Tesi sul tema "Vegetation change"

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.

The phasic, cyclical model of Calluna-dominated vegetation dynamics, proposed by A.S. Watt, is evaluated by investigation of the gap in the degenerate plant. Succession in the gap is analysed by Markov models and found to be non-Markovian, with a number of processes occurring simultaneously. Vegetation changes are better interpreted in terms of certain ecological attributes of the species concerned. The inter-relationships between three important moss species are investigated further. Regeneration of Calluna is very variable. Seedling establishment requires a safe site and sufficient moisture, and depends on wet summers. Vegetative layering occurs more frequently but varies between parent plants and substrate types. The presence of soil micro-organisms appears necessary for adequate adventitious root production. An outbreak of heather beetle was monitored and contrasted with outbreaks in the Netherlands. The population was reduced by a parasitoid which acted density independently. At these low densities, a mosaic of gaps is formed in the vegetation by spatial heterogeneity of heather beetle attacks. These gaps behave in a similar fashion to degenerate gaps, and most Calluna regeneration occurs by layering. The value and limitation of Watt's model is discussed, especially by reference to forest gap-dynamics theory.

Australia has experienced pronounced climate change since 1950, especially in forested areas where a reducing trend in annual precipitation has occurred. However, the interaction between forests and water at multiple scales, in different geographical locations, under different management regimes and in different forest types with diverse species is not fully understood. Therefore, some interactions between forests and hydrological variables, and in particular whether the changes are mediated by management or climate, remain controversial. This thesis investigates the responses of Australia’s terrestrial ecosystems to both historical and projected climate change using remote sensing data and ecohydrological models. The thesis is structured in seven chapters, and contains five research chapters. Vegetation dynamics and sensitivity to precipitation change on the Australian continent for the past long drought period (2002-2010) are explored in Chapter 2 using multi-resource vegetation indices (VIs; normalized difference vegetation index (NDVI) and leaf area index (LAI)) and gridded climate data. During drought, precipitation and VIs declined across 90% and 80% of the whole continent, respectively, compared to the baseline period of 2000-2001. The most dramatic declines in VIs occurred in open shrublands near the centre of Australia and in southwestern Australia coinciding with significant reductions in precipitation and soil moisture. Overall, a strong relationship between water (precipitation and soil moisture) and VIs was detected in places where the decline in precipitation was severe. For five major vegetation types, cropland showed the highest sensitivity to water change, followed by grassland and woody savanna. Open shrublands showed moderate sensitivity to water change, while evergreen broadleaf forests only showed a slight sensitivity to soil moisture change. Although there was no consistent significant relationship between precipitation and VIs of evergreen broadleaf forests, forests in southeastern Australia, where precipitation had declined since 1997, appear to have become more sensitive to precipitation change than in southwestern Australia. The attribution of impacts from climate change and vegetation on streamflow change at the catchment scale for southwestern Australia are described in Chapter 3. This region is characterized by intensive warming and drying since 1970. Along with these significant climate changes, dramatic declines in streamflow have occurred across the region. Here, 79 catchments were analyzed using the Mann-Kendall trend test, Pettitt’s change point test, and the theoretical framework of the Budyko curve to study changes in the rainfall-runoff relationship, and effects of climate and vegetation change on streamflow. A declining trend and relatively consistent change point (2000) of streamflow were found in most catchments, with over 40 catchments showing significant declines (p < 0.05, -20% to -80%) between the two periods of 1982-2000 and 2001-2011. Most of the catchments have been shifting towards a more water-limited climate condition since 2000. Although streamflow is strongly related to precipitation for the period of 1982 to 2011, change of vegetation (land cover/use change and growth of vegetation) dominated the decrease in streamflow in about two-thirds of catchments. The contributions of precipitation, temperature and vegetation to streamflow change for each catchment varied with different catchment characters and climate conditions. In Chapter 4, the magnitude and trend of water use efficiency (WUE) of forest ecosystems in Australia, and their response to drought from 1982 to 2014, were analyzed using a modified version of the Community Atmosphere Biosphere Land Exchange (CABLE) land surface model in the BIOS2 modelling environment. Instead of solely relying on the ratio of gross primary productivity (GPP) to evapotranspiration (ET) as WUE (GPP/ET), the ratio of net primary productivity (NPP) to Transpiration (ETr) (NPP/ETr) was also adopted to more comprehensively understand the response of vegetation to drought. For the study period, national average annual forest WUE was 1.39 ± 0.80 g C kg−1 H2O for GPP/ET and 1.48 ± 0.28 g C kg−1 H2O for NPP/ETr. The WUE increased in the entire study area during this period (with a rate of 0.003 g C kg−1 H2O yr-1 for GPP/ET; p < 0.005 and a rate of 0.0035 g C kg−1 H2O yr-1 for NPP/ETr; p < 0.01), whereas different trends were detected in different biomes. A significantly increasing trend of annual WUE was only found in woodland areas due to higher magnitudes of increases in GPP and NPP than ET and ETr. The exception was in eucalyptus open forest area where ET and ETr decreased more than reductions in GPP and NPP. The response of WUE to drought was further analyzed using 1-48 month scales standardised precipitation-evapotranspiration index (SPEI). More severe (SPEI < -1) and frequent droughts (over ca. 8 years) occurred in the north than in the southwest and southeast of Australia since 1982. The response of WUE to drought varied significantly regionally and across forest types. The response of WUE to drought varied significantly regionally and across forest types, due to the different responses of carbon sequestration and water consumption to drought. The cumulative lagged effect of drought on monthly WUE derived from NPP/ETr was consistent and relatively short and stable between biomes (< 4 months), but notably varied for WUE based on GPP/ET, with a long time lag (mean of 16 months). As Chapters 2-4 confirmed that climate change has been playing an important role in the water yield and vegetation dynamics in Australia, the response of water yield and carbon sequestration to projected future climate change scenarios were integrated using the Water Supply Stress Index and Carbon model (WaSSI-C) ecohydrology model in Chapter 5. This model was calibrated with the latest water and carbon observations from the OzFlux network. The performance of the WaSSI-C model was assessed with measures of Q from 222 Hydrologic Reference Stations (HRSs) in Australia. Across the 222 HRSs, the WaSSI-C model generally captured the spatial variability of mean annual and monthly Q as evaluated by the Correlation Coefficient (R2 = 0.1-1.0), Nash-Sutcliffe Efficiency (NSE = -0.4-0.97), and normalized Root Mean Squared Error by Q (RMSE/Q = 0.01-2.2). Then 19 Global Climate Models (GCMs) from the Coupled Model Intercomparison Project phase 5 (CMIP5), across all Representative Concentration Pathways (RCPs) (RCP2.6, RCP4.5, RCP6.0 and RCP8.5), were used to investigate the potential impacts of climate change on water and carbon fluxes. Compared with the baseline period of 1995-2015 across the 222 HRSs, the temperature was projected to rise by an average of 0.56 to 2.49 ˚C by 2080, while annual precipitation was projected to vary significantly. All RCPs demonstrated a similar spatial pattern of change of projected Q and GPP by 2080, however, the magnitude varied widely among the 19 GCMs. Overall, future climate change may result in a significant reduction in Q but may be accompanied by an increase in ecosystem productivity. Mean annual Q was projected to decrease by 5 - 211 mm yr-1 (34% - 99%) by 2080, with over 90% of the watersheds declining. On the contrary, GPP was projected to increase by 17 - 255 g C m-2 yr-1 (2% - 17%) by 2080 in comparison with 1995-2015 in southeastern Australia. A significant limitation of WaSSI-C model is that it only runs serially. High resolution simulations at the continental scale are therefore not only computationally expensive but also present a run-time memory burden. In Chapter 6, using distributed (Message Passing Interface, MPI) and shared (Open Multi-Processing, OpenMP) memory parallelism techniques, the model was parallelized (and renamed as dWaSSI-C), and this approach was very effective in reducing the computing run-time and memory use. By using the parallelized model, several experiments were carried out to simulate water and carbon fluxes over the Australian continent to test the sensitivity of the model to input data-sets of different resolutions, as well as the sensitivity of the model to its WUE parameter for different vegetation types. These simulations were completed within minutes using dWaSSI-C, and this would not have been possible with the serial version. Results show that the model is able to simulate the seasonal cycle of GPP reasonably well when compared to observations at 4 eddy flux sites in Australia. The sensitivity analysis showed that simulated GPP was more sensitive to WUE during the Australian summer as compared to winter, and woody savannas and grasslands showed higher sensitivity than evergreen broadleaf forests and shrublands. With the parallelized dWaSSI-C model, it will now be much easier and faster to conduct continental scale analyses of the impacts of climate change and land cover change on water and carbon. Overall, vegetation and water of Australian ecosystems have become very sensitive to climate change after a considerable decline in streamflow. Australian ecosystems, especially in temperate Australia, are projected to experience warmer and drier climate conditions with increasing drought risk. However, the prediction from different models varied significantly due to the uncertainty of each climate model. The impacts of different forest management scenarios should be studied to find the best land use pattern under the changing climate. Forest management methods, such as thinning and reforestation, may be conducted to mitigate the impacts of drought on water yield and carbon sequestration in the future.

Mestrado em Gestão e Conservação de Recursos Naturais - Instituto Superior de Agronomia
The present master's thesis, had as its main objective the application of a dynamic model of riparian habitats in a case study with pronounced mediterranean characteristics. he vegetation model used is based on the existence of water conditions (water height and distance to water) suitable for the development of each type of riparian vegetation in different stages of their development, modeling annually its space-time evolution. The rules underlying the model take into account the height of the flow, the shear stress and duration of flooding. The modeling of vegetation held in ArcGIS environment, bases on three general ohases: initial creation of landscape, simulation of temporal and spatial evolution of vegetation and the presentation of annual results.

Due to man made events and natural causes many regions are currently undergoing rapid and wide ranging changes in land cover globally including developing and developed countries. India is one of them where land use and land cover change are taking place at a rapid pace. Forests are the most valuable natural resources available to the mankind on planet earth. On the one hand, they are the essential source of livelihood for the poor and marginalized sections of the society; on the other hand they provide furniture and other items of desire for the rich. Forest land cover change is an important input for modeling ecological and environmental processes at various scales. Rapid delineation in naturally forested regions is one of the major environmental issues facing the world today. It has been estimated that vegetation change threatens about one sixth of the world's population and one quarter of global terrestrial land. Vegetation cover plays a key role in terrestrial biophysical process and is related to a number of ways to the dynamics of global climate. Monitoring seasonal changes in vegetation activity and crop phenology over wide areas is essential for many applications, such as estimation of net primary production, deciding time boundary conditions for crop yield modeling and supporting decisions about water supply. Vegetations are the major part of land cover and their changes have an important influence on the energy and mass biochemical cycles and are also a key indicator of regional ecological environment change. Urbanization, demand of land for agriculture and demand of timbers for industrial purposes are the main reasons of manmade natural forest destruction. Though we are planting trees through reforestation and afforestation programs but these new forests never can be the representative of natural forest. In order to understand and manage environment at large variety of temporal and spatial scales, up-to-date and reliable information is required all the time. Remote Sensing is a valuable data source which can provide us land-use/land-cover change information on a continuous basis with very high accuracy. Remotely sensed data like aerial photographs and satellite images are the only option that allows detecting land cover changes on a large scale. Satellite images have the potential of offering the most accurate and latest information compared to statistical, topographic or land use maps. In this study an attempt has been made in analyzing vegetation change detection that took place between 2000 and 2005 using Terra MODIS 32 day 500m time series data on a monthly basis. With the launch of National Aeronautics and Space Administration (NASA) onboard aqua and terra platform, a new generation of satellite sensor data is now available. Normalized Difference Vegetation Index method has been employed for accurate classification of images and has proved to be successful.

This thesis examined vegetation change over the last 43 years in Australia's largest contiguous alpine area, the Kosciuszko alpine zone in south-eastern Australia. Using historical and current data about the state of the most common vegetation community, tall alpine herbfield, this thesis addressed the questions: (1) what were the patterns of change at the species/genera and life form levels during this time period; (2) what were the patterns of recovery, if recovery occurred, from anthropogenic disturbances such as livestock grazing or trampling by tourists; (3) what impacts did natural disturbances such as drought have on the vegetation and how does it compare to anthropogenic disturbances; and (4) What are the benefits, limitations and management considerations when using long-term data for assessing vegetation changes at the species/genera, life form and community levels? The Kosciuszko alpine zone has important economic, cultural and ecological values. It is of great scientific and biological importance, maintaining an assemblage of vegetation communities found nowhere else in the world. It is one of the few alpine regions in the world with deep loamy soils, and contains endemic flora and fauna and some of the few periglacial and glacial features in Australia. The area also forms the core of the Australian mainland's most important water catchment and is a popular tourist destination, offering a range of recreational opportunities. The vegetation of the Kosciuszko alpine zone is recovering from impacts of livestock grazing and is increasingly exposed to pressures from tourism and anthropogenic climate change. At the same time, natural disturbances such as drought and fire can influence the distribution, composition and diversity of plants. Thus, there is a need for detailed environmental data on this area in order to: (1) better understand ecological relationships; (2) understand existing and potential effects of recreational and management pressures on the region; (3) provide data against which future changes can be assessed; and (4) provide better information on many features of this area, including vegetation, for interpretation, education and management. The research in this thesis utilised three types of ecological information: (1) scientific long-term datasets; (2) photographic records; and (3) a comparison of disturbed and undisturbed vegetation. This research analysed data from one of the longest ongoing monitoring programs in the Australian Alps established by Alec Costin and Dane Wimbush in 1959. Permanent plots (6 transects and 30 photoquadrats) were established at two locations that differed in the time since grazing and have been repeatedly surveyed. Plots near Mt Kosciuszko had not been grazed for 15 years and had nearly complete vegetation cover in 1959, while plots near Mt Gungartan showed extensive impacts of grazing and associated activities which only ceased in 1958. Some transect data from 1959 to 1978 have been analysed by the original researchers. The research presented in this thesis extends this monitoring program with data from additional surveys in 1990, 1999 and 2002 and applies current methods of statistical evaluation, such as ordination techniques, to the whole data set for the first time. Results indicated that the recovery from livestock grazing and the effects of drought have been the main factors affecting vegetation. Recovery from livestock grazing at the three transects at Gungartan was slow and involved: (1) increasing genera diversity; (2) increasing vegetation cover; (3) decreasing amounts of bare ground; and (4) a directional change over time in species composition. Patterns of colonisation and species succession were also documented. In 2002, 44 years after the cessation of grazing, transects near Mt Gungartan had similar vegetation cover and genera diversity to the transects near Mt Kosciuszko, but cover by exposed rock remained higher. A drought in the 1960s resulted in a temporary increase of litter and a shift in the proportional cover of life forms, as grasses died and herb cover increased at both locations. Proportions of cover for life forms reverted to pre-drought levels within a few years. The results also highlighted the spatial variability of tall alpine herbfield. The photoquadrats were surveyed in the years 1959, 1964, 1968, 1978 and 2001 and are analysed for the first time in this thesis. After comparing a range of methods, visual assessment using a 130 point grid was found to be the most suitable technique to measure vegetation cover and genera diversity. At the 18 quadrats near Mt Gungartan, there was a pattern of increasing vegetation cover as bare areas were colonised by native cudweeds and the naturalized herb Acetosella vulgaris. Revegetation from within bare areas largely occurred by herb species, while graminoids and shrub species predominately colonised bare ground by lateral expansion from the edges, eventually replacing the colonising herbs. At the 12 quadrats near Mt Kosciuszko, vegetation cover was almost complete in all years surveyed except 1968, which was at the end of a six year drought. Similar to the results from the transect study, the drought caused an increase in litter at both locations as graminoid cover declined. Initially herb cover increased, potentially as a result of decreased competition from the graminoids and a nutrient spike from decaying litter, but as the drought became more severe, herb cover also declined. Graminoid cover rapidly recovered after the drought, reaching pre-drought levels by 1978, and was at similar levels in 2001. Herb cover continued to decline after peaking in 1964. The photoquadrat study also documented the longevity and growth rates of several species indicating that many taxa may persist for several decades. It further provided insights into replacement patterns amongst life forms. In addition to assessing vegetation change following livestock grazing and drought at the long-term plots, recovery from tourism impacts was examined by comparing vegetation and soils on a closed walking track, with that of adjacent undisturbed tall alpine herbfield at a series of 22 paired quadrats. Fifteen years after the track was closed there was limited success in restoration. Over a quarter of the closed track was still bare ground with non-native species the dominant vegetation. Plant species composition differed and vegetation height, soil nutrients and soil moisture were lower on the track which had a higher compaction level than adjacent natural vegetation. The results presented in this thesis highlight that tall alpine herbfield is characterised by nearly entire vegetation cover which is dominated by graminoids, followed by herbs and shrubs in the absence of disturbance by livestock grazing, trampling or drought. The studies also showed that under quot;average" conditions, the relative cover of herbs and graminoids remained fairly stable even though there can be considerable cycling between them. Spatial variability in terms of taxa composition was high. The only common introduced species in unrehabilitated sites was Acetosella vulgaris, which was effective at colonising bare ground but was eventually replaced by other native species. However, in areas actively rehabilitated, such as on the closed track, non-native species introduced during revegetation efforts still persist with high cover 15 years after their introduction. Monitoring of vegetation change is also important at the landscape scale. This thesis provides a review of the potential use, the limitations and the benefits of aerial photography to examine vegetation change in the Kosciuszko alpine zone. Numerous aerial photography runs have been flown over the area since the 1930s for government agencies, industry and the military. Some of these records have been used to map vegetation communities and eroding areas at a point in time. Other studies compared different types and scales of photographs, highlighting in particular the benefits and potential of large scale colour aerial photography to map alpine vegetation. However, despite their potential to assess vegetation change over time, a temporal comparison of vegetation in the Kosciuszko alpine zone from aerial photographs has not been completed to this date. Historical photographs may not be easy to locate or access and difficulties with vegetation classification may restrict the practicality of using historical aerial photographs to assess vegetation change. Despite these issues, aerial photography may provide a very useful and efficient tool to assess changes over time when applied appropriately, even in alpine environments. The development of digital classification techniques, the application of statistical measures of error to both methodology and data, and the application of geographic information systems are likely to further improve the practicality of historical aerial photographs for the detection of vegetation change and assist in overcoming some of the limitations. The results presented in this thesis highlight the need for limiting disturbance, for ongoing rehabilitation of disturbed areas and for long-term monitoring in the Kosciuszko alpine zone. The results contribute to our understanding of how vegetation may change in the future and may be affected by new land use activities and climate change. This type of information, which otherwise would require the establishment of long-term studies and years of monitoring, can assist land managers of this and other important protected areas. The study highlights how the use and expansion of already existing datasets to gather ecological information can save considerable money and time, providing valuable data for current and emerging issues.

The plant communities of the Long Mynd plateau are the culmination of over 3000 years of human intervention that largely deforested the uplands, and subsequently maintained the generally treeless heath and grassland communities now extant. The capacity of these communities to respond to directional change is well known, indeed the traditional mode of heathland management, burning, depends on the regenerative capacity of the target species, generally heather (Calluna vulgaris), for its success. However, changes in post WW2 stocking practice; the loss of ponies followed by an increase in the numbers of sheep and a change to them being overwintered on the hill, led to excessive grazing and damage to the heath. This coincided with the spread over the hill by bracken (Pteridium aquilinum) and other changes in the distribution and nature of the vegetation. A sequence of vegetation surveys made by various individuals and organisations over the past 75 years or so has been analysed in an attempt to delineate spatial and temporal changes in the vegetation. This demonstrated the need for a standardised survey methodology to allow consistent monitoring. The analysis showed that bracken had been infiltrating most of the communities from its origins outside the lower limits of the Common as well as from some of the valley sides. Within the last decade, this expansion has apparently been contained in line with the current management plan for control. A survey of 730 quadrats in some 30 stands was made to characterise the variation of the vegetation on the plateau, and to relate it to some of the associated environmental factors. Classification, unconstrained ordination and ordination constrained by the abiotic environmental variables, showed that, a) the strongest trend in the vegetation distinguished water-flushed communities, b) non-wetland communities differentiate between heathland and grassland, c) this trend can be only partly be attributed to the measured abiotic environmental variables, d) the amount of pure Pteridietum [U20] is limited, although much of the heathland and grassland has bracken within it. There are indications that invasion by bracken often correlates with a loss of dominance of Calluna in favour of Deschampsia flexuosa and Vaccinium myrtillus. Difficulties in associating these trends with measured abiotic variables suggests, other factors probably management processes, are critical in driving this trend. Distribution of ‘heathland’ bryophytes was found to be associated more with the structure of their ‘host’ vascular communities rather than with abiotic factors. Finally, this investigation considers the practical implications with regard to the future encouragement of heather and the control of bracken. Cutting rather than burning appears to be the ecologically most suitable method for heather regeneration and bracken control.

During a three year field study (1997-2000) vegetation assemblages, collective vegetation variables, traits of dominant populations and hydrological and hydrochemical variables were repeat-sampled within seven wetland sites across Scotland and northern England. These ranged from the Irish Marshes, Inverness-shire in the north, to Tarn Moss, Cumbria at the southern extreme. Sampling was conducted at a total of fifty-six permanent sample stations located along a total of eleven transects. Vegetation groupings were defined using multivariate analyses, and were classified as various fen, mire, and swamp NVC community types. The various groups were characterised by the values for the range of variables measured, and significant differences were seen between a number of these variables for different groupings. In addition, certain separate groupings with the same community classification were also seen to have significant variations between them in terms of trophic status, and canopy height and biomass values. Collective vegetation variables and dominant population trait values were successfully predicted from physical and chemical variables measured within the groundwater and substrate during 1999. A number of specific models incorporating relatively large numbers of predictor variables were proposed alongside more general models incorporating fewer predictor variables. The greatest predictive power with R2 = 0.67 (p<0.001) for a model predicting stem density (m-2). Conversely, vegetation variables proved useful for predicting characteristics of the groundwater environment, for which specific and general models were against proposed. In this instance, the greatest predictive power was R2 = 0.79 (p<0.001) for a model predicting minimum water table level (i.e. maximum level of drawdown). The models were tested using data collected during 2000 from repeat sites and independent sites. Whilst some of the variables were predicted within noisy limits, predicted values generally corresponded well to observed values.

Over the last few decades global warming and human intervention have led to changes and deterioration in natural vegetation across the world. The Al Jabal Al Akhdar, in north east Libya, is one of those areas that have experienced changes in land cover. This region has environmental and economic importance in providing suitable habitat for wildlife and providing services for local communities and cities in the Libyan Desert. The overall aim of this thesis was to evaluate the factors which have affected vegetation cover change in the Al Jabal Al Akhdar region over the last 42 years. There were three key objectives to this research: (1) to assess changes in natural and semi-natural vegetation cover in the north-east of Libya using forty years of satellite image data, (2) to assess land cover change and the effects of human activities in the study area over a period of 42 years, (3) to assess the factors affecting vegetation change in the study area. A further objective was to assess climate change in the study area using the climate data which was available from three climatic stations as climate change may be responsible for vegetation cover change in the areas that have low human activity. To address these objectives, remote sensing techniques were used to assess vegetation cover change and the changes in human activity from 1972 to the present. Satellite images provide data that cannot be collected by traditional methods and provide a historical archive of what the landscape looked like in the past. This study used multi-temporal Landsat images, which are freely available, for the period from 1972 to the present and provide the key temporal record of vegetation change on the Earth. Vegetation Indices (NDVI, SAVI and EVI), derived from the spectral reflectance of leaves and canopies, were used to assess the changes in vegetation cover over time. Image classification was also used to characterise the nature of land cover change, in particular the impact of human intervention. A key finding related to Objective (1) was that some areas have experienced a statistically significant change in vegetation indices over the 42 years which was interpreted as a change in vegetation cover in the areas in question. A key conclusion related to Objective (2) was that land cover had changed in the study area over the period of study. The influence of human activities was exerted through increased land use and decreased areas of forest and shrubland in the region. The outputs of the above-mentioned objectives and the effects of climate change were used to assess Objective (3), to detect which factors caused vegetation cover change in the Al Jabal Al Akhdar region. The main factors causing vegetation change were the effects of human activities in the areas adjacent to human settlements, while in the sparsely populated areas in the south of the study area, vegetation cover changes may be related to recent climate change. In conclusion, although the number of available Landsat images used to delineate the changes in vegetation cover was limited, the methods used to interpret the images for vegetation indices and image classification were invaluable in determining important results for the objectives of the thesis. The results obtained from assessing vegetation cover and land cover change and patterns of changes are major steps towards filling the information gap and creating a database for monitoring land cover in the study area. This effort will contribute towards facilitating decision-making on mitigating the impact of land use dynamics on land cover as well as provide a basis for future research.

A study into the effects of vegetation change (particularly afforestation and reforestation) on the hydrology of an area requires a predictive investigative method. This is because empirical studies require a long time period to collate the necessary data and the results cannot be transferred to a remote site with any confidence. Physically based, distributed hydrological modelling offers a predictive capability but the use of effective parameters and lack of verification and validation casts doubts on the ability of the current generation of these models to be used in applications. In this thesis a new modelling scheme is developed that focuses directly on simulating the effects of afforestation on the storm event hydrology of small catchments in humid temperate regions. A mixed conceptual/physically based model (VSAS4), is developed to act as the hydrological base model. In order to parameterise the change in vegetation for VSAS4 a separate, pre-processing, forest growth model is developed. This model is a distance dependent, individual tree based, forest growth simulator. The combination of VSAS4 and the forest growth model is given the name LUCAS (Land Use Change, Afforestation, Simulator). Testing of LUCAS is carried out with verification and validation. The primary aim of these was to assessth e worth of the schemea nd highlight areasf or future research. The difficulty of verifying a complex modelling scheme such as LUCAS has led to the use of a scaled down sensitivity analysis. This is designed to test the schemes robustness and the influence of forest growth on the VSAS4 simulations. The forest growth model is verified and validated separately as an independent predictor of forest growth. The lack of a United Kingdom hydrological data set that spans the period of a forest growth on a catchment has led to only a limited validation of the scheme. The Tanllwyth catchment in Mid Wales was adjudged to be the best available (from a choice of four sites). The scheme cannot be considered a valid predictor of the effects of vegetation change on storm hydrology in humid temperate regions due to the lack of a full data set but the results suggest it cannot be considered invalid. The assessment of LUCAS as a predictor of the effects of vegetation change on storm event hydrology indicates that it has considerable potential but it is not able to be used directly in applications without further development. The validation of LUCAS using hydrograph reproduction and hypothetical scenarios has highlighted several avenues for future research within the study of the effects of vegetation change on stormflow hydrology. These are: the role of canopy closure in vegetation change; the need for soil water flow equations that account for more than just soil matrix flow; and the development of a probabilistic framework so that modelling schemes such as LUCAS can be used in applications without full verification and validation.

Includes bibliographical references.
The broad objective of these studies was to view present landuse (extensive small stock ranching) in the Karoo against the background of relationships between indigenous plants and animals and to indicate how modification of herbivore regimes might affect this arid environment. Specific objectives were to determine how grazing by domestic livestock brings about vegetation change, why such changes are sometimes irreversible, and whether existing conceptual models of vegetation dynamics adequately explain the impact of domestic livestock on Karoo vegetation. Three interrelated aspects of plant-animal interactions were considered: the influence of herbivores on the evolution of Karoo plants, food selection by indigenotis and introduced herbivores and the effects of herbivory, competition and rainfall on plant reproduction and recruitment. The results of these investigations are presented as 14 papers. The first three papers interpret plant morphology and biogeography to provide information on past spatial and temporal use of the landscape by herbivores, and the next seven provide new information on food selection by invertebrates, indigenous vertebrates and by domestic sheep. Three papers examine the hypothesis that the reproductive output, survival and abundance of some Karoo plant species are influenced by herbivory. The possible consequences of various land management options on diversity and productivity of Karoo rangelands are discussed in the concluding paper. It was inferred, from biogeographic trends in the relative abundance of plants with thorns or propagules adapted for epizoochoric dispersal, that densities of large mammalian herbivores decreased from the north eastern to the southwestern Karoo. Within the most arid parts of the Karoo, mammalian herbivory appears to have been concentrated along drainage lines and in pans.

Land degradation, a phenomenon referring to (drought) in arid, semi-arid and dry sub-humid regions as a result of climatic variations and anthropogenic activities most especially in the semi-arid lands of Sudan, where vast majority of the rural population depend solely on agriculture and pasture for their daily livelihood, the ecological pattern had been greatly influenced thereby leading to loss of vegetation cover coupled with climatic variability and replacement of the natural tree composition with invasive mesquite species. The principal aim of this study is to quantitatively examine the vigour of vegetation in Sudan through different vegetation indices. The assessment was done based on indicators such as soil adjusted vegetation index (SAVI). Cloud free multi-spectral remotely sensed data from LANDSAT imagery for the dry season periods of 1984 and 2009 were used in this study. Results of this study shows conversion of vegetation to other land use type. In general, an increase in area covered by vegetation was observed from the NDVI results of 2009 which is a contrast of that of 1984. The results of the vegetation indices for NDVI in 1984 (vegetated area) showed that about 21% was covered by vegetation while 49% of the area were covered with vegetation in 2009. Similar increase in vegetated area were observed from the result of SAVI. The decrease in vegetation observed in 1984 is as a result of extensive drought period which affects vegetation productivity thereby accelerating expansion of bare surfaces and sand accumulation. Although, increase in vegetated area were observed from the result of this study, this increase has a negative impact as the natural vegetation are degraded due to human induced activities which gradually led to the replacement of the natural vegetation with invasive tree species. The results of the study shows that NDVI perform better than by SAVI.

Vegetation stability was investigated within the Cape of Good Hope Nature Reserve by incorporating three vegetation surveys spanning a 44 year time period. The goals of this study were to investigate changes in overall species diversity within the reserve, stability of community composition within and between sites, and which factors are influencing stability. A survey was conducted on 26 vegetation plots across the reserve. Data from this survey and two previous surveys was used to compare species diversity between time periods. Vegetation compositional similarity was also compared between sites. I tested for the effect of difference in vegetation age and soil type as predictors of vegetation groups. Results showed an overall decrease in species diversity. Analysis ofvegetation groups showed that vegetation types are mostly stable as current vegetation can be predicted by groups based on 1966 data. Vegetation is influenced significantly by fire history and soil types.

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.

The Sunken Forest, located on Fire Island National Seashore, is a critically imperiled habitat and is one of only two known old-growth maritime holly forests in the world. Analysis of a dataset that dates back to nearly half a century has helped to identify major drivers influencing changes within the forest. These major drivers include; white-tailed deer herbivory, erosion, sea level rise, increased storm events, and canopy-gap dynamics. As of 2013, the Sunken Forest canopy is still analogues of 1967, but over the last 35 years vegetation recruitment within the forest has become limited due to white-tailed deer (Odocoileus virginianus) herbivory. The bayside of the Sunken Forest has also been eroding. Erosion with added pressure from sea-level rise is causing mortality of trees/understory vegetation, limiting seedling and herb recruitment, and shifting vegetation toward the bayside and low elevation areas within the interior of the forest.

African savannas cover roughly half of the continent, are home to a great diversity of wildlife, and provide ecosystem services to large populations. Savannas showcase a great diversity in vegetation structure, resulting from variation in climatic, edaphic, topographic, and biological factors. Fires play a large role as savannas are the most frequently burned ecosystems on Earth. To study how savanna vegetation structure shifts with environmental factors, it is necessary to gather site data covering the full gradient of climatic and edaphic conditions. Several earlier studies have used coarse resolution satellite remote sensing data to study variation in woody cover. These woody cover estimates have limited accuracy in drylands where the woody component is relatively small, and the data cannot reveal more detailed information on the vegetation structure. We therefore know little about how other structural components, tree densities, crown sizes, and the spatial pattern of woody plants, vary across environmental gradients.

This thesis aimed to examine how woody vegetation structure and change in woody cover vary with environmental conditions. The analyses depended on access to very high spatial resolution (<1 m) satellite imagery from sites spread across African savannas. The high resolution data combined with a crown delineation method enabled me to estimate variation in tree densities, mean crown size and the level of aggregation among woody plants. With overlapping older and newer imagery at most of the sites, I was also able to estimate change in woody cover over a 10-year period. I found that higher woody plant aggregation is associated with drier climates, high rainfall variability, and fine-textured soils. These same factors were also indicative of the areas where highly organized periodic vegetation patterns were found. The study also found that observed increases in woody cover across the rainfall gradient is more a result of increasing crown sizes than variation in tree density. The analysis of woody cover change found a mean increase of 0.25 % per year, indicating an ongoing trend of woody encroachment. I could not attribute this trend to any of the investigated environmental factors and it may result from higher atmospheric CO₂ concentrations, which has been proposed in other studies. The most influential predictor of woody cover change in the analysis was the difference between potential woody cover and initial woody cover, which highlights the role of competition for water and density dependent regulation when studying encroachment rates. The second most important predictor was fire frequency.

To better understand and explain the dominant ecosystem processes controlling savanna vegetation structure, I constructed a spatially explicit model that simulates the growth of herbaceous and woody vegetation in a landscape. The model reproduced several of the trends in woody vegetation structure earlier found in the remote sensing analysis. These include how tree densities and crowns sizes respond differently to increases in precipitation along the full rainfall range, and the factors controlling the spatial pattern of trees in a landscape.

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.

Long-term data are valuable for detecting changes in vegetation composition, and investigating how vegetation is responding to environmental change.  We use a range of approaches to quantify, characterise and interpret vegetation change in upland plant communities of the Scottish Highlands by comparing two datasets, collected in 1956-58 and 2007-08, on the species composition of representative plots in major vegetation types in the North-West Highlands and the East Central Highlands.  Firstly, we validate the methodology used for relocating plots that were not permanently marked, by showing that temporal change is greater than local spatial heterogeneity in the vegetation.  The results show evidence of biotic homogenisation in the vegetation, manifested through declining plant diversity and an increase in generalist species, particularly graminoids, at the expense of specialists, particularly dwarf-shrubs, lichens and forbs.  Climate change in the form of increased oceanicity, and grazing were found to have been important in driving these changes, whereas nitrogen deposition had a smaller effect.  These data were also applied to two issues in nature conservation management, showing that protected area status had no effect on the magnitude of vegetation change, and that indicator species of the key drivers of change could be identified.  Finally, we show how long-term vegetation change data can be used in hypothesis formulation for experimental work, by carrying out an ex-situ manipulative experiment on an important upland graminoid species, Trichophorum germanicum.  This thesis incorporates examples of the wide range of ecological questions and analytical approaches that can be investigated through revisitation studies.

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Grand Lake, Colorado has experienced a severe mountain pine beetle outbreak over the past twenty years. The aim of this study was to map lodgepole pine mortality and health decline due to mountain pine beetle. Multispectral data spanning a five-year period from 2006 to 2011 were used to assess the progression from live, green trees to dead, gray-brown trees. IKONOS data from 2011 were corrected to reflectance and validated against an Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) hyperspectral dataset, also collected during 2011. These data were used along with additional reflectance-corrected multispectral datasets (IKONOS from 2007 and QuickBird from 2006 and 2009) to create vegetation classification maps using both library spectra and regions of interest. Two sets of classification maps were produced using Mixture-Tuned Matched Filtering. The results were assessed visually and mathematically. Through visual inspection of the classification maps, increasing lodgepole pine mortality over time was observed. The results were quantified using confusion matrices comparing the classification results of the AVIRIS classified data and the IKONOS and QuickBird classified data. The comparison showed that change could be seen over time, but due to the short time period of the data the change was not as significant as expected.

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

Lake sediments from inland endoreic (saline) lakes in the semi-arid Ebro Basin, NE Spain have been analysed to provide a history of lake level, vegetation, catchment erosion and anthropogenic activity over the last 10,000 years. Analysis was undertaken for pollen, macrofossils (seeds, Cladocera ephippia, Chironomid head capsules etc), charcoal, geochemistry (total cations/trace metals, sulphate, carbonate & LOI) and sediment composition. Fourteen AMS radiocarbon dates provide dating control. Seven cores were investigated from 4 seasonal playa lakes, I shallow (<1.5m) semi-permanent salt lake, 1 deep (5.0m) permanent salt lake and 1 Medieval-age reservoir. Over 40 surface samples were also taken to investigate modern analogue environments. A review of the use and interpretation of saline lake macrophytes (seeds & pollen), Cladocera and Chironomids in palaeolimnology is provided, with special emphasis on those found in Spanish salt lakes. Taphonomic problems and nearshore-offshore facies were also investigated using a surface sample transect across a small playa lake. A surface sample pollen data set from 30 lakes in the Ebro Basin is presented and the implications for palaeo-interpretation discussed. The sensitivity of the pollen record as a climate indicator is investigated using 6 sites across a climatic gradient from sub-humid to semi-arid. Lake level reconstruction is based on an 8 stage semi-quantitative palaeohydrological model, developed from a surface sample data set from 32 lakes ranging from temporary to permanent, and hypersaline to freshwater. A simple hydrological model for groundwater fed lakes is also discussed which can be used to quantify these palaeohydrological changes. The early Holocene (<9.3-8.6Kyr BP) was characterised by semi-arid extreme continental conditions in the Ebro Basin, with an extensive Juniper thurifera woodland. Lake levels rose to their highest point in the Holocene between 8.6-7.2Kyr BP when evergreen oak and pine forest dominated. This is interrupted by a short recession in lake level between 8.2-7.6Kyr BP. A distinct regional early-Neolithic fire and clearance event occurs between 7.7-7.3Kyr BP. A drop in lake level and development of a monospecific pine forest (P. halepensis) indicates warmer and drier conditions in the mid Holocene (7.2-5.4Kyr BP). Evergreen oak reappears as forest cover declines after 5.4Kyr BP, although this is not marked by any increase in charcoal or cultivation indicators. Low groundwater but moderate lake levels (4.0-2.7Kyr BP) may be linked to high summer storm frequency and low winter rainfall. This coincides with catchment erosion and valley floor alluviation. Lake level rises again significantly between 2.7-1.8Kyr BP during Iberian and Roman times when archaeological evidence indicates a peak in population. Agriculture changes from small scale pastoral to small scale arable without any further decline in woodland cover. A rapid fall in lake level at ca. 1.8Kyr BP is followed by complete forest recovery (P. halepensis) and depopulation, until major deforestation around 1.4Kyr BP marks the arrival of the Visigoths/Arabs and extensive nomadic pastoralism. Intensive grazing pressure or lower temperatures resulted in Juniperus increasing again between 1.4-0.4Kyr BP. This also coincides with a second period of catchment erosion and valley floor alluviation. After ca. 0.4Kyr BP, lake levels have increased along with extensive olive cultivation and the development of modern (irrigated) arable agriculture.

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.

Thesis (M.A.)--Georgia State University, 2005.
Title from title screen. Zhi-Yong Yin, committee chair; Paul Knapp, Truman Hartshorn, committee members. Electronic text (135 p. : col. ill., col. maps)) : digital, PDF file. Description based on contents viewed Aug. 2, 2007. Includes bibliographical references (p. 125-133).

This thesis seeks to understand and explain the role of fire in land cover change, vegetation and carbon dynamics in the carbon-dense, tropical peat swamp forest ecosystem of Southeast Asia. Following a methodological review, earth observation and ground data are employed to investigate fire regime, post-fire vegetation recovery, and fire-driven carbon losses in 4,500 km2 of peatland in Central Kalimantan, Indonesian Borneo. Results reveal an increasing trend in deforestation (2.2% yr-1 forest loss rate, 1973-1996; 7.5% yr-1, 1997-2005) and identify fire as the principal cause. A step change in fire regime is identified, with increasing fire frequency and reduced return interval following land drainage for the Mega Rice Project (MRP). During the post-MRP period (1997-2005), ~45% of the area was subject to multiple fires; 37% burnt twice and 8% three or more times. Extensive fires in 1997 and 2002 were associated with ENSO droughts, but fires in non-ENSO years (i.e. 2004, 2005) indicate fire incidence has decoupled from ENSO. This study provides a novel approach to quantifying relative magnitude of burn severity using characteristics of the post-fire vegetation regrowth. Combined spectral and ground data are used to demonstrate that enhanced fire frequency and burn severity limit post-fire forest recovery, with fern-dominated communities replacing tree re-growth. The character of post-fire vegetation is an important factor defining burning conditions for a subsequent fire, whilst fire frequency, severity and return interval influence both rate and nature of vegetation regrowth. Methods are proposed for deriving fire-driven carbon losses. Over the period 1973-2005, losses are estimated at 79-113 Mt of carbon (53-83 Mt from peat; 26-30 Mt from vegetation), with the greatest loss occurring during the post-MRP era (65-94 Mt). This work identifies the processes linking fire regime in tropical peatland to changes in vegetation ecology and carbon stocks and assesses the implications for ecosystem rehabilitation.

This study develops understanding of vegetation change and water, sediment and carbon dynamics in semi-arid environments. Objectives were addressed using an integrated ecohydrological and biogeochemical approach. Fieldwork, over two contrasting grass-woody transitions at the Sevilleta National Wildlife Refuge, New Mexico, USA; quantified vegetation structure, soil structure and the spatial distribution of soil carbon resources. Over both transitions; woody sites showed a lower percentage vegetation cover and a greater heterogeneity in vegetation pattern, soil properties and soil carbon. Soil organic carbon differed in both quantity and source across the sites; with levels higher under vegetation, particularly at the woody sites. Biogeochemical analysis revealed soil organic carbon to be predominantly sourced from grass at the grassland sites. In contrast, at the woody sites soil organic carbon under vegetation patches was predominantly sourced from woody vegetation, whilst inter-patch areas exhibited a strong grass signature. Investigation of function focussed on the hydrological response to intense rainfall events. Rainfall-runoff monitoring showed woody sites to exhibit greater; runoff coefficients, event discharge, eroded sediment and event carbon yields. In contrast to grass sites, biogeochemical analysis showed the loss of organic carbon from woody sites to exhibit a mixed source signal, reflecting the loss of carbon originating from both patch and interpatch areas. To examine the linkages between vegetation structure and hydrological function, a flow length metric was developed to quantify hydrological connectivity; with woody sites shown to have longer mean flow pathways. Furthermore, in addition to rainfall event characteristics, flow pathway lengths were shown to be a significant variable for explaining the variance within fluxes of water, sediment and carbon. Results demonstrating increased event fluxes of sediment and carbon from woody sites have important implications for the quality of semi-arid landscapes and other degrading ecosystems globally. It is thus necessary to translate the understanding of carbon dynamics developed within this study to the landscape scale, so changing fluvial carbon fluxes can be incorporated into carbon budgets, research frameworks and land management strategies at policy-relevant scales.

Late Neolithic and Early Bronze Age Central China was the scene of important cultural developments, which impacted on agricultural practices and local vegetation. Using phytolith data from 4 archaeological -sites and a survey of a further 11 sites in Henan this project investigates changing crop choices, developments in crop processing and, to a lesser extent, differing local environments both temporally and spatially. Comparisons of relative levels of phytoliths from crop husks, Panicum, Setaria and Oryza, from each site and period were compared to provide evidence of changes in crop repertoire. Results from these investigations show crop variation both over time and in different parts of the region. Millet farming predominated in the more Northern sites in the Yellow River Valley, while rice was clearly the main crop at Baligang, the southernmost site within the Yangtze catchment. However, rice became more important in the Yellow River valley during the late Neolithic and interestingly despite a changing climate making rice farming more challenging in the Early Bronze Age farmers continued rice cultivation. Crop processing stages were interpreted by examination of differing proportions of phytoliths from crop husks, weed husks and crop and weed leaves which can illustrate differing cultivation systems, harvesting and processing practices. These can be seen especially clearly in the rice data from Baligang suggesting more successful agricultural practices and possible change in social organisation in the Late Neolithic. Evidence of local environmental variation was more challenging to unpick as all the phytolith samples available were from cultural contexts. However, the results of the investigation into the changing local environment reflect other proxy data. Differences in occurrence of specific key phytolith short cell morphotypes and changes in the levels of bulliforms, and cone shaped phytolith morphotypes from Cyperaceae indicating wetland were used to interpret local vegetational change, again both spatially and temporally. Rondel and bilobe shaped short cells represent Pooid and Panicoid grasses respectively and short cell morphotypes, such as rondels, bilobes, saddles, can also be used to track variation in levels of C3 and C4 grasses, so changes in proportions of these morphotypes can indicate larger vegetational change. A comparison between the dataset from Neolithic Central China and one from Neolithic India highlighted possible variations in arable systems with millet and wet rice farming in China contrasting with dry rice in India.

Parabolic dunes are one of a few common aeolian landforms that are highly controlled by eco-geomorphic interactions. Parabolic dunes, on the one hand, can be developed from highly mobile dune landforms, barchans for instance, in an ameliorated vegetation condition; or on the other hand, they can be reactivated and transformed back into mobile dunes due to vegetation deterioration. The development and transformations of parabolic dunes are also highly sensitive to changes in many environmental factors such as precipitation, temperature, wind regime, as well as changes in land management and other anthropogenic factors. The eco-geomorphic interrelationships and fundamental mechanisms controlling the dune transformations, however, are incompletely understood. This study combines fieldwork investigation, remote sensing, and Cellular Automaton modelling, to explore both: 1) the dune stabilisation and barchan-to-parabolic dune transformation, as well as 2) the dune reactivation and parabolic-to-barchan dune transformation, under the influence of climatic changes (e.g., drought stress and wind energy), and human disturbance (e.g., grazing activity). Extensive suites of simulations are used to explore boundary conditions, parameter controls, and external forces on both dune transformations. The results show that the characteristics of vegetation play an essential role in the processes of dune transformations, in particular, the species (annual grasses vs. perennial shrubs) and their capabilities of withstanding wind erosion and sand burial. This study has introduced a dune stabilising index (S*) that captures the interactions between key parameters and establishes the linkage between the system controls and the geometry of a stabilising dune. The dune surface erodibility significantly influences the threshold of climatic forces that reactivates an initial vegetated parabolic dune and transforms its lobe into a mobile barchan dune with arm remnants left behind. The Extended-DECAL can be easily adapted to a different dune system to explore various scenarios under the changes in both natural and anthropogenic controls and to assist in planning judicious land-management practices.

Vegetation forms the basis for the existence of animal and human. Due to changes in climate and human perturbation, most of the natural vegetation of the world has undergone some form of transformation both in composition and structure. Increased anthropogenic activities over the last decades had pose serious threat on the natural vegetation in Nigeria, many vegetated areas are either transformed to other land use such as deforestation for agricultural purpose or completely lost due to indiscriminate removal of trees for charcoal, fuelwood and timber production. This study therefore aims at examining the rate of change in vegetation cover, the degree of change and the application of Principal Component Analysis (PCA) in the dry sub-humid region of Nigeria using Normalized Difference Vegetation Index (NDVI) data spanning from 1983-2011. The method used for the analysis is the T-mode orientation approach also known as standardized PCA, while trends are examined using ordinary least square, median trend (Theil-Sen) and monotonic trend. The result of the trend analysis shows both positive and negative trend in vegetation change dynamics over the 29 years period examined. Five components were used for the Principal Component Analysis. The results of the first component explains about 98 % of the total variance of the vegetation (NDVI) while components 2-5 have lower variance percentage (< 1%). Two ancillary land use land cover data of 2000 and 2009 from European Space Agency (ESA) were used to further explain changes observed in the Normalized Difference Vegetation Index. The result of the land use data shows changes in land use pattern which can be attributed to anthropogenic activities such as cutting of trees for charcoal production, fuelwood and agricultural practices. The result of this study shows the ability of remote sensing data for monitoring vegetation change in the dry-sub humid region of Nigeria.

A series of anthropogenic disturbance conditions have altered the vegetation of the southern Appalachians during the past 200-years. The objective of this research was to identify the nature and timing of these vegetation changes in order to better understand the underlying causes. A total of 304 land surveys were collected for a small agricultural watershed from early settlement in 1787 through to the present day. Witness corners recorded tree species, shrubs, stumps, snags and non vegetative markers. Types of witness corners were tallied and tested for shifts in frequency across time periods. Tree species were also classified by silvical characteristics including sprouting capability, shade tolerance, and seed type and these groupings were tested for shifts in frequency across time periods. Landform bias of the witness corners was tested using references contained in the surveys. Results showed significant shifts in white oak (Quercus alba L.), chestnut (Castanea dentate Marsh. Borkh.), chestnut oak (Quercus prinus Wild.), black oak (Quercus velutina Lam.), red oak(Quercus rubra L.), black locust (Robinia pseudoacacia L.), yellow poplar (Liriodendron tulipifera L.), and scarlet oak (Quercus coccinea Muenchh.). The central change was a steady decline in white oak, probably due to the absence of fire and changes in soil properties. Chestnut replaced white oak as the dominant species, but was removed by chestnut blight in the 1930's. Sprouting capability appeared to be the most important silvical characteristic across all species.
Master of Science

Multispectral video (MSV) images were used to measure phenological changes in cultivated and uncultivated vegetation communities surrounding the Palo Verde Nuclear Generating Station (PVNGS). Multispectral video imagery was acquired from aircraft on seven dates between the middle of June and the end of September, 1986. Images representing three sites near the PVNGS were selected to calculate Ratio Vegetation Index (RVI) values for seven surface cover types. Mean RVI values were tested sequentially for change, plotted as a function of time, and then compared to a moisture index and the crop calendar. MSV detected changes in cultivated vegetation corresponding to the crop calendar. Changes in natural vegetation and the non-vegetated cover types were also detected, but did not correlate to the moisture index. There is insufficient evidence to determine if detected changes in uncultivated vegetation were the result of phenological changes or electronic noise.

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.

The Allideghi Wildlife Reserve in the Amibara District of Afar Regional State, Ethiopia, has international significance for harboring endangered Grevy's Zebra and other wildlife dependent on grasslands. The reserve is increasingly used by pastoral people and their herds. Impacts of livestock on native vegetation include direct effects of grazing and indirect effects from livestock-facilitated dispersal of an invasive plant, Prosopis juliflora. The main research objective was to determine effects of pastoralism and vegetation change on prospects for sustaining the Allideghi Wildlife Reserve as grassland habitat for Grevy's Zebra. Methods included use of driving surveys to quantify resource use by herbivores, vegetation analysis, and engagement with local people. Resource-use patterns of livestock across the Allideghi grassland were often positively affected by proximity of water, while that for wild ungulates was often negatively affected by proximity of people. Livestock concentration at a major borehole has created a large piosphere with concomitant reductions in herbaceous standing-crop, productivity, and species richness; plant species have shifted from grasses to forbs in severely grazed sites. Vegetation further from the borehole was resilient in response to moderate grazing pressure in terms of species composition and productivity. Since being introduced at a nearby commercial plantation in the 1970s, P. juliflora has been dispersed to the Allideghi Wildlife Reserve via livestock; cattle, sheep, and goats eat the pods and deposit seeds in manure at settlements and favored foraging areas. Prosopis juliflora greatly reduced species richness and basal cover of native herbaceous vegetation in the Allideghi grassland. Analysis of remotely sensed images from the past 30 years indicated major land-use change in the district due to agricultural expansion as well as land-cover change due to Prosopis encroachment and heavy grazing. Recent efforts have been undertaken by various agencies to control P. juliflora, via harvest in the district, but this has yielded variable and often negative results. Without a concerted effort to limit livestock grazing and control spread of P. juliflora, the future for the grassland and wildlife at the Allideghi Wildlife Reserve is grim. Agencies and policy makers need to promote science- and community-based approaches to help rectify the situation. (311 pages)

Bibliography: p. 155-175.
Palynological analysis of organic sediments from a freshwater lake near Port Elizabeth (34°S,25°30'E) has provided a high- resolution vegetation history of the area for the last 2200 years. Detailed identification and counting of the fossil pollen resulted in the generation of a pollen diagram. Changing frequencies in fossil pollen over time are represented, and inferences are made regarding environmental conditions which influenced the vegetation. A detailed narrative of vegetation history in response to environmental change is presented, and this is compared to results from related studies. The significance of the Lake Farm study site has been noted in terms of its location as a 'zone of convergence' for a variety of vegetation types. Results of fossil pollen analysis indicate that environmental conditions prior to 1 500BP were drier than at present. Forest and fynbos vegetation were not well-represented in the pollen spectrum at this time, and it is suggested that they were not favoured by these conditions. Environmental conditions ameliorated after 1500BP, becoming more mesic, which favoured the proliferation of both forest and fynbos vegetation types. At present xeric and grassland elements are declining, while shrubs increase, indicating an enhanced human-induced disturbance regime. It is suggested that the partial decline in forest elements at present 1s most likely attributable to human-induced disturbance of the environment. The introduction of exotic trees has been noted (approx. 280BP) and is seen to have coincided with the influx of european settlers to the region. Principal Components Analysis has revealed that the vegetation distribution in the area has been most heavily influenced by human activity and moisture availability. The necessarily subjective interpretation of the statistical results, however, casts some doubt on the validity of the conclusions drawn. The validity of the conclusions drawn from this study becomes apparent not only in terms of what is learned about the history of forests, but also the form any future management should take.

This thesis examines the role of autogenic and allogenic forces in determining the timing and development of blanket peat initiation and how the occurrence and growth of blanket peat subsequently constrains human activities. A number of factors involved in the formation of blanket peat have been defined in the literature, in particular the roles of climate change, soil processes and anthropogenic effects, tested in this thesis from a typical peat-covered upland in south west Scotland. Tests are developed from a multi-proxy approach and by comparing peat-stratigraphic and palaeoecological records from a series of nine 14C dated peat profiles from a single hillside. A detailed examination of the sequence and timing of blanket peat initiation in the Holocene Epoch is presented from a case study from the head of the Glen App valley, Lagafater, south west Scotland. The evidence was taken from a total of nine peat transects at 215m OD, 300m OD and 400m OD where agricultural, hydrological and micro-climatic effects are expected to have differed and had different impacts on soils and vegetation. Samples were retrieved from the top, middle and bottom of a gently undulating slope at each altitude. This has allowed a localised picture of peat initiation to be obtained from each locality and with changes in altitude, allowed for an analysis of the factors responsible up and down slope and the identification of synchronous autogenic forces. A number of analytical techniques have been used. Pollen analysis was undertaken as the principal method of vegetation reconstruction at all sites, particularly through the initiation horizon. Variations in mire-surface wetness, determined through dry bulk density and humification analysis, were also employed to generate a record of probable changes in effective precipitation and the effect these may have had on the accumulation rate of the blanket peat. In order to attempt to answer the question of when blanket peat was initiated and to establish the synchroneity of changes, twenty nine AMS 14C dates were obtained. The evidence suggests that blanket peat developed during the Mesolithic period, through to the early Bronze Age. It substantiates an anthropogenic forcing factor for palaeohydrological changes, with early landscape management and cereal cultivation accelerating the process of blanket peat initiation. With progressive changes in precipitation interacting with factors such as weathering of bedrock and vegetation cover, the local landscape at Lagafater was covered by blanket peat by the early Bronze Age. The radiocarbon chronology obtained from the multi-proxy records allowed the synthesis of these data sets and the definition of Holocene vegetation change, climate change and the history of human impact throughout the early prehistoric era across this landscape.

Contemporary research concerning wildlands and wildlife of the American West increasingly calls for greater complexity in understanding human-environmental relationships. This dissertation investigates a culturally diverse portion of Greater Yellowstone in order to complicate these dialogues. It explores a riparian corridor along the Wind River, a region permanently settled by Eastern Shoshone, Northern Arapaho and Euro-American residents in the late 1800s and early 1900s. Using the Wind River Basin as a case study, this research observes the landscape through three different lenses: settlement geography, place identity, and vegetation change. By incorporating a variety of methods to understand regional change (including historical research, interviews with residents, and comparative aerial and ground photography), it finds that riparian change relates to a complex cultural-ecological mosaic. Not only is change perceived differently by a variety of communities in the Wind River Basin; change relates to century-old settlement geographies, government policies and cultural preferences, shifting economies and power relationships, and evolving relationships formed by interrelationships of people and environment. This dissertation argues that investigations of environmental change must not oversimplify dynamic relationships between people and place. Indeed, the complexity of these places may relate to why Greater Yellowstone has remained one of the largest intact ecosystems in the lower 48 states.

New Zealand's South Island tussock grasslands have been highly modified by human activities, including burning, grazing and introductions of exotic plants for pastoralism. Studies suggest that tussock grasslands are degraded, in that native species have declined, and exotic species have increased in both diversity and abundance. These trends are primarily thought to be related to the impacts of grazing and subsequent grazing removal. Few studies have assessed long-term changes that have occurred in tussock grasslands, and those that have are generally limited to one particular location. This thesis aimed to investigate temporal changes in community structure in tussock grasslands, and relate these changes to environmental variables and land tenure. Data were used from 90 permanently-marked vegetation transects, which were set up on 19 geographically widespread properties in areas of tussock grassland across Canterbury and Otago in the South Island of New Zealand. The transects were on land in both conservation and pastoral tenure. Each transect was 100 m, and consisted of 50 0.25 m² quadrats. The transects were measured between 1982 and 1986 (first measurement), were re-measured between 1993 and 1999 (second measurement) and again between 2005 and 2006 (third measurement). A total of 347 vascular species were observed over the 90 transects and three measurement times. Species richness declined between the first and second measurements (first time interval), and increased between the second and third measurements (second time interval), at both the small (quadrat) and large (transect) scales. Both native and exotic species declined in mean quadrat species richness during the first time interval, and then increased during the second time interval. Changes in mean quadrat species richness were similar on transects in both conservation and pastoral tenure. Multivariate analysis of species' occurrences in quadrats identified a long gradient in species composition for these 90 transects. Four key plant communities were identifed along this gradient and differed in their mean elevation: (1) Highly-modified pastoral community, (2) Short-tussock grassland community, (3) Tall-tussock grassland community, (4) Alpine mat-forming species community. A detailed investigation into temporal changes that occurred on 53 transects that occurred in short- and tall-tussock grassland communities showed that changes in species composition were not consistent over time. Transects on different properties changed in species composition by different amounts. Specifically, in ordination space, transects on two properties changed in composition significantly more than transects on one other property. The property that a transect was on also affected the way that it changed in composition, i.e. native species were more likely to have increased on transects on some properties. Transects in conservation tenure did not change in species richness or composition differently from those in pastoral tenure. Considering that many native plants in tussock grasslands are relatively slow-growing, and that these areas have been grazed and burned for more than a century, we may expect it to be some time before we can detect differences in vegetation dynamics on conservation land from that on pastoral land. The changes in the community structure of these tussock grasslands were related to a combination of environmental factors, such as soil chemistry, climate, and management factors. This study has allowed greater understanding of vegetation change in tussock grasslands, and demonstrates the importance of long-term ecological monitoring in making reliable and accurate predictions about landscape-scale changes in tussock grassland community structure.