Tesis sobre el tema "Vegetation dynamics"

Rapid climate change in Arctic regions is of concern due to important feedbacks between the Arctic land surface and the global climate system. A large amount of organic carbon (C) is currently stored in Arctic soils; if decomposition is stimulated under warmer conditions additional release of CO2 could result in an accelerating feedback on global climate. The strength and direction of Arctic C cycle - climate feedbacks will depend on the growth response of vegetation; if plant growth increases some or all of the extra CO2 emissions may be offset. Currently the Arctic is thought to be a small net sink for CO2, the expected balance of terrestrial C sinks and sources in the future is unknown. In this thesis I explore some of the critical unknowns in current understanding of C cycle dynamics in Arctic vegetation. Quantifying gross primary productivity (GPP) over regional scales is complicated by large spatial heterogeneity in plant functional type (PFT) in Arctic vegetation. I use data from five Arctic sites to test the generality of a relationship between leaf area index (LAI) and canopy total foliar nitrogen (TFN). LAI and TFN are key drivers of GPP and are tightly constrained across PFTs in Low Arctic Alaska and Sweden, therefore greatly simplifying the task of up-scaling. I use data from Greenland, Barrow and Svalbard to asses the generality of the LAI-TFN relationship in predicting GPP at higher Arctic latitudes. Arctic ecosystems are unique among biomes in the large relative contribution of bryophytes (mosses, liverworts and hornworts) to plant biomass. The contribution of bryophytes to ecosystem function has been relatively understudied and they are poorly represented in terrestrial C models. I use ground based measurements in Northern Sweden to fill an existing data gap by quantifying CO2 fluxes from bryophytes patches in early spring and summer, and develop a simple model of bryophyte GPP. Using the model I compare bryophyte GPP to that of vascular plants before, during and after the summer growing season, finding that productive bryophyte patches can contribute up to 90 % of modelled annual GPP for typical vascular plant communities at the same site, and that the relative magnitude of bryophyte GPP is greatest in spring whilst the vascular plant canopy is still developing. Understanding how GPP relates to plant growth is important in relating remotely sensed increases in Arctic ‘greenness’ to changes in plant C stocks. I use a 13C pulselabelling techniques to follow the fate of recently fixed C in mixed vascular and bryophyte vegetation, with a focus on quantifying the contribution of bryophytes to ecosystem carbon use efficiency (CUE). I show that bryophytes contribute significantly to GPP in mixed vegetation, and act to increase ecosystem CUE. I highlight the importance of including bryophytes, which do not have roots, in aboveground: belowground partitioning schemes in C models. To further explore C turnover in bryophytes, I use the results of a second 13C labelling experiment to develop a model of C turnover in two contrasting Arctic mosses (Polytrichum piliferum and Sphagnum fuscum). I find significant differences in C turnover between Polytrichum piliferum which respires or translocates about 80 % of GPP, while Sphagnum fuscum respires 60 %. This analysis is the first to explicitly model differences in C partitioning between Arctic bryophyte species. Finally, I discuss the implications of each chapter for our understanding of Arctic C dynamics, and suggest areas for further research.

The corpus of available vegetation knowledge is characterised by its fragmented form and by the way in which relationships between different ecological quantities tend to be expressed non-quantitatively. Much of the corpus is only held informally and composed of deterministic factual or conditional statements. Despite its form, this thesis demonstrates that available ecological knowledge can be usefully employed for predictive modelling of vegetation dynamics under different conditions. The thesis concentrates on modelling Mediterranean vegetation dynamics. Using a mixture of concepts and techniques from deterministic state transition and functional attributes modelling. Qualitative Reasoning and knowledge-based systems, three ontological distinct modelling systems are developed to demonstrate the utility of available knowledge for modelling vegetation dynamics. All three systems use declarative, rule-based approaches based on first-order logic and are composed of a set of representational constructs along with a separate system for reasoning with these constructs to make predictions. A method for reasoning about change in non-quantitative model variables is developed based upon time and direction of change. This ‘temporal reasoning system’ provides a solution to the state variable problem and may offer a general way of modelling with non-quantitative knowledge. To illustrate, a different model of Mediterranean vegetation dynamics is developed and run under different conditions for each system. The capabilities and possible problems of each system in terms of ecological validity, knowledge representation and reasoning are discussed. The general utility of rule-based approaches to modelling vegetation dynamics are also discussed along with the implications of the modelling systems developed for the activities of decision-support and ecological theory development.

This thesis addresses the effects of seasonality of grazing on vegetation dynamics. Background to the thesis is provided by the Hill Sheep and Native Woodland (HSNW) project, a system-scale experiment with the long-term aim of integrating upland sheep husbandry within native woodlands. Sheep husbandry in the HSNW project has involved a change from year-round grazing to off-wintering (grazing on upland areas from April to October only). The potential impact of this change on individual plants, plant communities and the landscape is evaluated. A cutting experiment was run to simulate herbivory in different seasons at the level of the individual plant. In grassland communities, where most species are wintergreen graminoids, interaction between species and seasons in response to cutting was minimal. In mire communities, where species with a range of life-forms and phenologies are present, there was considerable interaction between species and lifeform in response to cutting in different seasons. Non-wintergreen graminoid species recovered more rapidly than wintergreen graminoid species following cutting in spring, but the reverse occurred following cutting in autumn. Dwarf shrubs and forbs were slow to recover from cutting in all seasons in both mires and grasslands. The impacts of three grazing treatments in the HSNW project (unchanged year-round grazing, change to off-wintering, and change to zero grazing) on sward structure and species abundance were monitored. Sward height increased in the ungrazed treatment, but changes in species relative abundance were minimal in the short time-scale of the study. Literature suggests that change in plant species abundance in grazed systems is driven by herbivore selection preference for (or avoidance of) plant species and plant responses to grazing. Experts were interviewed to generate a set of seasonal data on herbivore selection preferences and plant responses, and levels of agreement between experts were assessed. Information was also collated on plant response to herbivory from the cutting experiment (above) and plant trait information from the literature. A qualitative model was developed to predict change in species abundance. It was used to: i) explore patterns of interaction between herbivore preferences and plant responses to grazing; ii) compare short-term predictions of change in abundance made using each of the information sources available; iii) make longer-term predictions using plant trait information. The short-term predictions were tested against the small changes in species abundance that had occurred in the HSNW project. Predictions made with the plant trait information were more accurate than those made with other information. A study of levels of browsing on regeneration was carried out in mature sheep-grazed birch woodlands. It was rare to find regeneration at sites grazed only by sheep, but tree regeneration did occur in the presence of sheep. Seasonal variation in browsing was found to be more related to the presence of cattle than to the presence of sheep. Finally, the findings are collated to make management recommendations for the HSNW project and other systems where seasonal grazing regimes could be beneficial.

The overall aim of this thesis is to investigate long-term (multi-century) vegetation development in a high-latitude setting, with a particular emphasis on the emergence and persistence of spatial structure in plant communities. The spatial distribution of plants within a community influences vegetation dynamics and the functioning of terrestrial ecosystems. Knowledge of the spatiotemporal dynamics of vegetation is therefore crucial to understanding ecosystem response to disturbance, and to successful ecosystem management. Studies of spatiotemporal dynamics from high-latitude settings are rare, despite these regions being among the most sensitive to warming and subject to ongoing environmental change. The study was based on a primary succession on Mt Hekla in south-central Iceland. The chronosequence approach was used to infer 850 years of vegetation development from a suite of 14 lava flows (five of which had been disturbed by the deposition of volcanic tephra). The thesis is organised around four main research themes: 1) Trajectories of development- How do the global (i.e. non-spatial) properties of a community (e.g. species diversity) change with terrain age in a primary succession? Plant species frequency data from 12 transect surveys (each comprising 400 contiguous 10 cm x 10 cm quadrats) were used to test the predictions of classical models of primary succession; 2) Initial colonisation- How do plants first establish on newly-created terrain? Photographic surveys and point-pattern analysis were used to assess the processes by which pioneer species colonised ‘safe sites’ on lava surfaces six and fifteen years old; 3) Spatial scale and structure- How and why does the spatial structure of vegetation vary over long timescales? Vegetation and soil data from transect surveys (item 1, above) were analysed using a variety of spatial statistics, in order to test three models of spatiotemporal dynamics; 4) Temporal changes in environmental gradients- To what extent does biotic reaction feed-back into the small- (metre-) scale distribution of plants? Randomised sampling of vegetation on 42 sites was combined with continuous microclimatic monitoring to assess the changing steepness of environmental gradients, and the impact this process had on vegetation development. The analyses revealed a robust, multi-century trajectory of vegetation development on undisturbed sites. The development of vegetation on newly-emplaced flows was rapid, as colonists randomly exploited small- (millimetre) scale surface irregularities in the lava. A thick (up to 20 cm), spatially homogeneous ‘carpet’ of moss formed within 50 years of flow emplacement. Thereafter, the vegetation became progressively more structured with increasing terrain age, as patches of vegetation formed and expanded. On the oldest sites, differentiation of the vegetation according to metre-scale variations in elevation was apparent. Overall, the results emphasise the limitations of classical models of succession in high-latitude habitats and suggest widespread applicability for the nucleation model of primary succession. The study also suggests that the spatiotemporal response of vegetation to environmental gradients is mediated by short-range positive feedback. These findings have implications for modelling vegetation development on other primary substrates (e.g. glacial forelands) and the response of spatially patchy, high-latitude vegetation to future climate change.

Management of the southern lowland heaths is usually for conservation, and aims to create structural diversity, prevent succession to scrub and maintain the low nutrient status of the system. The main aim of this study was to investigate the effects of burning, fire temperature and intensity on seed banks, nutrients, vegetative regeneration and seedling demography. In the first experiment (1994), vegetation fuel loads were manipulated in stands representing two different growth phases of vegetation (mature and degenerate) at two sites (Arne in Dorset and Aylesbeare in Devon) to test the hypothesis that frre temperature and intensity depend chiefly on fuel load rather than other factors associated with the pre-burn age of the stand. There were two burning treatments of low and high fuel loads. Fire temperatures were measured with temperature-sensitive paints on tiles ('pyrometers'). Intensity (the heat released per unit area) was highly correlated with fuel load. Fire temperatures were higher in the high fuel load treatments than in the low fuel load treatments at all stands except the Aylesbeare degenerate stand. where temperatures were low in both treatments. In the second experiment (1995). paraffin was added to treatment plots at the mature stand at Aylesbeare in an attempt to produce higher temperatures. but this aim was not achieved. Thermocouples and pyrometers were used to measure temperatures and indicated that these fires were hotter than the fires at the same stand in the first experiment. There was no effect of burning or of temperature on the size of the seed banks in the soil and litter in the frrst experiment. The litter seed bank of Erica tetralix was depleted by a similar quantity in both treatments in the second experiment. There was no effect of burning on the nutrients (P. Ca. K) in the humic soil in either of the experiments. The concentration of nutrients was higher in the ash than in the litter of control plots after burning (except at the Aylesbeare degenerate stand). There were significant but inconsistent effects of temperature on the quantities of nutrients. Vegetative regeneration after burning depended chiefly on the pre-bum age of the stand and possibly on grazing. but fire temperature had no effect. There was rapid resprouting at both the mature stands, but regeneration at the degenerate stands was sparse. Calluna rapidly regained dominance at the Arne mature stand but was relatively less abundant at the Aylesbeare mature stand after burning than it was before. Agrostis curtisii. Ulex gallii and Erica tetralix were co-dominant in the regrowth at this stand. However. there was a decline in the relative abundances of U. gallii and A. curtisii by the third growing season after burning. At the mature stands. seedlings affected by resprouting vegetation were sheltered from microclimatic extremes and experienced higher survivorship than seedlings in areas of bare ground. However. seedling density under resprouts decreased over time as continued growth of resprouts prevented seedling germination and establishment. Seedling populations on bare ground at both types of stand oscillated widely, and there was a rapid turnover. Although bare ground persisted at the degenerate stands, there was no invasion of non-heathland species. A canopy of ericaceous species should eventually develop via gradual accumulation of seedling density and vegetative spread from the few stem bases that were able to resprout. The range of temperatures generated by the fuel loads tested did not have significant effects on regeneration from seed or from stem bases. However. the removal of vegetation by burning had a large impact on vegetation dynamics by enabling seedling emergence and resprouting from stem bases. The balance between regcneration from seed and from stcm bases depended on the pre-bum age of the stand.

The vegetation dynamics of submersed Charophyta vegetation were studied in three shallow lakes, located around 25 km southeast of Gävle, close to theBaltic Sea. The samples were taken with a plastic tube used as a homemade sediment core sampler. The samples were then divided into a lower, middle and an upper layer so that temporal patterns inferred from sedimentation order could be studied. For making the statistical analyzes the program R version 2.12.1 for windows was used. The aims of this study were to describe how the vegetation dynamic changes over time in the three different lakes selected and develop methods on how to use the spore bank to make temporal surveys. Altogether three species were found at the study sites, Chara intermedia, Chara aspera and Chara tomentosa. The results showed that Chara in general behave differently between the communities, i.e. the lakes have different vegetation dynamics. Directional changes were shown for C. aspera and C. intermedia whereas C. tomentosa showed clear signs of patch dynamics. The results are discussed from different viewpoints, such as eutrophication, salinity variation, intraspecific competition and founder or dominance- controlled communities. Stoneworts are sensitive to eutrophication, and many of the Swedish species are today red listed. Understanding the vegetation dynamics of stonewort species are therefore of great relevance. This study show that it is possible to make a temporal vegetation dynamics survey using a spore bank.
Vegetationsdynamiken hos Charophytavegetation har studerats i tre grunda sjöar. De studerade sjöarna ligger ungefär 25 km sydost om Gävle, nära Östersjön. Sedimentproverna togs upp med hjälp av ett plaströr som fungerade som en sedimentprovtagare. Varje prov skivades sedan upp i tre skikt, undre, mellan och övre för att kunna studera vegetationsförändringar över tid. För de statistiska analyserna användes programmet R version 2.12.1 för windows. Syftet med denna studie var att beskriva hur vegetationsdynamiken förändras över tid i de tre utvalda sjöarna och utveckla metoder för att kunna använda sporbanken till dessa undersökningar. Totalt hittades tre arter i de studerade sjöarna, Chara intermedia, Chara aspera och Chara tomentosa. Resultaten visar att Chara generellt uppför sig olika mellan de olika samhällena, sjöarna har olika vegetationsdynamik. C. aspera och C. intermedia visade sig ha riktade förändringar medan C. tomentosa visade tydliga tecken på patch dynamik. Resultaten diskuteras utifrån olika tolkningar, som eutrofiering, salthaltsvariation , intraspecifik konkurrens och founder eller dominance- kontrollerade samhällen. Kransalger är känsliga för eutrofiering och många av de svenska arterna är idag rödlistade. Förståelsen för vegetationsdynamiken hos kransalgsarter är därför av stor relevans. Denna studie visar att det är möjligt att göra en undersökning av vegetationsdynamikens förändring över tid genom att använda sig av en sporbank.

Coastal regions are vulnerable to various natural processes, ranging from normal waves to extreme events. Given the flourishing development and large population along coastlines, various measures have been taken to mitigate the water-induced damage. Nature-based coastal protection, especially vegetation, has attracted unprecedented studies over the past two decades. To enhance understanding of this subject, this dissertation evaluates the impact of patchy vegetation on wave and runup dynamics along coastlines. Selecting from a prototype in Dalehite Cove, Galveston Bay, TX, results from a Boussinesq model (COULWAVE) showed patchy vegetation reduced up to 75% mean shoreward current in the mound-channel wetland systems. These vegetation patches also reduced the primary circulation around mounds, with a power-form relation between circulation size and various parameters (i.e., bathymetry, incident wave and vegetated roughness). Substituting spectral waves for regular waves in the similar wetlands, more energy was transferred into the higher frequencies. The impact of patchy vegetation on wave energy was frequency- and space-dependent, with increased energy observed in specific harmonics and locations. Comparison with unvegetated horizontal bathymetry demonstrated that mound-channel bathymetry was the dominant factor in transferring and dissipating wave energy, while vegetation patches added a fair contribution. As for extreme events, such as tsunamis, laboratory experiments and numerical simulations were conducted to assess the effectiveness of patchy vegetation with various roughness levels, spacings and sizes. Overall, vegetation patches reduced the most destructive loads onshore by up to 80%. Within-patch roughness variation only caused uncertainty on the hydrodynamics around the seaward patches, while the mitigation of extreme loads was not undermined. A logarithmic relation was observed between the protected area from extreme loads and the vegetated coverage. These findings will fill the knowledge gap of hydrodynamics in the presence patchy vegetation, and improve the engineering practice of coastal protection using nature-based infrastructure.
Ph. D.

The Cederberg Wilderness Area, in the Cape Floristic Region, South Africa, contains over 2000 plant species, 280 of which are endemic. The area has been subject to various forms of land use for millennia ranging from hunter-gatherers, herders, and farmers to visitors today. This study used palaeoecological techniques to investigate the impacts of past land use, specifically the transition from hunter-gathering to farming and herding in order to provide a baseline for current wilderness management. A sediment core was extracted from a wetland adjacent to the De Rif farmstead, analysed for fossil pollen and charcoal and dated using AMS radiocarbon dating. Historical records were used to link changes with land use history. A vegetation survey of the site focussed on the grass component of the vegetation. The largest impacts on vegetation during the last 2300 years are due to grazing and agriculture during the 1800s to 1940. Fire-sensitive taxa have not declined, apart from possibly Ericaceae, suggesting that changes in fire have not exceeded a threshold that affects the community at a family level. Changes in the fire regime, combined with disturbance by ploughing and grazing have increased the abundance of Poaceae and Cyperaceae, resulting in a decrease in Restionaceae. Ploughing affected the height structure and species composition of the site, and allowed the invasion and persistence of exotic grasses which now make up 43% of total grass cover on the previously ploughed area. Few indigenous fynbos grasses were found suggesting that the grass community is depauperate due to disturbance. The higher grass abundance preceded the largest fire recorded in the charcoal record suggesting a grass fire cycle has started at De Rif. Ploughing, grazing and invasive grasses, rather than changes in fire regime or resource extraction, are the main causes of vegetation change at De Rif and still affect the site today. Wilderness management will need to mitigate the impacts of livestock and agriculture on De Rif and monitor the recovery of this and other previously farmed areas to ensure that they do not become as a source of invasive species in the future under novel disturbances such as anthropogenic climate change.

Conservation of biodiversity in urban vegetation fragments is of increasingly high importance with growing urbanisation globally, particularly so in the biodiversity hotspot of the Mediterranean southwest of Western Australia. Relationships between fragmentation (connectivity, fragment size, time since isolation) and urban disturbances (weed invasion, human disturbance, rubbish) relative to the soil seed bank of a representative sample of Perth’s urban woodland fragments (N=36) were examined. In total, 182 seedling types comprising 57 invasive, 105 native and 20 unknowns were identified. Approximately 65% of the 21,770 seedlings counted were native, 33% invasive and 2% unidentified. The average soil seed bank seedling density was 2,787 germinates per m2. Community analysis (ordination) showed that the soil seed bank composition correlated with fragment age. Subsequent quantitative analysis (linear regression) did not find evidence for species or functional trait groups being vulnerable to decline due to isolation or fragment age, indicating that species extinction is not evident within Perth’s urban fragments. More invasive annual herbs were found within older fragments and the soil seed banks of smaller fragments were found to have lower native species abundance compared to larger fragments. A positive relationship was found between the number of footpaths within a fragment and the abundance of annual invasive herbs within the soil seed bank. High weed cover in a fragment resulted in high weed presence within the soil seed bank. It was also found that high weed cover did not necessarily indicate low native species diversity or abundance within the soil seed bank, suggesting caution in using weed cover to classify sites as suitable/unsuitable for topsoil transfer in restoration activities. The soil seed bank of the study sites contained a diverse range of native species, indicating that topsoil from these areas may be useful in restoration of degraded sites.

The impact of the small stock industry on Succulent Karoo vegetation has long been acknowledged, and there is a need for researchers and managers to better understand the dynamics and processes leading to vegetation degradation and recovery. Despite the fact that these arid systems tend to hold high demographic inertia, as well as being prone to sudden and unpredictable events, there are few data-sets that are long enough (>50 yrs) to adequately distinguish 'noise' from true changes. This study examines an extensive data set of plant % cover, recorded using a rapid transect step-point technique, for the period between 1971 and 2002 throughout the Ceres Karoo. Correspondence analysis (CA) ordinations were used to show plant community changes from year to year at two sites: one that has been rested for many decades, and one that has used the Group Camp system since 1970. CA ordinations were also used to depict changes between 1992 and 2002 on three farms using different grazing systems. Results are discussed against the backdrop of the Stock Reduction Scheme initiated in the 1970's. Of the two farms examined from 1971 to 2002, the one using the group camp approach has shown an initial lag-period of about 10 years, and a subsequently steady and directional turnover of plant communities, increasing in cover of desirable species, until 2002. The rested farm showed no identifiable change. Of the farms studied between 1992 and 2002, a clear separation was found between the one using the Group Camp system and the others that are only grazed in winter. The former farm appeared to be showing the greatest amount of change. These results challenge the opinion that rested arid region veld is unlikely to recover. On the contrary, there has been a move toward more desirable veld since the 1970's in land that has not even been rested, but has had relatively reduced stock numbers. The more rested lands appear to be healthier than those that have been more frequently grazed, but they are not showing clear signs of change, supporting theories that arid region vegetation dynamics are characterised by a state and transition type of model. An important pattern to note is that changes from a degraded to a more desirable veld are characterised by a long lag period of more than 10 years, with subsequent changes occurring throughout a 20-year period. Thus the importance of allowing rangelands sufficient time to recover is highlighted, as is the importance of establishing, and continuing existing, long-term data sets.

This thesis investigates new late Quaternary vegetation records from four sites in the Craven District of the Yorkshire Dales. The chosen sites fall along an east-west transect broadly following the line of the south Craven Fault. The rationale for site selection was not based on conventional palynological considerations of potential for rich core samples, rather to provide a range of different locations within a distinct micro-region each existing in some specific proximity to known archaeological features. The logic was to attempt to get beyond broad 'natural' climatological and vegetational inferences to understand the nature and level of potential anthropogenically produced change at a local scale as a sub-set of natural change in a broader regional zone over time. The sites reveal varied vegetation histories from the Late Glacial period to the present day and all show signs of being influenced by changes in their arboreal structure at some time, although no two sites have exactly the same vegetation communities until around 5000 BP when the tree canopy is opened to allow an open grassland to dominate. The results indicate the possibility that Betula values, in particular, might indicate cooling events found in the Greenland ice cores for Greenland Interstadial 1 as well as the Pre-boreal Oscillation and the Holocene 9.3 ka BP Event. Closer chronological control of such values could help to determine whether vegetational dynamics were synchronous with fluctuations in temperature and the speed with which trees respond to severe temperature fluctuations. Various hiatuses identified during analysis of the cores may be caused by human influence on the wetlands, given that archaeological evidence from caves shows human occupation of the Craven area from the late Upper Palaeolithic onwards.

The machair systems of the Outer Hebrides of Scotland are renowned for the diversity and conservation value of their plant communities. However, machair systems have a restricted distribution and are prone to disturbance from both anthropogenic and environmental sources. The effects of two major anthropogenic and environmental disturbances (agriculture and burial by wind-blown sand) on the Kildonan and Drimsdale machair systems on South Uist, and the dynamic responses of machair vegetation to these human and natural agencies were described and quantified. The investigated machair systems comprised a diversity of sand dune, grassland and marginal vegetation types which were spatially distributed along a primary environmental gradient of soil organic matter, soil moisture and soil pH. Vegetation types influenced by the effects of cultivation, notably potato-bed successions, abandoned field systems and fallow areas were also a conspicuous component of the Kildonan and Drimsdale machairs. Re-vegetation following cultivation occurred primarily through vegetative processes. Sexual reproduction via seed within different machair vegetation types was limited. The numbers of viable seeds recorded from machair seed banks ranged from 7 mˉ² soil in foredune vegetation to 58 mˉ² soil in newly-ploughed cereal land. Densities of viable seeds recorded from the machair seed rain ranged from 2 mˉ² soil in the dune slack to 24 mˉ² soil in a potato patch, fallow for two years. These values represent the first estimates of the size of machair seed budgets. Glasshouse-based burial experiments examined the effects of inundation by sand on four machair vegetation types. Foredune grassland, dune slack, three-year fallow grassland and unploughed grassland vegetation exhibited the capacity to survive and re-emerge from both intermittent and single depositions of sand. In terms of effects on plant frequency, intermittent burial by five depositions of 1cm of sand was more damaging than a single deposition of 5cm. However, all vegetation types responded to both intermittent and single burial events primarily through a change in the number of individual plants, rather than through a change in the number of plant species. Five different responses to burial by sand were identified for machair species. The suspension of photosynthetic activity and the maintenance of a low dark respiration rate were identified as important physiological responses to burial. Photosynthetic activity of machair vegetation was resumed on denudation, indicating the existence of an elastic physiological response to burial. Ecological processes in the dune slack habitat and the responses of slack vegetation to its dynamic environment were subtly different to those characterizing the other investigated sub-communities. These differences were attributed to the fact that machair slacks are characterized by an historical ecology distinct from that of other areas of the machair system. It is suggested that the formulation of effective management plans and conservation strategies for the machair systems of the Outer Hebrides ultimately lies in an understanding of their historical ecology, and in their past management and environmental histories.

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

It seems to be obvious that precipitation has a major impact on greening during the rainy season in semi-arid regions. First results1 imply a strong dependence of NDVI on rainfall. Therefore it will be necessary to consider specific rainfall events besides the known ordinary annual cycle. Based on this fundamental idea, the paper will introduce the development of a rain adjusted vegetation index (RAVI). The index is based on the enhancement of the well-known normalized difference vegetation index (NDVI2) by means of TAMSAT rainfall data and includes a 3-step procedure of determining RAVI. Within the first step both time series were analysed over a period of 29 years to find best cross correlation values between TAMSAT rainfall and NDVI signal itself. The results indicate the strongest correlation for a weighted mean rainfall for a period of three months before the corresponding NDVI value. Based on these results different mathematical models (linear, logarithmic, square root, etc.) are tested to find a functional relation between the NDVI value and the 3-months rainfall period before (0.8). Finally, the resulting NDVI-Rain-Model can be used to determine a spatially individual correction factor to transform every NDVI value into an appropriate rain adjusted vegetation index (RAVI).

Quantitative monitoring of vegetation change over time is essential in understanding the environmental processes of which are important in climate change and global warming models, because vegetation change is an indicator of environmental variability. However, obtaining such information has been a challenge especially for vegetation phenology due to the lack of appropriate methods for quantitative assessment. There is therefore a need to derive methods to quantitatively characterize vegetation dynamics in order to monitor the effect of climate change on the biosphere and as inputs to global change models. The aim of this research was to test the relationships between ground-based measurement of leaf area index (LAI) and vegetation indices (VI) derived from satellite remote sensing instruments to quantitatively monitor vegetation dynamics in a broadleaf and coniferous forest in the UK. This research has four key hypotheses. First, phenological changes (which is the timing of recurring biological events in plants) in broadleaf and coniferous forest canopies may be characterized using ground-based measurement of LAI, because LAI is good proxy for vegetation phenology. Second, cloud cover frequency in the UK leads to a requirement for higher temporal resolution remote sensing data to monitor changes in vegetation phenology. Third, data from the Disaster Monitoring Constellation (DMC) satellites provides a sufficiently high temporal resolution for monitoring vegetation phenology in the UK. Fourth, vegetation indices derived from atmospherically corrected DMC data may be used to monitor vegetation phenology in the UK. Analysis of Advanced Very High Resolution Radiometer (AVHRR) and Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask showed that the average of number of cloud free days at the UK test sites in the year 2005 was five days per month with a minimum of one cloud free day per month implying that high temporal resolution satellites like the DMC will be appropriate for monitoring vegetation change. Nine DMC satellite images were acquired over 2005/2006 for the study sites plus one coincident Landsat ETM+ in 2005. Four vegetation indices (VI) were derived from the satellite data sets and were related to LAI/PAI. PAI is the plant area index defined as the total surface area of both photosynthetic and nonphotosynthetic part of plant per unit ground area. A regression model was used to predict LAI/PAI and the root mean square error (RMSE) was determined for both sites. The RMSE of the observed and predicted LAI values show that the levels of errors at Risley Moss were 0.51 for LAI, 0.52 for overstorey PAI and 0.8 for total canopy while PAI was 1.1 for Charter's Moss. Therefore, the DMC and one Landsat ETM+ data set related to LAI/PAI can adequately retrieve biophysical parameter in the deciduous woodland. However, in the coniferous canopy the numbers of observations was fewer and the measurement errors larger leading to a requirement for more data in order to establish statistically significant and ecologically useful relationships. Improvements in the accuracy of ground-based LAI/PAI measurements, radiometric and atmospheric correction of satellite data are expected to increase the accuracy of such LAI/PAI estimates in future.

Thesis (Ph. D.)--University of North Carolina at Chapel Hill, 2006.
Title from electronic title page (viewed Oct. 10, 2007). "... in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Biology." Discipline: Biology; Department/School: Biology.

Fire has not always been so elusive in the northern European forest yet forest management and active fire suppression has created an ecosystem almost free of fire. This absence of fire is thought to have contributed to the widespread dominance of Picea abies as well as the decline in deciduous species and subsequent loss of floristic diversity. Forest fires in general are driven by a complex interplay between natural (climate, vegetation and topography) and anthropogenic disturbance and through palaeoecology we are able to explore spatio-temporal variability in the drivers of fire and changing fire dynamics. This thesis explores spatial and temporal variability in Holocene vegetation and disturbance dynamics through stand-scale palynology. Pollen and macroscopic charcoal are used to reconstruct past vegetation and fire dynamics with local- and regional-scale pollen-derived quantitative vegetation reconstructions able to identify both large-scale ecosystem response and local-scale disturbance. Spatio-temporal heterogeneity and variability in biomass burning is explored to identify the drivers of fire and palaeo-vegetation reconstruction is compared to process-based, climate-driven dynamic vegetation model output to test the effect of fire frequency as a driver of vegetation composition and dynamics. Early-Holocene fire was driven by natural climate variations and fuel availability. The establishment and spread of Picea abies (Norway spruce) appears to be driven by an increase in continentality although local disturbance cannot be ruled out. The expansion of Picea led to a step-wise reduction in regional biomass burning and the now widespread dominance of Picea abies is responsible for the low fire frequency observed through Fennoscandia. The mid-Holocene decline in deciduous species was primarily driven by localised anthropogenic disturbance and may have been assisted by the shift to cooler, wetter climate conditions. There is an underlying natural fire frequency of approximately 400 years observed in southern Finland and without intensive anthropogenic disturbance floristic diversity may have remained locally, to the present day. Stand-scale palynology is able to record past local disturbance at a high spatial precision however more than one site is required to understand regional disturbance and the variable controls of fire dynamics.

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

Warm Springs Mountain (WSM), a priority conservation area for The Nature Conservancy in Bath County, Virginia, is home to a rare montane pine barren and large tracts of uninterrupted mixed pine and deciduous forest extending east into the George Washington National Forest. Limited documentation of past disturbances and their influence on WSM forests presents challenges for land managers desiring to understand historic conditions for these ecosystems. The only formal study of vegetation dynamics on WSM noted an absence of pitch pine (Pinus rigida Mill.) regeneration and an increase in fire-intolerant species during recent decades in the pine barren community that is probably linked to fire suppression. Dendrochronological studies of disturbance history in the central and southern Appalachians have mostly focused on ridgetop and southwestern-facing slopes. This study examines long-term forest dynamics in the pine- and oak-dominated forests on southeastern slopes of Warm Springs Mountain and downslope from the higher elevation pine barren using dendrochronology and vegetation analysis. We studied trees in six 20 x 50 m plots to develop a tree ring chronology and document changes in stand composition and structure through time. We found an increase in fire-intolerant species and decline in fire-dependent pines and oaks through time. Pitch pines have not recruited since 1954 in our sites due to a lack of burning, while Acer rubrum L. has produced high numbers of seedlings in recent years. This study of vegetation dynamics over space and time will provide insights for land managers and inform fire restoration practices.
Master of Science

Thesis (M.S.)--West Virginia University, 1999.
Title from document title page. Document formatted into pages; contains viii, 63 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references.

Abstract Succession is a compositional change of species and other ecosystem characteristics over time. Mire development, i.e., long-term mire succession is basically driven by an increase in peat layer height, promoting changes in hydrology, vegetation and nutrient status of a particular mire. Due to this, ecosystem processes, such as production and loss of carbon due to decomposition (i.e. carbon gas functions), change with increasing successional mire stage. An adequate method for studying the changes in ecosystem C functions is to measure CO2 and CH4 fluxes between the ecosystem and atmosphere. Succession and carbon dynamics of boreal pristine mires have been much studied. However the link between these phenomena is largely unknown. Further, if and how the C gas functions of mires change during mire succession it is rather poorly understood. The main objective of this thesis was to study how ecosystem functions, measured as CO2 and CH4 exchange, change during mire development. The study also aims to explore the drivers of succession in mire development, i.e., mire succession. Successional mire C dynamics were studied along an eight-kilometer-long successional sequence of primary paludified mires located in the land uplift coast of the Bothnian Bay. Due to the short distance between sites, they all have been under the same climatic control for most of their development. The gradual replacement of plant species with different photosynthetic potential, phenology and assimilating green area resulted in lower-level and temporal variation of CO2 exchange patterns at the later successional stages. Similar to this, CH4 also had the lowest interannual variation in the later stages. In general, CH4 emissions increased with mire age even though this trend did not emerge during the rainy season. Further, this study showed that the wintertime C function pattern was related to the C pattern during the previous summer confirming the important effect of growing season patterns on wintertime C dynamics. In addition to the fundamental effect of vegetation as a driver of succession which was also confirmed in this study, the role of hydrological conditions appeared to be equally important. More constant hydrological conditions at later successional stages resulted in lower temporal variation in CH4 and CO2 fluxes. The present results suggest that the stability of ecosystem C gas functions increases during mire development due to increasing autogenic control
Tiivistelmä Sukkessio on ekosysteemin lajistossa ja sen muissa ominaisuuksissa ajan kuluessa tapahtuva muutos. Suon kehitystä eli pitkäaikaista suosukkessiota vie eteenpäin turpeen paksuuskasvu, joka saa aikaan muutoksia suoekosysteemin hydrologiassa, kasvillisuudessa ja ravinnetilassa. Tästä johtuen myös suoekosysteemin erilaiset prosessit, kuten tuotanto sekä hajoamisen kautta tapahtuva hiilen vapautuminen eli hiilikaasutoiminta muuttuu suon ikääntyessä. Ekosysteemin hiilikaasutoiminnassa tapahtuvia muutoksia voidaan tutkia muun muassa mittaamalla ekosysteemin ja ilmakehän välisiä hiilidioksidi- ja metaanivirtoja. Boreaalisten luonnontilaisten soiden sukkessiota ja hiilidynamiikkaa on tutkittu runsaasti, mutta niiden välistä yhteyttä ei sen sijaan juuri tunneta. Tämän vuoksi ei tiedetä kuinka soiden hiilikaasutoiminta mahdollisesti muuttuu suon kehityksen aikana eli suosukkession edetessä. Tämän tutkimuksen päätavoitteena oli tutkia kuinka hiilidioksidin ja metaanin vaihdolla mitattu ekosysteemitoiminta muuttuu suon kehityksen aikana. Tutkimus pyrki myös selvittämään suosukkessiota kontrolloivat tekijät. Eri-ikäisten soiden hiilikaasudynamiikkaa tutkittiin mittaamalla hiilikaasuja Perämeren maankohoamisrannikolla kahdeksan kilometrin pituisella sukkessiogradientilla, joka koostuu primaarisoistumisen kautta syntyneistä soista. Soiden lyhyestä keskinäisestä etäisyydestä johtuen ne ovat olleet saman ilmastollisen kontrollin alaisena suurimman osan kehityksestään. Vaiheittainen kasvilajien muutos sukkessiogradientilla yhdessä kasvilajien erilaisen yhteyttämispotentiaalin, fenologian ja yhteyttävän lehtipinta-alan kanssa johti hiilidioksidivaihdon alhaisempaan tasoon sekä pienempään ajalliseen vaihteluun vanhemmilla sukkessiovaiheilla. Myös metaanin vaihdolla oli alhaisimmat vuosien väliset vaihtelut vanhemmilla vaiheilla. Yleisesti ottaen metaanipäästöt kasvoivat suon iän myötä, vaikkakaan tätä trendiä ei havaittu sateisena kasvukautena. Lisäksi tutkimus osoitti, että talviaikaiset hiilivirrat (CO2, CH4) seurasivat kesäaikaisen hiilidynamiikan vaihtelua. Kasvillisuuden keskeinen rooli ekosysteemin sukkessiossa havaittiin myös tässä tutkimuksessa. Kasvillisuuden ohella merkittäväksi suosukkessiota sääteleväksi tekijäksi osoittautui hydrologisten olojen vaikutus. Tasaisemmat hydrologiset olot vanhemmilla sukkessiovaiheilla johtivat vähäisempään ajalliseen vaihteluun metaani- ja hiilidioksidivirroissa. Tutkimuksen tulokset viittaavat siihen, että ekosysteemin hiilidynamiikka stabilisoituu suon kehityksen aikana lisääntyvän autogeenisen kontrollin kautta

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

Mubwindi Swamp in the Rukiga Highlands of south west Uganda is today surrounded by dense stands of moist lower montane forest. A survey of the present-day vegetation combined with an investigation of the surface pollen spectra indicate pollen accumulating on the swamp surface mainly result from taxa within the catchment. This survey also highlights that a combination of climatic, edaphic, demographic and human factors control the present-day composition and distribution of montane forest. Mubwindi Swamp comprises deep accumulations of peat-rich deposits. Pollen data from three cores of sediment provide a partial record of montane vegetation history since at least 43,000 yr B.P. This record is broken by at leat two, and possibly three, sedimentary hiati. Twelve radiocarbon ages, charcoal data from two cores using combination of three methods, and investigations into particle size characteristics place this record of montane vegetation dynamics within a wider palaeoenvironmental debate concerning Pleistocene climates, forest refugia, and human induced impacts on montane forests. Pollen deposited before and soon after the last glacial maximum represents a vegetation composition very different to that of the modern vegetation within the Mubwindi Swamp catchment. Specifically there were increased amounts of Artemisia, Ericaceae, Faurea and Stoebe. However, some elements found within the modern vegetation persisted during these periods, these include Ilex, Olea, Podocarpus and Zanthoxylum. These taxa may have persisted possibly because of favourable topography or soils. The uppermost 5m of deposits in core MB6 from the site are dated to the last 2100 years, and thus provide an opportunity, so far unique for Uganda, and much of central Africa, to record variations in the composition of montane vegetation during the late Holocene. Results of analyses of pollen indicate that lower montane forest has been present in the catchment for Mubwindi Swamp throughout the last 2100 years, despite extensive clearance in other parts of the region over the same period. A transition to a more open - and possibly drier - form of forest is apparent from around 680 yr B.P. This montane forest composition is thought to stem from a recent period of climatic change to less humid conditions. This change is followed by an increased occurrence of pollen from plants presently associated with areas of degraded forest from around 200 yr B.P. and a decrease in pollen from important sources of timber. Some recovery of timber trees is apparent from around 50 yr B.P. Changes in forest composition around 200 years ago could be interpreted as representing slightly increased humidity. However, in view of the nature of these changes, a low level of human impact was probably the most important causal factor. The gazetting of Bwindi-Impenetrable Forest as a Forest Reserve in the early 1930s may have facilitated a recovery of timber trees during the present century.

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

This case study used Zhongwei City in northwest China to quantify the urbanization and revegetation processes (1990-2011) through a unified sub-pixel measure of vegetation cover. Research strategies included: (1) Conduct sub-pixel vegetation mapping (1990, 1996, 2004, and 2011) with Random Forest (RF) algorithm by integrating high (OrbView-3) and medium spatial resolution (Landsat TM) data; (2) Examine simple Dark Object Subtraction (DOS) atmospheric correction method to support temporal generalization of sub-pixel mapping algorithm; (3) And characterize patterns of vegetation cover dynamics based on change detection analysis. We found the RF algorithm, combined with simple DOS, showed good generalization capability for sub-pixel vegetation mapping. Predicted sub-pixel vegetation proportions were consistent for "pseudo-invariant" pixels. Vegetation change analysis suggested persistent urban development within the city boundary, accompanied by a continuous expansion of revegetated area at the city fringe. Urban development occurred at both the suburban and urban core areas, and was mainly shaped by transportation networks. A transition in revegetation practices was documented: the large-scale governmental revegetation programs were replaced by the commercial afforestation conducted by industries. This study showed a slight increase in vegetation cover over the time period, balanced by losses to urban expansion, and a likely severe degradation of vegetation cover due to conversion of arable land to desert vegetation. The loss of arable land and the growth of artificial desert vegetation have yielded a dynamic equilibrium in terms of overall vegetation cover during 1990 to 2011, but in the long run vegetation quality is certainly reduced.
Master of Science

This thesis is an examination of human impact on plant species composition and vegetation structure of Narok District, Kenya. This is done by comparing the vegetation on disturbed and undisturbed sites. A literature review confirms the influence of administation, abiotic and biotic factors on vegetation dynamics. A macro-scale approach to the research, based on satellite imagery, confirms rapid expansion of cultivation. A subsequent micro-scale approach, based on field sampling, suggests that present human activities are becoming increasingly permanent and predominantly large-scale. This trend is accounted by increasingly human population and the change in Maasai lifestyles. The description of vegetation data using indices of similarity suggests that samples asociation is determined by proximity factors and human disturbance. Alpha diversity indices suggest that medium human disturbances increase species diversity. Examination of floristic and structural characteristics suggests that high human disturbances reduce the number of woody species and the volume of wood while low disturbances favour the predominance of woody species. On the contrary, medium human disturbances attract both successional and climax species. Also, climatic and soil factors by determining plant lifeforms and the height of plants, affect the amount of wood in the different site categories. An inhibition model is suggested for the change in species composition at different height categories. This is either due to the exclusion of seedling recruitment by resource monopolization in undisturbed sites or due to suppression of recruitment of plants to the next stage in disturbed sites. PCO ordination of floristic data based on species association shows samples pattern along axis 1 which varies with moisture gradient, organic carbon, nitrogen and zinc while the pattern along axis 2 suggests the influence of human disturbance, mainly between disturbed and undisturbed forest and bushland samples. Classification of vegetation data using both average linkage and furthest neighbour algorithms produce clusters on the basis of sample closeness suggesting a continuous, rather than a discrete, distribution of species. Lack of earlier fusing of samples of the same disturbance characteristics and from the same plant community is attributed to two factors. First, is the relatively recent human influence which has had little influence on plant composition. Second, is plant resilience which resists and/or reduces human impact on vegetation. In view of the findings, it is recommended that vegetation management should aim at reducing extreme disturbances. Low disturbances, it is postulated, reduce re-establishment potential of a site by promoting resource monopolization while high disturbances degrading site conditions for recolonization. A defined land use policy is suggested as a pre-requisite to implementation of the proposals offered.

Thesis (M.S.) -- University of Maryland, College Park, 2005.
Thesis research directed by: Marine, Estuarine, Environmental Sciences Graduate Program . Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

In urban riparian areas, vegetation composition may be affected by urban land use changes at both the stream reach and the watershed scale. Moreover, the mechanisms by which seeds disperse may be affected both by reduction in seed sources due to vegetation removal and by the urban stream syndrome that produces flashier hydrographs and incised channels. I hypothesized that vegetation communities with high cover of native and hydrophilic species would be found in watersheds with high forest cover, while more limited cover of these species would be found in highly developed watersheds. Additionally, to examine the dispersal mechanisms contributing to these patterns, I hypothesized that 1) more seeds would be deposited in riparian areas by water than by wind; 2) the number of seeds deposited by streams would decrease as watershed urbanization increased; and 3) seeds deposited in the most urbanized sites would be primarily from species with traits favoring deposition by water, including large seed size and presence of a dispersal appendage. To investigate relationships between urban land cover types and riparian vegetation, I surveyed 30 randomly-selected riparian forests in the Portland-Vancouver metro area and related vegetation assemblages to watershed land cover. Vegetation was mapped to the nearest 1cm along three transects in each site. Land cover was characterized both within a 500m buffer around each site, and within the entire watershed. Relationships between land cover and vegetation assemblages were investigated using nonmetric multidimensional scaling and classification trees. To investigate the effect of watershed urbanization intensity on riparian seed deposition, I collected seeds deposited in nine riparian sites along a gradient of watershed total impervious area (TIA). I used a stratified-random approach to select sites. In each site, wind-deposited seeds were collected in funnel traps three times, and water-deposited seeds were collected in turf traps four times, over a 15-month period, spanning both wet and dry seasons. Consistent with my first hypothesis, communities dominated by native understory species were found exclusively in watersheds that were at least 15% forested by evergreen canopy. These findings suggest that native understory communities can persist in urban areas if adequate surrounding forest cover is maintained. Regarding my second major hypothesis, significantly more seeds were deposited by water than by wind (p < 0.05; mean of 155 seeds per turf trap; mean of 30 seeds per funnel trap). For shrubs, for species primarily dispersed by animals, and for species under 15m tall, hydrochory significantly increased delivery to riparian areas over the background seed delivery rate measured in funnel traps. There was a significant reduction in the number of seeds deposited by streams as TIA increased (adjusted R² = 0.74; p < 0.01). Deposition of shade-tolerant seeds decreased significantly, while deposition of non-native seeds increased significantly (p < 0.05) with watershed TIA and with development within 500m from the site, likely due to alterations of seed source pools of these species. Findings indicate that in an urban setting, small streams have the capacity to act as dispersal vectors, connecting fragmented populations that may otherwise be seedlimited. Riparian forests with diverse understory assemblages maintained by ongoing seed deposition may persist in urban areas with sufficient watershed forest, as well as with low development cover, in both the whole watershed and the near-stream area. Total seed deposition by streams, as well as deposition of shade-tolerant species, can be expected to decrease with increased watershed development. Results suggest that passive approaches to restoration of riparian forest understories in urban watersheds will only likely be successful with sufficiently high forest cover and with restricted development.

A comprehensive understanding of the hydrodynamics in vegetated open-channels and flow-vegetation interaction is of high interest to researchers and practitioners alike for instance in the content of river and coastal restoration schemes. The focus of this study was to investigate the effect of the presence of vegetation on flow resistance, turbulence statistics, and the instantaneous flow in open channels by performing three-dimensional computational-fluid-dynamics (CFD) simulations. Firstly, fully developed turbulent flow in fully-vegetated channel was analyzed by employing the method of high-resolution Large-Eddy Simulation (LES). Flow through a staggered array of rigid, emergent cylinders was simulated and the LES was validated through experiments. After validation, numerical simulations were performed at an extended parameter range of two different cylinder Reynolds numbers (ReD = 500 and 1340) and three different vegetation densities (φ = 0.016, 0.063, and 0.251). Flow structures and statistics were analyzed on the instantaneous flow and the effect of the vegetation density and cylinder Reynolds number was assessed. Moreover, drag forces exerted on the cylinders were calculated explicitly, and the effect of both ReD and φ on the drag coefficient was quantified. Secondly, two new alternative simulation strategies, a RANS based strategy with a vegetative closure model and a low-resolution Large-Eddy Simulation, were devised. They were evaluated by simulating several experimental cases with diverse conditions of the cylinder arrangement (i.e., staggered vs. random distribution), vegetation densities (φ = 0.016, 0.022, 0.063, 0.087, 0.091, 0.150, and 0.251), and cylinder Reynolds number (ReD = 170 - 1700). For the RANS based strategy, the importance of a-priori knowledge was assessed, and for the low-resolution LES, the efficiency and accuracy was demonstrated. Finally, a numerical strategy based on a porosity approach was developed and applied to open-channel flow through a natural plant. The simulated velocities were compared with experimentally acquired ones and results showed reasonable agreement. The results obtained in this research contribute to the understanding of fundamental mechanism of flow-vegetation interaction in vegetated open-channels, resolving turbulent flow-vegetation interaction explicitly. In addition, the new numerical strategies developed as part of this research are expected to allow describing the behavior of turbulent flow through artificial and natural vegetation with high efficiency and accuracy.

Understanding the relationships between modern pollen deposits and their surrounding vegetation is an important tool to improve the quality of reconstructions and interpretations of past vegetation changes from fossil pollen records. The overall aim of this research project is to validate and calibrate mathematical models of relationships for assemblages deposited in peatland and forest landscapes in southeast China, which will form an essential basis for quantitative reconstruction of past land cover from Quaternary peat deposits in the region. This field study area presents great challenges, being spatially large in extent compared to studies in northwestern Europe and of difficult and inaccessible mountainous terrain, which makes the vegetation survey (10m-100m) time-consuming. Firstly, a study to decide whether to use moss or soil surface samples is presented. 42 paired moss and soils sample were collected in the five main forest types. Similar levels of variation in the pollen spectra are seen, but there are systematic differences in the mean values of key groups of taxa. Moss polsters are chosen since they record the most accurate representation of the contemporary vegetation. Secondly, the behaviour of several pollen dispersal and deposition models is tested against a grassland-forest transect. The Prentice-Sugita model passed the test and is therefore considered suitable for use. Thirdly, the first estimates of relative pollen productivity (RPP) for 9 key taxa (Castanea, Cryptomeria, Cyclobalanopsis, Liquidambar, Pinus, Poaceae, Quercus, Rosaceae and Theaceae) are presented. Two alternative methods (modified Davis method and iteration method) for estimating RPP are also developed, which have great potential for use in wider areas. Fourthly, wetland herb taxa are important in the pollen spectra from mire records, therefore surface samples were taken from a mire surface. They show that there are inter-annual differences in pollen from wetland herbs. Finally, the discussion presents suggestions for how these findings can be best applied to land-cover reconstruction, explores the strengths and limitations of the study and identifies future directions which such work could take.