responses in ten ecoregions within the basin. Results show significant common oscillations of five and 15-year between ENSO, PDO and annual precipitation in the basin, with wetter years during common ENSO and PDO positive phases and dryer years during common negative phases. Precipitation also shows an increase in variability in the last 20 years of record. Highly correlated responses between seasonally detrended NDVI and precipitation were also identified in each ecoregion, with distinctive delays in vegetation response ranging from one month in the southern deserts (in the fringe of the monsoon precipitation regime), to two months in the mid latitudes and three months to the north, affected by seasonal precipitation. These results were used to downscale precipitation and temperature from two GCMs that perform well in the basin and have a distinctive ENSO-like signal (MPI-ECHAM5 and UKMO-HADCM3) and to extend the downscaling to estimate vegetation responses based on their significant correlations with precipitation.

Agriculture is one of the most important human activities providing food and more agricultural goods for seven billion people around the world and is of special importance in sub-Saharan Africa. The majority of people depends on the agricultural sector for their livelihoods and will suffer from negative climate change impacts on agriculture until the middle and end of the 21st century, even more if weak governments, economic crises or violent conflicts endanger the countries’ food security. The impact of temperature increases and changing precipitation patterns on agricultural vegetation motivated this thesis in the first place. Analyzing the potentials of reducing negative climate change impacts by adapting crop management to changing climate is a second objective of the thesis. As a precondition for simulating climate change impacts on agricultural crops with a global crop model first the timing of sowing in the tropics was improved and validated as this is an important factor determining the length and timing of the crops´ development phases, the occurrence of water stress and final crop yield. Crop yields are projected to decline in most regions which is evident from the results of this thesis, but the uncertainties that exist in climate projections and in the efficiency of adaptation options because of political, economical or institutional obstacles have to be considered. The effect of temperature increases and changing precipitation patterns on crop yields can be analyzed separately and varies in space across the continent. Southern Africa is clearly the region most susceptible to climate change, especially to precipitation changes. The Sahel north of 13° N and parts of Eastern Africa with short growing seasons below 120 days and limited wet season precipitation of less than 500 mm are also vulnerable to precipitation changes while in most other part of East and Central Africa, in contrast, the effect of temperature increase on crops overbalances the precipitation effect and is most pronounced in a band stretching from Angola to Ethiopia in the 2060s. The results of this thesis confirm the findings from previous studies on the magnitude of climate change impact on crops in sub-Saharan Africa but beyond that helps to understand the drivers of these changes and the potential of certain management strategies for adaptation in more detail. Crop yield changes depend on the initial growing conditions, on the magnitude of climate change, and on the crop, cropping system and adaptive capacity of African farmers which is only now evident from this comprehensive study for sub-Saharan Africa. Furthermore this study improves the representation of tropical cropping systems in a global crop model and considers the major food crops cultivated in sub-Saharan Africa and climate change impacts throughout the continent.
Landwirtschaft ist eine der wichtigsten menschlichen Aktivitäten, sie stellt Nahrungsmittel und andere landwirtschaftliche Produkte für weltweit 7 Milliarden Menschen zur Verfügung und ist in den Ländern Afrikas südlich der Sahara von besonderer Bedeutung. Die Mehrheit der afrikanischen Bevölkerung bestreitet ihren Lebensunterhalt in der Landwirtschaft und wird von Klimaänderungen stark betroffen sein. Die Doktorarbeit ist durch die Frage motiviert, wie sich von Klimamodellen vorhergesagte Temperaturerhöhungen und sich verändernde Niederschlagsverteilungen auf die landwirtschaftliche Vegetation auswirken werden. Die Forschungsfragen in diesem Kontext beschäftigen sich mit regionalen Unterschieden von Klimaänderungen und ihren Auswirkungen auf die Landwirtschaft und mit möglichen Anpassungsstrategien die mit geringem technischem Aufwand genutzt werden können. In diesem Zusammenhang wird schnell deutlich, dass Daten über die komplexen landwirtschaftlichen Systeme in Afrika südlich der Sahara häufig nur selten vorhanden sind, aus fragwürdigen Quellen stammen oder von schlechter Qualität sind. Die Methoden und Modelle zur Untersuchung der Auswirkungen von Klimaänderungen auf die Landwirtschaft werden zudem ausschließlich in Europa oder Nordamerika entwickelt and häufig in den temperierten Breiten aber seltener in tropischen Gebieten angewendet. Vor allem werden globale, dynamische Vegetationsmodelle in Kombination mit Klimamodellen eingesetzt um Änderungen in der landwirtschaftlichen Produktion auf Grund von Klimaänderungen in der zweiten Hälfte des 21.Jahrhunderts abzuschätzen. Die Ergebnisse der Arbeit zeigen einen mittleren Ertragsrückgang für die wichtigsten landwirtschaftlichen Pflanzen um 6% bis 24% bis 2090 je nach Region, Klimamodell und Anpassungsstrategie. Dieses Ergebnis macht deutlich, dass Landwirte die negativen Folgen von Klimaänderungen abschwächen können, wenn sie die Wahl der Feldfrucht, die Wahl des Anbausystems und den Aussaattermin an geänderte Klimabedingungen anpassen. Die Arbeit stellt methodische Ansätze zur Berechung des Aussaattermins in temperierten und tropischen Gebieten (Kapitel 2) sowie zur Simulation von Mehrfachanbausystemen in den Tropen vor (Kapitel 3). Dabei werden wichtige Parameter für das globale, dynamische Vegetationsmodell LPJmL überprüft und neu berechnet. Es zeigt sich, dass das südliche Afrika und die Sahelregion die am stärksten betroffenen Regionen sind, vor allem aufgrund von Niederschlagsänderungen, weniger aufgrund von Temperaturerhöhungen. In den meisten anderen Teilen, vor allem Zentral- und Ostafrikas bedingen Temperaturerhöhungen Rückgänge der Erträge (Kapitel 4). Diese Arbeit leistet einen wichtigen und umfassenden Beitrag zum Verständnis der Auswirkung von Klimaänderung auf die landwirtschaftliche Vegetation und damit zu einem großen Teil auf die Lebensgrundlage von afrikanischen Landwirten.

Models of climate change predict that temperature will increase during the 21th century and the largest warming will take place at high northern latitudes. In addition to warming, predictions for northern Europe include increased annual precipitation and a higher proportion of the precipitation during winter falling as rain instead of snow. These changes will substantially alter the hydrology of rivers and streams and change the conditions for riverine communities. The warming is also expected to result in species adjusting their geographic ranges to stay within their climatic tolerances. Riparian zones and wetlands are areas where excess water determines the community composition. It is therefore likely that these systems will be highly responsive to alterations in precipitation and temperature patterns. In this thesis we have tested the predicted responses of riparian vegetation to climate-driven hydrologic change with a six year long transplant experiment (I). Turfs of vegetation were moved to a new elevation with shorter or longer flood durations. The results demonstrate that riparian species will respond to hydrologic changes, and that without rare events such as unusually large floods or droughts, full adjustment to the new hydrological regime may take at least 10 years. Moreover, we quantified potential effects of a changed hydrology on riparian plant species richness (II) and individual species responses (III) under different climate scenarios along the Vindel River in northern Sweden. Despite relatively small changes in hydrology, the results imply that many species will become less frequent than today, with stochastic extinctions along some reaches. Climate change may threaten riparian vegetation along some of the last pristine or near-natural river ecosystems in Europe. More extensive loss of species than predicted for the Vindel River is expected along rivers in the southern boreal zone, where snow-melt fed hydrographs are expected to be largely replaced by rain-fed ones. With a seed sowing experiment, we tested the differences in invasibility between open wetlands, forested wetlands and riparian zones (IV). All six species introduced were able to germinate and survive in all habitats and disturbance levels, indicating that the tested wetlands are generally invisible. Germination was highest in open wetlands and riparian zones. Increasing seed sowing density increased invasion success, but the disturbance treatments had little effect. The fact that seeds germinated and survived for 2 to 3 years in all wetland habitats indicates that wetland species with sufficiently high dispersal capacity and propagule pressure would be able to germinate and establish here in their respective wetland type. Our results clearly demonstrate that a changed climate will result in substantial changes to functioning, structure and diversity of boreal wetland and riparian ecosystems. To preserve species rich habitats still unaffected by dams and other human stressors, additional protection and management actions may have to be considered.

In cities, microclimates are created by local mixtures of vegetation, constructed materials, vertical structure, and moisture, with significant consequences for human health, air quality, and resource use. Vegetation can moderate microclimates through evapotranspiration, however this function is dependent on local conditions so its effect may vary over space and time. This dissertation used hyperspectral and thermal remote sensing imagery to derive key observations of urban physical and biophysical properties and model urban microclimates across the megacity of Los Angeles. In Chapter 1, I used Multiple Endmember Spectral Mixture Analysis (MESMA) to map sub-pixel fractions of different vegetation types, as well as other types of urban cover, at 4 m and 18 m resolution over Santa Barbara, California (Wetherley et al., 2017). Fractional estimates correlated with validation fractions at both scales (mean R² = 0.84 at 4 m and R² = 0.76 at 18 m), with accuracy affected by image spatial resolution, endmember spatial resolution, and class spectral (dis)similarity. Accuracy was improved by using endmembers measured at multiple spatial resolutions, likely because they incorporated additional spectral variability that occurred across spatial scales. In Chapter 2, I applied this methodology to derive sub-pixel cover for the greater Los Angeles metropolitan area (4,466 km²) (Wetherley et al., 2018). Further improvement in quantifying sub-pixel vegetation types was achieved by modifying the MESMA shade parameter. Land surface temperature (LST), derived from thermal imagery, was used to model temperature change along vegetation fractional gradients, with slopes of LST change showing significant differences between trees and turfgrass (p < 0.001). Expected per-pixel LST was derived from these gradients based on sub-pixel composition, and when compared to measured LST was found to deviate with a standard deviation of 3.5 °C across the scene. These deviations were negatively related to irrigation and income, while building density was observed to affect tree LST more than it affected turfgrass LST. In Chapter 3, I used the map of Los Angeles landcover, along with data from LiDAR, GIS, and WRF climate variables, to parameterize an urban climate model (Surface Urban Energy and Water Balance Scheme: SUEWS) for 2,123 neighborhoods (each 1 km²) across Los Angeles. Modeled latent fluxes were correlated with remote sensing LST (R² = 0.39) collected over a period of 5 hours, with an overall diurnal pattern modified by irrigation timing. Spatial variability across the study area was related to local landcover, with albedo and vegetation fraction strongly influencing latent and sensible fluxes. A strong regional climatic gradient was observed to affect latent fluxes based on coastal proximity. Overall, this dissertation quantifies the key drivers of urban vegetation function in a large city, and further demonstrates the potential of hyperspectral and thermal imagery for observing city scale surface and microclimate variability.

Doutoramento em Engenharia Florestal - Instituto Superior de Agronomia - UL
Vegetation fires are an important component of the earth’s system land processes and have a significant impact on the vegetation and CO2 dynamics. The global fire patterns are not thoroughly explored and the drivers of fire regimes in global scale are interconnected. However, several modelling assumptions are contradicted by exploring those relationships partially. At global scale, fire extent is fuel limited, with climatic variables showing both positive and negative influence on fuel moisture conditions, and humans showing a negative net effect. When isolating the influence of population density and assuming spatial nonstationarity, the human impact is very detailed and reflects the main land use activities with emphasis on cropland and rangeland management at continental scale. The footprint of fire into the Earth system can be measured in terms of radiative forcing from pre and post-fire albedo changes, with the forest biomes driving the extremes on annual basis. Additionally this thesis explores the patterns and the trends of contemporary fire activity. Contrary to previous studies, the results show non-monotonic patterns at grid cell level. The findings of this thesis give a better insight into the spatial variability and the controls of fire at global scale using satellite derived datasets with a focus to the anthropogenic land use activities

Cette thèse étudie la dynamique de la végétation Holocène pour Burmarrad dans le NW Malte et fournit une reconstruction paléoclimatique quantitative à base de pollen pour cet archipel méditerranéen situé au centre. Le record de pollen donne un nouvel aperçu l'échange végétation 7280-1730 cal BP qui correspondent bien avec other régional dossiers. La reconstruction du climat fournit également forte corrélation avec les sites du sud (en dessous de 40oN) de la Méditerranée. L'interprétation suggère un paysage initialement ouvert au début du néolithique, se développer en un dense Pistacia brousse ca. 6700 calBP. Depuis environ 4450 calBP le paysage devient de nouveau ouvert, coïncidant avec le début de l'âge du bronze sur l'archipel. Cette période coïncide avec une instabilité accrue du climat (entre 4500 et 3700 calBP) qui est suivie par une diminution progressive de la disponibilité de l'humidité de l'été à la fin de l'Holocène. Durant la période romaine début de l'occupation (1972-1730 calBP) le paysage reste généralement ouvert avec une augmentation modérée de Olea. Cette augmentation correspond à des preuves archéologiques pour la production de l'huile d'olive dans la région, avec l'augmentation des taxons cultivés des cultures et des espèces rudérales associés, ainsi que d'une hausse des cas d'incendies. Cette thèse propose également une synthèse des résultats d'un autre noyau (BM1) provenant de la même zone de chalandise, ainsi que les résultats d'une étude préliminaire de la pluie de pollen de surface moderne. L'archipel fournit des indications sur la végétation, les impacts humains et les changements climatiques dans un contexte de l'île au cours de l'Holocène
This thesis investigates the Holocene vegetation dynamics for Burmarrad in north-west Malta and provides a pollen-based quantitative palaeoclimatic reconstruction for this centrally located Mediterranean archipelago. The pollen record from this site provides new insight into the vegetation changes from 7280 to 1730 cal BP which correspond well with other regional records. The climate reconstruction for the area also provides strong correlation with southern (below 40oN) Mediterranean sites. The interpretation suggests an initially open landscape during the early Neolithic, surrounding a large palaeobay, developing into a dense Pistacia scrubland ca. 6700 cal BP. From about 4450 cal BP the landscape once again becomes open, coinciding with the start of the Bronze Age on the archipelago. This period is concurrent with increased climatic instability (between 4500 and 3700 cal BP) which is followed by a gradual decrease in summer moisture availability in the late Holocene. During the early Roman occupation period (1972 to 1730 cal BP) the landscape remains generally open with a moderate increase in Olea. This increase corresponds to archaeological evidence for olive oil production in the area, along with increases in cultivated crop taxa and associated ruderal species, as well as a rise in fire events. This thesis also provides a synthesis with the results from another core (BM1) taken from the same catchment area, as well as results of a preliminary modern surface pollen rain study. The Maltese archipelago provides important insight into vegetation, human impacts and climatic changes in an island context during the Holocene

Anthropogenic emissions of the greenhouse gases carbon dioxide and methane have stimulated a rise in global surface temperature of 0.76 ºC since the turn of the 20th Century. Such climate warming has already had significant impacts on the terrestrial biosphere, raising concerns that ecosystems will feed back to future climate by altering the balance of carbon flow between the land and atmosphere. It is well established that warming can directly affect rates of photosynthesis and ecosystem respiration, which together dictate the carbon balance of most ecosystems. However, warming is also causing shifts in the productivity and composition of vegetation, and there is growing recognition that this can have indirect effects on carbon cycling via its influence over soil properties and the activity of the soil microbial community. Despite this, much uncertainty currently surrounds the effects of warming on vegetation, both between individuals of the same species and at the plant community scale. Furthermore, the consequences of vegetation change at either scale for carbon dynamics are not well understood when considered in tandem with warming. Northern peatlands are of particular relevance to ecosystem climate feedbacks, holding one third of global soil carbon in regions vulnerable to rapid temperature change. The aim of this thesis was to explore how different scales of vegetation change regulate peatland carbon cycle responses to warming. This was achieved using field experiments across Europe, including manipulations, reciprocal transplants and gradient studies, which integrated a range of approaches for detecting responses from the genetic to ecosystem scale. By measuring the growth responses of dominant plant species to warming and the presence or absence of different plant functional types (Chapter 2), I reveal interdependencies between peatland plants that will cause community change over decades to centuries. I suggest that such responses occur due to both plant-plant interactions and the influence of vegetation over microclimate, the latter of which I also show to be regulated by vegetation composition (Chapter 3). Additionally, I demonstrate that warming at different temporal scales has contrasting effects on the metabolism, photosynthesis and growth of Eriophorum vaginatum (Chapter 5), causing decreases in growth over seasons to years through phenotypic plasticity and increases in growth over centuries to millennia through natural selection. I show that such adaptation of individual plants to rising temperature has potential consequences for peatland carbon dynamics. Moreover, I reveal that changes in vegetation composition at the community level could destabilise the peatland carbon stock under warming by accelerating decomposition of ancient carbon (Chapter 4). Through these findings, I provide novel insight into the scales and mechanisms by which vegetation responses to warming impact carbon dynamics. Given the key role of northern peatlands in the global carbon cycle, I suggest that warming effects on peatland vegetation may have considerable consequences for future climate by controlling liberation of peatland carbon into the atmosphere.

Mapping and explaining the distribution of vegetation helps land managers to make systematic conservation planning decisions. This is typically achieved using models that correlate the distribution of species with environmental factors, and can predict the vegetation at unsurveyed locations. These Species Distribution Models (SDMs) have numerous unresolved issues, but serve as a useful first-pass approximation for planning purposes.This thesis investigates some of the uncertainties of SDMs, including the impact of data accuracy, the incorporation of spatial processes, the evaluation of alternative models, and the benefits and challenges of producing models at the landscape scale. The research was conducted on the Illawarra Escarpment, 80 km south of Sydney, Australia (34.4 oS, 150.9 oE). The escarpment contains a north-south trend in eucalypts (Eucalyptus spp.) that cannot be explained in terms of elevation or geology. It also exhibits a patchy distribution of rainforest communities, some unique to the Illawarra. It is not known which environmental factors determine the distribution of either the eucalypts or rainforest species, or how they may respond to a changing climate.Species distributions are sensitive to the accuracy of data used, and yet many models only use elevation as a surrogate for temperature, or use simple elevation sensitive interpolations from weather stations. I collected hourly temperature data from 40 sites on the Illawarra Escarpment, and investigated whether elevation was an adequate surrogate for temperatures in this landscape. I then investigated whether temperature surfaces could be improved by considering other topographic and geographic factors, including exposure to wind, distance to coast, radiation, and the average conditions in the surrounding neighbourhood. Elevation was well correlated with moderate seasonal temperatures (e.g. summer minima and winter maxima), but was poorly correlated with the extreme temperatures (summer maxima, winter minima) that are physiologically limiting for many species. Using neighbourhood influences, exposure to wind and distance to coast improved the accuracy of temperature surfaces, and increased the explanatory performance of vegetation models. I concluded that elevation was not always an adequate surrogate for temperature. Temperatures are also affected by other topographic and geographic factors, and these should be considered when developing models for systematic conservation planning activities.Species distribution models are typically based solely on niche factors. Where spatial processes are included, it is typically by employing autologistic regression, or other techniques that use survey data as a predictor. This precludes the models being used to make predictions in times or places where survey data is unavailable, and reduces ecological explanation because it is an interpolation technique. I used neighbourhood (contextual) indices based on environmental factors as an alternative method to overcome these problems. I demonstrate that contextual indices improve SDMs over purely niche-based models, and are capable of predicting unknown populations in unsurveyed areas. I conclude that contextual indices have numerous advantages over autologistic regression, and can capture a continuum between niche and dispersal limited species.Models that predict how species will respond to climate change either use coarse-scale climate surfaces, or idealised predictions of uniform warming. These methods may dramatically over-estimate extinction risk because they neglect fine-scale variations in warming, and refugia where species can persist despite unfavourable regional conditions. I created fine-scale estimates of warming by combining 35 years of Bureau of Meteorology observations with one year of intensive fine-scale temperature monitoring. I found that warming was greatest at inland locations, at lower elevations, away from streams, and at sites exposed to hot, dry northwesterly winds. As species are biased in the geographic and topographic positions they occupied, some species have experienced more warming than others and are at greater threat from climate change. I concluded that it was important to continue developing methods to downscale coarse-grained climate surfaces, and suggest that the accuracy of this process could be improved by using a range of topographic factors.There are many methods for selecting predictors in SDMs, and the competing models often make highly variable predictions. I addressed this uncertainty by comparing the performance of models with and without a given environmental factor. I found that there was relatively strong support for the geology and winter minimum temperature predictors, as well as predictors based on contextual indices, as there was a significant drop in model performance when these predictors were excluded. In contrast, there was less support for summer maximum temperature, as other temperature predictors could combine to produce similar model performance. Model performance varied more between models for different species than between different predictor combinations for the same species. I concluded that it was inappropriate to assess models based on subjective benchmarks, such as an AUC of more than 0.7. A comparison between competing models for the same species gives a better indication of the validity of the model building procedure.The results of this research provide important insights into the benefits and challenges of creating SDMs at the landscape scale (extent of 10–200 km). It is a major challenge to obtain spatially and thematically accurate environmental predictors and biotic data at this scale, and studies should include the collection of data to ensure models are adequate. Landscapes will not have as much environmental variation as coarse-scale models, and this will limit the ability to transfer the models to new study areas. However, there are a number of benefits that justify these studies. Producing accurate temperature surfaces at the landscape scale will result in less pseudoreplication and less predictor colinearity. This will improve the robustness of models. Landscape scale studies also allow modellers to capture fine-scale refugia, and this will improve the accuracy of climate change predictions. Finally, many ecological processes operate at a scale that is too fine to be detected with coarse-scale models. Landscape scale models may be the only alternative to detect these processes. There is no optimal scale for SDMs, however, and a future challenge is to better integrate coarse and fine-scale models to make more ecologically robust predictions.

In arctic tundra ecosystems mosses dominant the vegetation in terms of productivity and diversity.  Despite this, mosses are often overlooked in studies of tundra ecology.  However, evidence from this thesis suggests that mosses maybe integral to the functioning of these systems.  Mosses insulate soil keeping it cooler than air temperature, an effect more apparent under deeper moss.  The effects of the moss layer on soil characteristics alter conditions for microbial populations resulting in higher nitrogen availability in soil under shallow moss.  This thesis shows that the role of mosses in determining vascular plant success may dictate many higher plant interactions.  There are both positive and negative effects of the moss layer on vascular plant growth.  The relationship between positive and negative impacts of the moss layer on vascular plants is species specific, meaning that moss cover may be a key determinate of vascular plant community structure.  Climatic warming and herbivory are important drivers of vegetation change in the Arctic.  This thesis shows that grazing by reindeer and grubbing by geese is detrimental to moss cover.  As mosses insulate the soil, a reduction in depth or integrity increases soil temperatures and enhances microbial activity and thus nitrogen availability.  This in conjunction with addition of nutrients from faeces enhances vascular plant productivity to the further detriment of mosses.  Warming increases soil temperature and accelerates decomposition, but has little affect on either biomass of moss or vascular plants.  Moss grubbing has a greater negative effect on mosses in a warmed environment.  This thesis concludes that mosses are integral to the current functioning of tundra heaths.

Thesis (Ph. D.)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2008.
Stieglitz, Marc, Committee Member ; Guillas, Serge, Committee Member ; Fu, Rong, Committee Member ; Curry, Judith, Committee Member ; Dickinson, Robert, Committee Chair.

Large amounts of carbon (C) are stored in tundra soils. Global warming may turn tundra ecosystems from C sinks into sources or vice versa, depending on the balance between gross primary production (GPP), ecosystem respiration (ER) and the resulting net ecosystem exchange (NEE). We aimed to quantify the summer season C balance of a 27 km2 tundra landscape in subarctic Sweden. We measured CO2 fluxes in 37 widely distributed plots across five tundra vegetation types and in 7 additional bare soil plots, to assess effects of abiotic and biotic components on C exchange. C fluxes in bare soils were low and differed to all vegetation types. Thus, accounting for differences between bare soils and vegetated parts is crucial for upscaling a C balance using a landcover classification map. In addition, we found that both NEE and ER, varied within and across different tundra vegetation types. The C balance model for the growing season 2016 revealed a net C loss to the atmosphere. Most vegetation types acted as CO2 sources, with highest source strength in dense shrub vegetation at low elevations. The only considerable C sinks were graminoid-dominated upland meadows. In addition, we found a shift in C balance between different heath vegetation types, ranging from C source in dense deciduous shrub vegetation (Mesic Heath and Dry Heath) to C sink in low growing shrub vegetation (Extremely Dry Heath). These results highlight the importance to account for differences between vegetation types when modelling C fluxes from plot to landscape level.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2000.
Includes bibliographical references (p. 211-224).
The climate of West Africa exhibits significant variability at the time scale of decades. The persistent drought of the past three decades is an example of such variability. This study investigates the role of vegetation dynamics in shaping the low-frequency variability of the climate over West Africa. A zonally symmetric, synchronously coupled biosphere-atmosphere model (ZonalBAM) which includes explicit representation of vegetation dynamics has been developed, and has been validated using observations on both the atmospheric and biospheric climate. The model is then used to study the dynamics of the coupled biosphere-atmosphere system over West Africa. Based on the model sensitivity to initial conditions and the resilience of the coupled system with respect to perturbations, we demonstrate that the coupled biosphere-atmosphere system over West Africa has multiple equilibrium states, with reversible transitions between different equilibria. The two-way biosphere-atmosphere feedback is a significant process in both climate persistence and climate transition. Based on long-term climate simulations using ZonalBAM driven with the observed sea surface temperature (SST) variations, our study shows that vegetation dynamics is a significant process in shaping the climate variability of West Africa. The response of the regional climate system to large-scale forcings is significantly regulated by vegetation dynamics. The relatively slow response of vegetation to changes in the atmosphere is a significant mechanism that acts to enhance the low-frequency rainfall variability. Climate transitions between different equilibria act as another mechanism contributing to the low-frequency rainfall variability - multi-decadal fluctuations can take place as a collective reflection of climate persistence at one equilibrium and climate transition towards another. Vegetation dynamics seems to play an important role in the development and persistence of the current Sahel drought. The most likely scenario for the triggering mechanism of the Sahel drought would involve a combination of several processes including regional changes in land cover as well as changes in the patterns of global and regional SST distributions. However, regardless of the nature of the triggering mechanism, the response of the natural vegetation to the atmospheric changes is the critical process in the development and persistence of the observed drought.
by Guiling Wang.
Ph.D.

In the southwest United States, the current prolonged warm drought is similar to the predicted future climate change scenarios for the region. This study aimed to determine patterns in vegetation response to the early 21st century drought across multiple biomes. We hypothesized that different biomes (forests, shrublands, and grasslands) would have different relative sensitivities to both climate drivers (precipitation and temperature) and legacy effects (previous-year's productivity). We tested this hypothesis at eight Ameriflux sites in various Southwest biomes using NASA Moderate-resolution Imaging Spectroradiometer Enhanced Vegetation Index (EVI) from 2001 to 2013. All sites experienced prolonged dry conditions during the study period. The impact of combined precipitation and temperature on Southwest ecosystems at both annual and sub-annual timescales was tested using Standardized Precipitation Evapotranspiration Index (SPEI). All biomes studied had critical sub-annual climate periods during which precipitation and temperature influenced production. In forests, annual peak greenness (EVImax) was best predicted by 9-month SPEI calculated in July (i.e., January-July). In shrublands and grasslands, EVImax was best predicted by SPEI in July through September, with little effect of the previous year's EVImax. Daily gross ecosystem production (GEP) derived from flux tower data yielded further insights into the complex interplay between precipitation and temperature. In forests, GEP was driven by cool-season precipitation and constrained by warm-season maximum temperature. GEP in both shrublands and grasslands was driven by summer precipitation and constrained by high daily summer maximum temperatures. In grasslands, there was a negative relationship between temperature and GEP in July, but no relationship in August and September. Consideration of sub-annual climate conditions and the inclusion of the effect of temperature on the water balance allowed us to generalize the functional responses of vegetation to predicted future climate conditions. We conclude that across biomes, drought conditions during critical sub-annual climate periods could have a strong negative impact on vegetation production in the southwestern United States.

La fenologia és la clau per a controlar els processos fisicoquímics i biològics, especialment l’albedo, la rugositat superficial, conductància de les fulles, fluxos de carboni, aigua i energia. Per tant, l’estimació de la fenologia és cada vegada més important per a comprendre els efectes del canvi climàtic en els ecosistemes i les interaccions biosfera-atmosfera. La teledetecció és una eina útil per a caracteritzar la fenologia, encara que no existeix un consens sobre el tipus de sensor satel·litari i metodologia òptims per a extreure mètriques fenològiques. Els objectius principals de la meva investigació van ser (i) millorar l’estimació de la fenologia vegetal a partir de dades satel·litàries, (ii) validar les estimacions fenològiques amb observacions terrestres i teledetecció propera a la superfície, i (iii) comprendre les relacions entre les variables climàtiques i la fenologia en un context de canvi climàtic, així com avaluar les respostes de la vegetació en esdeveniments extrems. Aquests objectius s’exploren en els següents tres capítols de la tesi. En el capítol 2, vaig investigar la sensibilitat de la fenologia a (I) la variable de vegetació: índex de vegetació NDVI, índex d’àrea foliar (LAI), fracció de radiació fotosintèticament activa absorbida (FAPAR) i fracció de coberta vegetal (FCOVER); (II) el mètode suavitzat per a derivar trajectòries estacionals; i (III) el mètode d’estimació fenològica: llindars, funció logística, mitjana mòbil i primera derivada. El mètode basat en llindars aplicat a la sèrie temporal Copernicus Global Land LAI V2 suavitzada va donar resultats òptims al validar-los amb observacions terrestres, amb errors quadràtics mitjans de ~10 d i ~25 d per a l’inici d’estació fenològica i la senescència respectivament. En el tercer capítol vaig utilitzar mesures fenològiques continues de PhenoCam i FLUXNET a alta resolució temporal (30 minuts). Això permet una comparació més robusta i precisa amb la fenologia estimada a partir de satèl·lit, evitant problemes relacionats amb les diferències en la definició de mètriques fenològiques. Vaig validar la fenologia estimada a partir de sèries de temps de LAI amb PhenoCam i FluxNet en 80 boscos caducifolis. Els resultats van mostrar una forta correlació (R2 > 0,7) entre la fenologia obtinguda mitjançant teledetecció i les observacions terrestres per a l’inici d’estació i R2 > 0,5 per al final d’estació. El mètode basat en llindars va funcionar millor amb un error quadràtic mitjà de ~9 d amb PhenoCam i ~7 d amb FLUXNET per a l’inici de l’estació, i ~12 d i ~10 d, respectivament, per a la senescència. En el quart capítol vaig investigar els patrons espai – temporals de la resposta fenològica a les anomalies climàtiques en l’hemisferi nord utilitzant la fenologia estimada en el Capítol 2 i validat en el Capítol 2 i Capítol 3, i conjunts de dades climàtiques de múltiples fonts per a 2000-2018 a resolucions de 0,1º. També vaig avaluar l’impacte de les onades de calor extremes i les sequeres en la fenologia. Les anàlisi de correlació parcial de les mètriques fenològiques estimades amb satèl·lit i les variables climàtiques van indicar que els canvis en la temperatura pre estacional van tenir major influència sobre les anomalies fenològiques que la precipitació: com més alta és la temperatura, més aviat es l’inici estacional en la majoria de boscos caducifolis (coeficient de correlació mitjà de -0,31). Tant la temperatura com la precipitació van contribuir a l’avanç i retard del final d’estació. Un retard en la senescència es va correlacionar significativament amb un índex de precipitació – evapotranspiració estandarditzat (SPEI) positiu (~ 30% dels boscos). El final i inici d’estació va canviar > 20 d en resposta de l’onada de calor en la major part d’Europa en 2003 i als Estats Units d’Amèrica l’any 2012.
La fenología es clave para controlar los procesos fisicoquímicos y biológicos, especialmente el albedo, la rugosidad superficial, conductancia de las hojas, flujos de carbono, agua y energía. Por lo tanto, la estimación de la fenología es cada vez más importante para comprender los efectos del cambio climático en los ecosistemas y las interacciones biosfera-atmósfera. La teledetección es una herramienta útil para caracterizar la fenología, aunque no existe consenso sobre el tipo de sensor satelital y metodología óptimos para extraer métricas fenológicas. Los objetivos principales de mi investigación fueron (i) mejorar la estimación de la fenología vegetal a partir de datos satelitales, (ii) validar las estimaciones fenológicas con observaciones terrestres y teledetección cercana a la superficie, y (iii) comprender las relaciones entre las variables climáticas y la fenología en un contexto de cambio climático, así como evaluar las respuestas de la vegetación a eventos extremos. Estos objetivos se exploran en los siguientes tres capítulos de la tesis. En el capítulo 2, investigué la sensibilidad de la fenología a (I) la variable de vegetación: índice de vegetación NDVI, índice de área foliar (LAI), fracción de radiación fotosintéticamente activa absorbida (FAPAR) y fracción de cubierta vegetal (FCOVER); (II) el método de suavizado para derivar trayectorias estacionales; y (III) el método de estimación fenológica: umbrales, función logística, media móvil y primera derivada. El método basado en umbrales aplicado a la serie temporal Copernicus Global Land LAI V2 suavizada dio resultados óptimos al validarlos con observaciones terrestres, con errores cuadráticos medios de ~10 d y ~25 d para el inicio de estación fenológica y la senescencia respectivamente. En el tercer capítulo, utilicé medidas fenológicas continuas de PhenoCam y FLUXNET a alta resolución temporal (30 minutos). Esto permite una comparación más robusta y precisa con la fenología estimada a partir de satélite, evitando problemas relacionados con las diferencias en la definición de métricas fenológicas. Validé la fenología estimada a partir de series de tiempo de LAI con PhenoCam y FluxNet en 80 bosques caducifolios. Los resultados mostraron una fuerte correlación (R2 > 0,7) entre la fenología obtenida mediante teledetección y las observaciones terrestres para el inicio de estación y R2 > 0,5 para el final de estación. El método basado en umbrales funcionó mejor con un error cuadrático medio de ~9 d con PhenoCam y ~7 d con FLUXNET para el inicio de estación, y ~12 d y ~10 d, respectivamente, para la senescencia. En el cuarto capítulo, investigué los patrones espacio-temporales de la respuesta fenológica a las anomalías climáticas en el hemisferio norte utilizando la fenología estimada en el Capítulo 2 y validado en el Capítulo 2 y Capítulo 3, y conjuntos de datos climáticos de múltiples fuentes para 2000-2018 a resoluciones de 0.1°. También evalué el impacto de las olas de calor extremas y las sequías en la fenología. Los análisis de correlación parcial de las métricas fenológicas estimadas con satélite y las variables climáticas, indicaron que los cambios en la temperatura pre estacional tuvieron mayor influencia sobre las anomalías fenológicas que la precipitación: cuanto mayor es la temperatura, más temprano es el comienzo estacional en la mayoría de los bosques caducifolios (coeficiente de correlación medio de -0,31). Tanto la temperatura como la precipitación contribuyeron al avance y retraso del final de estación. Un atraso en la senescencia se correlacionó significativamente con un índice de precipitación-evapotranspiración estandarizado (SPEI) positivo (~ 30% de los bosques). El final e inicio de estación cambió >20 d en respuesta a la ola de calor en la mayor parte de Europa en 2003 y en los Estados Unidos de América en 2012.
Phenology is key to control physicochemical and biological processes, especially albedo, surface roughness, canopy conductance and fluxes of carbon, water and energy. High-quality retrieval of land surface phenology (LSP) is thus increasingly important for understanding the effects of climate change on ecosystem function and biosphere–atmosphere interactions. Remote sensing is a useful tool for characterizing LSP although no consensus exists on the optimal satellite dataset and the method to extract phenology metrics. I aimed to (i) improve the retrieval of Land Surface Phenology from satellite data, (ii) validate LSP with ground observations and near surface remote sensing, and (iii) understand the relationships between climate variables and phenology in a climate change context, as well as to assess the responses of vegetation to extreme events. These three main research objectives are explored in the three chapters of the thesis. In chapter 2, I investigated the sensitivity of phenology to (I) the input vegetation variable: normalized difference vegetation index (NDVI), leaf area index (LAI), fraction of absorbed photosynthetically active radiation (FAPAR), and fraction of vegetation cover (FCOVER); (II) the smoothing and gap filling method for deriving seasonal trajectories; and (III) the phenological extraction method: threshold, logistic-function, moving-average and first derivative based approaches. The threshold-based method applied to the smoothed and gap-filled Copernicus Global Land LAI V2 time series agreed the best with the ground phenology, with root mean square errors of ~10 d and ~25 d for the timing of the start of the season (SoS) and the end of the season (EoS), respectively. In the third chapter, I took advantage of PhenoCam and FLUXNET capability of continuous monitoring of vegetation seasonal growth at very high temporal resolution (every 30 minutes). This allows a more robust and accurate comparison with LSP derived from satellite time series avoiding problems related to the differences in the definition of phenology metrics. I validated LSP estimated from LAI time series with near-surface PhenoCam and eddy covariance FLUXNET data over 80 sites of deciduous broadleaf forest. Results showed a strong correlation (R2 > 0.7) between the satellite LSP and ground-based observations from both PhenoCam and FLUXNET for the timing of the start (SoS) and R2 > 0.5 for the end of season (EoS). The threshold-based method performed the best with a root mean square error of ~9 d with PhenoCam and ~7 d with FLUXNET for the timing of SoS, and ~12 d and ~10 d, respectively, for the timing of EoS. In the fourth chapter, I investigated the spatio-temporal patterns of the response of deciduous forests to climatic anomalies in the Northern Hemisphere using LSP derived in Chapter 1 and validated in Chapter 1 and Chapter 2, and multi-source climatic data sets for 2000–2018 at resolutions of 0.1°. I also assessed the impact of extreme heatwaves and droughts on deciduous forest phenology. Analyses of partial correlations of phenological metrics with the timing of the start of the season (SoS), end of the season (EoS), and climatic variables indicated that changes in preseason temperature played a stronger role than precipitation in the interannual variability of SoS anomalies: the higher the temperature, the earlier the SoS in most deciduous forests in the Northern Hemisphere (mean correlation coefficient of -0.31). Both temperature and precipitation contributed to the advance and delay of EoS. A later EoS was significantly correlated with a positive standardized precipitation-evapotranspiration index (SPEI) at the regional scale (~30% of deciduous forests). The timings of EoS and SoS shifted by >20 d in response to heat waves throughout most of Europe in 2003 and in the United States of America in 2012.
Universitat Autònoma de Barcelona. Programa de Doctorat en Ecologia Terrestre

Cette thèse présente les données des pollens et micro-charbons fossiles couvrant la période des 50 000 dernières années à partir de sites sélectionnés transversalement nord-sud du désert de Namib. Dans le cadre de cette thèse, on utilise le rock hyrax middens, l’accumulation des boulettes et des urines fossilisés du Procavia capensis, représentant une excellente archive pour archives pour la préservation des pollens et micro-charbons à long-terme. Trois sites des hyrax middens ont été sélectionnés pour l'analyse: au sud du désert de Namib (Pella), la marge est des dunes de sable de Namib (Zizou) et le centre de la Namib (Spitzkoppe). En plus, le régime pluvial de ces sites se caractérise par une forte variabilité annuelle et interannuelle. En conséquence, tous ces sites se situent au long de l’écotone du Désert et du biome Nama-Karoo, ainsi qu'à l’est (biome de Savane). Alors que ces sites sont répertoriés dans des écosystèmes similaires, l’écotone, lui, est considéré comme une zone potentiellement très sensible au changement du système climatique régional. Un intérêt spécifique de ces enregistrements terrestres est pour évaluer s’ils corroborent ou s’opposent avec les résultats provenant ceux des sédiments marins de la côte namibienne, en particulier la conclusion : l’abondance des taxa dominants du Fynbos Biome du Cape peut indiquer significativement une expansion vers le nord de la flore du Cape pendant les périodes plus froides glaciaires. Selon les sites d’études sélectionnées, les conclusions principales de ce travail sont les suivantes:Les hyrax middens de Pella fournissent le premier enregistrement pollinique continué au sud du désert de Namib durant la période des 50 000 dernières années. Ces données polliniques ont permis de reconstruire le changement de la végétation et d'estimer la température et l'aridité. Les résultats indiquent que la période glaciaire se caractérise par une augmentation de la disponibilité de l'eau sur le site par rapport à l'Holocène. Les changements de la température et de l'évapotranspiration potentielle semblent avoir joué un rôle important dans la détermination de l'équilibre hydrologique.L'enregistrement de Zizou hyrax midden met en évidence des changements de la végétation à la marge l'est des dunes de sable depuis 38 000 ans cal BP. La végétation de la période glaciaire se caractérise par les pourcentages relativement élevés des Astéracées pollen, et plus particulièrement par des taxa du climat plus froid: Stoebe et Artemisia¬-type. En accord avec les données de Pella, le réchauffement au début de l'Holocène indiqué par la dominance de pollen des graminées dans l'assemblage pollinique suggère une expansion du biome de Désert.Les hyrax middens de Spitzkoppe enregistrent les changements de la végétation dans le centre du désert de Namib au cours des 32 000 dernières années. Les résultats sont globalement cohérents en comparant aux autres enregistrements terrestres dans la région. L'analyse de ces données n'est cependant pas encore terminée.Dans tous ces sites, une variabilité significative a été observée à la fois dans la dernière période glaciaire et l'Holocène. Les conditions plus froides de l'ère glaciaire semblent être caractérisées par une augmentation de la disponibilité de l'eau le long de la totalité de notre zone d'étude. Au contraire des résultats provenant des carottes marines, nos enregistrements indiquent aucune expansion de la végétation de Fynbos biome, et seulement des traces de Restionaceae pollen dans le site extrêmement au sud à Pella (pas plus de 1%), mais aucun trace de ce pollen n'ayant été observé à Zizou ainsi qu’à Spitzkoppe
This thesis presents fossil pollen and microcharcoal data during the last 50,000 years from a north-south transect of the Namib Desert. The arid environment of the Namib precludes the development of permanent wetlands, and as a result few palaeoenvironmental records exist from the region. In this study, we employ rock hyrax middens – fossilised accumulations of the faecal pellets and urine of the Procavia capensis. Hyrax middens from three sites were selected for analysis: the southern Namib (Pella), the eastern margin of Namib Sand Sea (Zizou), and the central Namib (Spitzkoppe). The results from these terrestrial sites are the extent to which they may corroborate or conflict with findings from pollen records obtained from marine sediments of the Namibian coast.The Pella hyrax middens provide the first continuous pollen record from the southern Namib Desert since the last 50,000 years, and are used to reconstruct vegetation change and quantitative estimates of temperature and aridity. Results indicate that the last glacial period was characterised by increased water availability relative to the Holocene. Changes in temperature and potential evapotranspiration appear to have played a significant role in determining the hydrologic balance. The record can be considered in two sections: 1) the last glacial period, when low temperatures favoured the development of more mesic Nama-Karoo vegetation at the site, with periods of increased humidity concurrent with increased coastal upwelling, both responding to lower global/regional temperatures; and 2) the Holocene, high temperatures and potential evapotranspiration resulted in increased aridity and an expansion of the Desert Biome.Considered in the context of discussions of forcing mechanisms of regional climate change and environmental dynamics, the results from Pella stand in clear contrast with many inferences of terrestrial environmental change derived from regional marine records. Observations of a strong precessional signal and interpretations of increased humidity during phases of high local summer insolation in the marine records are not consistent with the data from Pella. Similarly, while high percentages of Restionaceae pollen has been observed in marine sediments during the last glacial period, they do not exceed 1% of the assemblage from Pella, indicating that no significant expansion of the Fynbos Biome has occurred during the last 50,000 years.The Zizou hyrax midden highlights vegetation changes on the eastern margin of the Namib Sand Sea since 38,000 cal BP. Results show the different vegetation compositions between the last glacial period and the Holocene. Glacial vegetation characterised with relatively high percentages of Asteraceae pollen, particularly cool climate taxa such as Stoebe and Artemisia types. Similar to the data from Pella, with the onset of Holocene warming grass pollen comes to dominate the assemblage, suggesting an expansion of the Desert Biome. We suggest that the climate during the last glacial period was more humid, and supported the development of shrubs/small trees. Arid conditions during the Holocene saw the depletion of this resource, and the development of grasslands that could exploit the rare rains that the region experiences today. In common with the Pella record, no elements of the Cape flora are found in the Zizou middens.The Spitzkoppe hyrax middens record vegetation changes in the central Namib during the last 32,000 years. The last glacial vegetation compositions composed of Olea, Artemisia¬-type, Stoebe¬-type and grasses. In the Holocene, the arboreal taxa such as Olea was replaced by others like Eculea, Dombeya, Commiphora, and Croton¬-type with relative higher percentage of grasses at early Holocene

Climate change, with a higher temperature, is making the snow covered period shorter in the Swedish mountain region. This represents a threat towards plant species at high altitudes which, due to their sensitivity to temperature changes, will likely lead to a change in plant species composition. The purpose of this study was to determine if there has been any change in vegetation cover in the Swedish mountain region and if temperature has changed, based on data from a long-term monitoring program called National Inventory of Landscapes in Sweden (NILS), and data collected from the Swedish Meteorological and Hydrological Institute (SMHI). The NILS program has conducted three different inventories from 2003 – 2018 in which Sweden is divided into 10 stratums. Further, NILS divided Sweden into 631 survey quadrates (5x5 km), which of 145 were distributed across the Swedish mountain region. Linear regression analysis was used to determine if there had been any change in the average vegetation cover, or in the cover of lichens, mosses and herbs, and to assess if there had been any change in temperature during the sample years 2003 – 2018. The results showed that the vegetation cover in both the sample blocks of 20 m radius and smaller sample areas 0.252 m had no significant change. Neither did the cover of herbs or the average temperature. The period of 2003 – 2018 might have been too short of a period to see any kind of difference in the temperature and the herbs might have a difficult time surviving due to a long drought period and hence they have not been able to increase their average cover. Results of lichens and mosses showed a significant increase in cover which could be because they can withstand drought better than herbs and therefore have a better chance of surviving the rise in temperature. In conclusion I observed no change in the cover of vegetation but there is an increasing cover of lichens and mosses in the Swedish mountain regions.
Klimatförändring med en högre temperatur leder till att den snötäckta perioden är kortare i det svenska fjällområdet. Detta är ett hot mot växtarter som redan befinner sig på gränsen av sin extrema klimatzon och kommer med stor sannolikhet leda till en förändring i kompositionen av växtarter. Syftet med denna studie var att fastställa om det har blivit någon förändring i vegetationstäcket i det svenska fjällområdet och om det finns en förändring i temperaturen med hjälp av data från Nationell Inventering av Landskapet i Sverige (NILS) och data insamlat av Sveriges Meteorologiska och Hydrologiska Institut (SMHI). NILS programmet har i dagsläge utfört inventering vid 3 olika perioder fördelade mellan 2003 – 2018 och dom har delat upp Sverige i 10 olika stratum där stratum 10 är fjällområdet. Vidare har dom delat in Sverige i 631 inventerings rutor, varav 145 ligger i fjällområdet. Lineär regression analys användes för att se om det blivit någon förändring i täckningen av vegetationen, lavar, mossor och örter, samt för att se om temperaturen förändrats under inventeringsåren 2003 – 2018. Resultatet av täckningen på vegetationen i 20 m radie samt de små provområdena på 0,252 visade inte någon antydan på signifikant skillnad. Det gjorde inte heller resultatet på täckningen av örter eller medeltemperaturen. 2003 – 2018 kan ha varit alldeles för kort för att avgöra om det har blivit någon förändring i medeltemperaturen och örter kan ha de svårare att överleva då torrperioden blivit längre på grund utav den kortare snötäckta perioden. Mossor och lavar hade dock båda en ökning i sin täckning vilket kan förklaras med att många arter kan lagra vätska under en lång tid och har därmed en större chans att överleva en stigande temperatur. Slutsatsen är att sedan 2003 förekommer det inte någon förändring i vegetationstäcket, däremot finns det en ökad täckning av lavar och mossor i det svenska fjällområdet.

Growing concerns over the possible effects of greenhouse-gas-related global warming on North American forests have led to increasing calls to address climate change effects on forest vegetation in management and planning applications.  The objectives of this project are to model contemporary conditions of soils and climate associated with the presence or absence and abundance of five southern pine species: shortleaf pine (Pinus echinata Mill.), slash pine (P. elliottii Engelm.), longleaf pine (P. palustris Mill.), pond pine (P. serótina Michx.), and loblolly pine (P. taeda L.).  Classification and regression based Random Forest models were developed for presence-absence and abundance data, respectively.  Model and diagnostics such as receiver operating curves (ROC) and variable importance plots were examined to assess model performance.  Presence-absence classification models had out-of-bag error rates ranging from 6.32% to 16.06%, and areas under ROC curves ranging from 0.92-0.98.  Regression models explained between 13.76% and 43.31% of variation in abundance values.  Using the models based on contemporary data, predictions were made for the future years 2030, 2060, and 2090 using four different greenhouse gas emissions scenarios and three different general circulation models.  Maps of future climate scenarios showed a range of potential changes in the geographic extent of the conditions consistent with current presence observations.  Results of this work will be incorporated into eastern U.S. variants of the Forest Vegetation Simulator (FVS) model, similar to work that has been done for FVS variants in the West.
Master of Science

Recent (post-1950s) climate change impacts on society and ecosystems have been recognised globally. However these global impacts are not uniform at regional or local scales. Despite research progress on such scales there are still gaps in the knowledge as to 'what' is happening and 'where'? The goal of this study addresses some of these gaps by analysing climate variability and vegetation response at the furthest south westerly peninsula of the United Kingdom. This research is focused on West Cornwall (South West England) - an area dominated by a strong maritime influence. The first part of this PhD research analysed archive and contemporary instrumental data in order to detect any trends in climate variability. The weather data was retrieved from the Met Office archive for Camborne 1957-2010 and Culdrose 1985-2011 stations; Trengwainton Garden (1940-2010), and from the Royal Cornwall Polytechnic Society, for Falmouth (1880-1952) and Helston (1843-1888). The data showed positive trends in mean annual and maximum temperature with the largest trend magnitude in the 20th and 21st century. Seasonal temperature change varies locally with the highest increase in autumn spring and summer. Precipitation trends were only positive for the 19th century for Helston. Correlation between precipitation data and North Atlantic Oscillation (NAO index) was negative, however the opposite result was detected when the NAO index was correlated with temperatures. Surprisingly, return period analysis showed a decrease in the frequency and intensity of extreme precipitation events post 1975 for Camborne and Trengwainton Garden stations. The second part of this study analysed changes in vegetation distribution in West Cornwall using historical and contemporary vegetation records. Historical vegetation records were used from the Flora of Cornwall collection of herbarium records and contemporary vegetation records which were available online, containing mainly the 'New Atlas of British and Irish flora'. Data sets were geo-referenced using ArcGIS in order to analyse changes in species geographical distribution pre and post-1900. Analysis showed that historical vegetation records can be used to assess any changes in geographic distributions of vegetation. Analysis for the area of West Cornwall showed a loss of range for 18 species, for 6 species this loss was larger than 50% of the area, and there was no change in overall range area for 10 species. Ellenberg values and environmental indicator values showed that they can be used as an indicator of environmental change, showing a decrease in species with lower January temperatures. Analysis also showed an increase in moderate wetter species, where species with extreme low and high precipitation environmental indicator values showed a greater loss. Furthermore species with a higher requirement for light showed a loss as well as species with lower nitrogen values. To analyse the loss of species at the local scale, West Cornwall was divided into three areas (North Border Cells, Central West Cornwall Cells and South Border Cells). The highest loss of 11 species was detected for South Border Cells, where the loss for Central West Cornwall Cells was 6 and for North Border Cells 8 species. It was found that 17 species were experiencing loss on different local sites. For 9 of these 17 species, change at the local scale was different to the national scale change at the individual species level, group level and habitat level. Furthermore, the whole area of West Cornwall lost two species post-1900, with a different loss locally. This showed that species could be protected locally in appropriate microclimate refugia, which will be of benefit for the preservation of regional identity ecosystem services and overall genetic pool of the species.

Recent attempts to elucidate a climatic effect on erosion rates at the catchment scale have generally found little or no correlation between precipitation and erosion rates, yet climate has been shown to exert a significant control on landscape properties such as drainage density, slope and relief. That erosion rates to do not directly reflect climatic conditions may not come as a surprise, since erosion rates will tend to keep pace with uplift rates in a tectonically active landscape. The interplay between erosion rates and climate may therefore be better understood with reference to the erosional efficiency of the landscape. Erosional efficiency governs how steep the landscape must become to balance uplift rates, and has also recently been postulated to affect the width to length (or spacing) ratio of first order basins, and the distribution of hillslopes within a landscape, via the relative inputs of diffusive and advective transport. This study constrains the efficiency of sediment transport along a climatic transect spanning a precipitation range of over two orders of magnitude in the Chilean Coastal Cordillera (26˚-41˚S), combining long-term erosion rates derived from concentrations of cosmogenic Be-10 in quartz in fluvial sediments with topographic metrics. The effects of changes in the relative input of diffusive and advective processes is investigated by studying the basin spacing ratios and distribution of hillslopes for a variety of natural landscapes and landscapes generated using the CHILD model. Sediment transport efficiency was found to peak at the transition between arid and semiarid climates, where herbaceous vegetation has almost entirely replaced bare ground, and to level off as climate becomes more humid, providing a background sediment transport efficiency value which will be applicable in both semi-arid and humid landscapes. Basin spacing ratios in natural landscapes show little variation along the transect, suggesting that changes in climate have little effect on this apparently universal catchment property, although maximum basin length attained appears to be linked to sediment transport efficiency. Slopes are consistently lower in the southern region where vegetation and sediment transport efficiency are uniform; here, lower slopes are maintained despite relatively high erosion rates thanks to higher sediment transport efficiency than in the north. Results from the CHILD landscapes show an increase in width to length ratio with decreasing sediment transport efficiency; this relationship is at odds with both the data from the study area and with data from previous studies. Results therefore indicate that, in natural landscapes, climate and vegetation cover exert a first order control on sediment transport efficiency. While climate and vegetation play little or no part in controlling the ratio of catchment dimensions, they may exert some control on the maximum dimensions of catchments and may help to modify the distribution of mean basin slope via their effects on hillslope processes.

In the last few decades, we have been witness to unprecedented changes in precipitation and temperature. Such alterations to our climate system have important implications for terrestrial ecosystems that billions of people depend on for their livelihood. The situation is especially tenuous for those living directly off the landscape via resources from natural ecosystems or subsistence agriculture as in much of tropical Africa. Studies of past climates provide potential analogues and help validate models essential for elucidating mechanisms that link changes in climate mean and variability and how they may affect ecosystem distribution and productivity. However, despite the importance of the paleo-record for insight into the future, tropical proxy records are rare, low resolution, and too short to capture important intervals that may act as analogs, such as the Last Interglacial (MIS 5e; ~130-115ka).Long, high-resolution drill cores from Lake Malawi, southeast Africa, provide a record of tropical climate and vegetation that extends back ~1.2mya, comprising many continuous glacial-interglacial cycles. My primary research involves conducting pollen analyses on these cores. First, I analyzed a high-resolution interval of the shortest Malawi core in order to better understand abrupt vegetation transitions during the Last Deglaciation. Further analysis was conducted on the longest Malawi core, beginning with an interval covering all of the Penultimate Glacial through the Last Interglacial. The resultant pollen data has shown that abrupt, large-scale landscape transitions from forest to desert follow local insolation and lake levels at the site, with a strong dependence of forest/woodland vegetation types on mean rainfall as well as rainfall seasonality. The interpretation of paleodata requires a good understanding of modern processes, thus another project has focused on using model simulations of the Last Interglacial and modern satellite NDVI time series to highlight dynamical and statistical relationships between vegetation and climate change. This work suggests that despite suggested links between monsoon intensity and SSTs in the southern African tropics, insolation controls on atmospheric circulation are the primary drivers of vegetation reorganization. In addition, this work highlights the importance of rainfall seasonality and dry season length in addition to precipitation controls on vegetation.

During the mid- to late Neogene (20 - 2.5 million years ago), episodic retreat of the Antarctic Ice Sheet (AIS) coincided with periods of higher-than-present atmospheric CO2, indicating ice sheet sensitivity to climatic conditions similar to those projected for the coming decades. Understanding Antarctic climate and vegetation during such a period of AIS retreat is crucial for our fundamental understanding of high latitude environments in warmer-than-present climate scenarios. This thesis presents a detailed geochemical study of sediments and plant fossils from the terrestrial Sirius Group of Oliver Bluffs, Transantarctic Mountains, located at 85 °S today and during the Neogene. Biomarker analysis of the sediments show strong evidence for a warmer Antarctica, where summer temperatures reached 5 °C. These relatively favourable conditions sup- ported a low diversity mixed vegetation. In contrast to the macrofossil record, there is geochemical evidence for conifers, suggesting that Antarctic vegetation was strongly controlled by local environmental variability. The warmer conditions are associated with a dynamic carbon cycle, evidenced by anomalously high and variable atmospheric 13C and possibly linked to atmospheric CO2 levels. Precipitation isotopes are reconstructed from plant compound isotope analysis of the fossils, and indicate markedly different hydrological cycling. This result is supported by climate modelling experiments which suggest that Antarctic hydrological cycling is most strongly governed by the extent of the ice sheet rather than by greenhouse gas radiative forcing. This thesis presents a new approach to exploring Antarctic climate and vegetation and provides important novel information on this crucial region of the world.

Vegetation dominated by heather and bracken, two common species of the UK uplands, is often nutrient limited and heavily influenced by climate. Thus, changes in climate or nutrient availability might be expected to have pronounced effects on growth and competition between these species. Mature heather and bracken turfs, transplanted from the field into 1 x 1 by 0.5 m deep plots, were subjected to factorial experimental treatments of root competition, shoot competition, summer drought, increased nitrogen supply and increased temperature for four consecutive years. The effects of root competition on the growth of heather and bracken were as great as those caused by the environmental treatments alone. Shoot competition had little effect on the growth of the two species, and thus, competition was concluded to occur predominately for below ground resources. Heather, in the building phase, was a superior competitor to bracken due to its extremely fine and invasive root system. Heather was able to compete with and deplete water from the roots of established bracken plants. Measurement of integrated of water use efficiency () and water use by droughted heather and bracken showed that the predicted environmental change scenarios are likely to cause an increase in the intensity of competition for water. There was no evidence of competition for nitrogen, despite nitrogen clearly limiting the growth of both species. The effects of the treatments on shoot phenology, morphology, photosynthetic physiology, biomass and below ground biomass have been examined. Above ground, heather was more responsive to the treatments imposed than bracken, having greater increases in shoot growth in favourable conditions, but greater decreases in shoot growth, and greater physiological acclimation, in stressed conditions, particularly drought in combination with increased nitrogen supply. Below ground, growth of bracken was extremely responsive whilst that of heather was not. However, even when bracken below ground growth was most stimulated, by increased nitrogen supply, it was still held in check by heather.

Detecting climate-induced effects in forest ecosystems become increasingly important as more evidence of greenhouse-gas-related climate change were founded. The Forest Vegetation Simulator (FVS) is an important growth and yield model used to support management and planning on public forest lands over the southern United States, however its prediction accuracy was challenged due to its climate- insensitive nature. The goal of this study was to develop species-specific prediction models for eastern U.S. forest tree species with climate and soil properties as predictors in order to incorporate the effects of climate and soils-based variables on forest growth and yield into FVS-Sn. Development of climate- sensitive models for site index, individual-tree mortality and diameter increment were addressed separately, which were all developed using Random Forests on the basis of USDA Forest Service Forest Inventory and Analysis program linked to contemporary climate data and soil properties mapped in the USDA Soil Survey Geographic SSURGO database. Results showed climate was a stronger driver of site index than soils. When soils and climate were used together, site index predictions for species grouped as conifers or hardwoods were almost as precise as species-specific models for many of the most common eastern forest tree species. Model comparison was conducted to pursue the most suitable individual-tree mortality prediction model for 20 most important species among Logistic Regression, Random Forests, and Artificial Neural Networks. Results showed that Random Forests with all indicators involved generally performed well, especially sound for species with medium and high mortality. At a chosen threshold, it frequently achieved the equally highest value of sensitivity and specificity among chosen candidates. To evaluate the prediction ability of Random Forests model on individual-tree diameter increment, Multiple Linear Regression model was built as baseline on each of most common 20 species eastern U.S. area. Comparison results showed that Random Forests gained advantages in model validation and future projection under climate change. Using the developed climate-sensitive models, multiple maps were produced to illustrate how forest tree growth, yield, and mortality of individual tree may change in the eastern U.S. over the 21st century under several climate change scenarios.
Ph. D.

>Magister Scientiae - MSc
This work aimed at assessing the response of wetland vegetation productivity to the 2014-2017 climate-induced drought at the Soetendalsvlei wetland system in the Western Cape province of South Africa. To achieve this objective, firstly a literature review on the progress of remotely sensed data applications in assessing and monitoring wetland vegetation productivity was conducted. The review elaborates on the role of remote sensing in monitoring and assessing wetland vegetation productivity, with a detailed discussion of the climate change and variability impacts on wetland vegetation productivity. Accurate assessment results are produced when suitable processing techniques are selected as well as appropriate spatial and spectral resolution for extracting spectral information of wetland vegetation productivity. Secondly, wetland vegetation changes and productivity status was assessed using multi-temporal resolution Landsat series imagery and Normalized Difference Vegetation Index (NDVI) during the wet and dry seasons for the period between 2014 and 2018.

The end of the Pleistocene marked a turning point for the Earth system, as climate gradually emerged from millennia of severe glaciation in the Northern Hemisphere. It is widely known that the deglacial climate change then was accompanied by an unprecedented decline in many species of large terrestrial mammals, featuring among others the near-total eradication of the woolly mammoth. Due to a herbivorous diet that involved the grazing of a large number of trees, their extinction is thought to have contributed to the rapid and well-documented expansion of dwarf deciduous trees in Siberia and Beringia, which in turn would have resulted in a significant reduction in surface albedo, leading to an increase in global temperature.In this study, we use the UVic ESCM to simulate various scenarios of the megafaunal extinctions, ranging from the catastrophic to more realistic cases, in order to quantify their potential impact on the climate system, and investigate the associated biogeophysical feedbacks between the growing vegetation and rising temperatures. The more realistic experiments include sensitivity tests based on the timing of extinction, tree clearance ration, and size of habitat, as well as a gradual extinction and a simulation involving free (non-prescribed) atmospheric CO2. Overall, most of the paleoclimate simulations and the sensitivity tests yield results that correspond well with our intuition. For the maximum impact scenario, we obtain a surface albedo increase of 0.006, which translates into a global warming of 0.175°C; these numbers are comparable in magnitude to those in similar studies.
La fin de l'époque du Pléistocène est une étape importante de l'histoire climatique de la Terre. En effet, c'est lors de cette période mouvementée que notre planète s'est pour une ultime fois libérée des conditions glaciales qui perduraient depuis des dizaines de millénaires, et souvent marquées par la présence d'imposante calottes glaciaires dans l'hémisphère nord. Il est bien connu que ce changement climatique fut également accompagné d'un déclin sans précédent de plusieurs espèces de grands mammifères terrestres, y compris une extermination rapide et brutale du mammouth laineux. En raison d'une diète composée en partie de végétaux provenant d'arbres prolifiques durant cette période, il y a de fortes raisons de croire que les ceux-ci auraient pu contribuer au maintien d'une faible densité forestière au sein de leur habitat. Par conséquent, leur extinction aurait contribué à une rapide émergence d'une variété de petits arbres feuillus tant en Sibérie qu'en Béringie, provoquant par la même occasion une réduction considérable de l'albédo de surface, qui à son tour aurait entrainé une augmentation globale de la température.L'objectif visé par cette étude est de quantifier l'impact potentiel qu'aurait pu avoir une extinction majeure de la mégafaune sur le climat de la Terre, par le biais d'une modification de la carte végétale menant à une hausse de la température. Afin d'examiner en détail la rétroaction de processus biogéophysiques à ce changement de température, nous employons le modèle de complexité intermédiaire de l'Université de Victoria (UVic) avec des scénarios plus ou moins réalistes, dont une catastrophe aux proportions exagérées servant à déterminer les limites de que peut offrir le modèle UVic. Parmi les cas plus terre-à-terre figurent quelques tests de sensibilité menés sur des paramètres tels que le taux de déboisement des mammouths, la grandeur de leur habitat, ainsi que l'année de leur extinction. D'autres expériences ayant été menées portent sur un étalement graduel d'un déclin des populations de mégaherbivores, ainsi qu'une simulation laissant libre cours aux échanges de carbone entre l'atmosphère et les autres constituants du système climatique, en autres mots une libre variation du niveau de CO2 dans l'atmosphère.En général, nous obtenons des résultats qui se conforment assez bien avec ceux d'études similaires. Dans le cas d'un scénario catastrophique, nous enregistrons une baisse de l'albédo terrestre équivalent à un peu moins de 0.006, donnant lieu à une hausse de la température se chiffrant à 0.175°C globalement. Quant aux expériences plus réalistes, les résultats en très grande majorité confirment notre intuition.

In this thesis, the "green" McGill Paleoclimate Model (MPM) is developed by interactively coupling the five-component physical MPM with a Dynamic Global Vegetation Model (DGVM) known as VECODE (VEgetation COntinuous DEscription model). Three applications to the pre-industrial Holocene climate, ocean, vegetation and terrestrial carbon cycle dynamics are presented, after a new land surface scheme is introduced. In these applications, orbital (Milankovitch) forcing and prescribed atmospheric CO2, starting from eight thousand years before present (8 kyr BP), are applied. In addition, a prescribed retreat of the Laurentide Ice Sheet (LIS) from 8 to 6 kyr BP is introduced. [Note: All acronyms used in this thesis are given in Appendix A.]
The first application, in which the atmospheric CO 2 is fixed at 280 ppmv, shows that the vegetation-albedo feedback together with the retreating LIS allows the global annual mean surface air temperature to increase starting from 8 kyr BP and reach a maximum at around 6 kyr BP. The decreasing Northern Hemisphere summer insolation (orbital forcing) together with the vegetation-albedo feedback can explain the gradual cooling during the past 6 kyr. The southward shift of the boreal forest treeline from 6 to 0 kyr BP and the desertification of northern Africa from 8 to 2 kyr BP are also simulated, in good agreement with paleoclimatic reconstructions.
In the second application, the reconstructed (Taylor Dome) atmospheric CO2 is used as a variable radiative forcing, and an inverse method is introduced to investigate the global carbon cycle dynamics. The model results indicate that the retreating LIS, in association with the vegetation-albedo and vegetation-precipitation (biogeophysical) feedbacks, causes the terrestrial carbon store to reach its maximum at around 6 kyr BP. Based on the inverse method, it is inferred that the first 10 ppmv atmospheric CO 2 increase from 8 to 6 kyr BP comes from the ocean carbon pool, which includes sedimentation processes. However, the land carbon release of about 68 PgC (95 PgC without CO2 fertilization) from 6 to 0 kyr BP can only contribute about 5 to 7 ppmv increase in atmospheric CO2; additional carbon sources are needed from the ocean. The simulated desertification results in a 70-PgC decrease in total carbon in the Sahara desert. This decrease is partially compensated by a 40-PgC increase in total carbon in the Southern Hemisphere.
Finally, in the third application, the total volume of meltwater/freshwater from the retreating LIS is estimated, and four discharge scenarios are proposed to investigate the impact of this freshwater on the Holocene ocean, climate and terrestrial carbon cycle. During each freshwater perturbation, the simulated maximum Atlantic meridional overturning circulation (MOC) intensity is reduced, by amounts of up to 8 Sv. However, it rebounds to a higher level than the original state, within 10 to 20 years after the termination of the freshwater input. During the time of a weakened MOC, the SST is reduced in the high-latitude North Atlantic and increased in the Southern Ocean due to decreased northward oceanic heat transport. Only a large freshwater perturbation (>0.1 Sv) has a significant impact on the Holocene climate and terrestrial carbon cycle; it results in an enhanced cooling of about 1°C in the Northern Hemisphere (caused by the appearance of the North Atlantic sea ice) and notable drops in the global net primary productivity (2 PgC/yr) and total land carbon storage (40 PgC).

More than two-thirds of the terrestrial carbon is found within the first meter of the soils, which is determined by the imbalance between fraction of net primary production entering soil and decomposition of organic matter. Changes in soil C contents in response to climate change may trigger a feedback that can influence the trend and scale of the climate change. Nitrogen can also limit primary production and influence decomposition of organic matter. In this thesis, I assess the relationships of soil C and N with measures of aboveground productivity, abundances of vegetation and climate in conifer-dominated forests across regional climates in British Columbia and Alberta. I also evaluate the variation in aboveground productivity with respect to climate and N availability. Soil samples were collected from the F, H , 0-30 (M1), and 30-60 cm (M2) layers at each study site. The concentrations of organic C and its fractions, and available N pools including inorganic, organic and microbial N, and potential rates of net nitrogen mineralization were measured. Fine-litter mass production was used as an estimate of productivity. The vertical projection of understory vegetation types including herbs, bryophytes, and shrubs were measured and used as abundances of understory vegetation types. Climatic factors were estimated using the ClimateBC model. In Chapter 2, Climate factors representing precipitation and heat-to-moisture indices combined with litter production explained 60 and 50% of the variations in C contents in organic and mineral layers, respectively. The potential net N mineralization rate was primarily related to the quality of organic C and concentrations of soluble organic (SON) and microbial N. Results indicated accumulation of SON in mineral layers with increased precipitation, and influence of the shrub abundance on available N in all layers. Fine-litter production was positively correlated with precipitation and heat-to-moisture indices, but was not consistently correlated with available N pools. The results indicated an overriding influence of precipitation and water availability on productivity and accumulation of C in soil in forests across BC. The abundances of herbs and shrubs were also related to soil C and N availability.

Flow regime, the magnitude, duration and timing of streamflow, controls the development of floodplain landforms on which riparian vegetation communities assemble. Streamflow scours and deposits sediment, structures floodplain soil moisture dynamics, and transports propagules. Flow regime interacts with environmental gradients like climate, land-use, and biomass-removing disturbance to shape riparian plant distributions across landscapes. These gradients select for groups of riparian plant species with traits that allow them to establish, grow, and reproduce on floodplains – riparian vegetation guilds. Here I ask, what governs the distributions of groups of similar riparian plant species across landscapes? To answer this question, I identify relationships between riparian vegetation guilds and communities and environmental gradients across the American West. In Chapter One, I discuss guild-based classification in the context of community ecology and streams. In Chapter Two, I identified five woody riparian vegetation guilds across the interior Columbia and upper Missouri River Basins, USA, based on species’ traits and morphological attributes. I modeled guild occurrence across environmental gradients, including climate, disturbance, channel form attributes that reflect hydrology, and relationships between guilds. I found guilds’ distributions were related to hydrology, disturbance, and competitive or complementary interactions (niche partitioning) between co-occurring guilds. In Chapter Three, I examine floodplain riparian vegetation across the American West, identifying how hydrology, climate, and floodplain alteration shape riparian vegetation communities and their guilds. I identified eight distinct plant communities ranging from high elevation mixed conifer forests to gallery cottonwood forests to Tamarisk-dominated novel shrublands. I aggregated woody species into four guilds based on their traits and morphological attributes: an evergreen tree guild, a mesoriparian shrub guild, a mesoriparian tree guild, and a drought and hydrologic disturbance tolerant shrub guild. Communities and guilds’ distributions were governed by climate directly, and indirectly as mediated through streamflow. In Chapter Four, I discuss the utility of guild-based assessments of riparian vegetation, current limitations to these approaches, and potential future applications of the riparian vegetation guild concept to floodplain conservation and management. The classification of vegetation into functional trait-based guilds provides a flexible, framework from which to understand riparian biogeography, complementing other models frameworks for riparian vegetation.

This study presents a phytolith record from a late Holocene sedimentary core from the Hluhluwe-iMfolozi area in northeastern KwaZulu-Natal. Radiocarbon dating showed the oldest sediments to be from 23 80 ± 40 BP. Phytoliths are present throughout the record. There was a high degree of unclassifiable phytoliths (>79% per sub-sample), but preliminary trends identified include a dominance of C₄ grasses throughout (Chloridoideae and Panicoideae; adapted to higher temperatures and greater insolation), and slightly more C₃ grasses (Pooideae; adapted to cooler temperatures and/or winter rainfall) towards the base. The tree cover density index (D/P) does not reflect increased bush encroachment at this site, but appears to show a closed forest at 36cm depth and relatively densely-wooded savannas throughout, while the humidity-aridity index (lph) and water stress index (Fs) suggest a move towards a xerophytic short grass savanna, and increased water stress at the top of the sequence, which could reflect the climate becoming hotter and drier due to global warming. Future studies should be done to identify phytoliths specific to the vegetation of the study site and to calibrate the indices used to confirm their utility for this area.

Changing sub‐Arctic plant communities can be an important feedback to climate change, via shifts in quantity and quality of litter production. Litter inputs to soil have appreciable influence on soil organic matter and microbial dynamics and consequently may provide a feedback to climate change in the sub‐Arctic. As permafrost peatlands thaw in response to climate change, the community composition of vegetation has been observed to shift from smaller and woodier shrubs to larger, more biodegradable sedges. We tested the hypothesis that carbon (C) stored in plant biomass increases across a permafrost thaw gradient by sampling both above‐ and below‐ground biomass in a permafrost‐underlain palsa, partially thawed bog, and fully thawed fen, all at Stordalen Mire in northern Sweden. Surprisingly, we found that total above‐ and below‐ground biomass together do not significantly change from the intact to the fully‐thawed habitats, despite previous research showing that net ecosystem productivity (NEP) appears to be higher in the fully thawed inundated fen. The lack of observed biomass increase despite the increase in NEP observed in other studies could be explained if the higher productivity sedges in fen sites have higher turnover, and transfer that productivity to SOM through high root exudation and/or litter deposition. We also observed a shift in plant community composition associated with loss of plant biodiversity across the gradient. These results suggest that plant community succession alters the quantity, type, and diversity of plant litter inputs to the soil. Such changes in litter quantity and type may be important drivers of decomposition rates and therefore the status of the ecosystem as a source versus sink for atmospheric C.