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Vegetation Dynamics (vegetation + dynamics)

Distribution by Scientific Domains

Earth and Environmental Science	45%
Life Sciences	43%
Humanities and Social Sciences	9%

Selected Abstracts

Impact of CO2 concentration changes on the biosphere-atmosphere system of West Africa

GLOBAL CHANGE BIOLOGY, Issue 12 2002
GUILING WANG
Abstract Vegetation dynamics plays a critical role in causing the decadal variability of precipitation over the Sahel region of West Africa. However, the potential impact of changes in CO2 concentration on vegetation dynamics and precipitation variability of this region has not been addressed by previous studies. In this paper, we explore the role of CO2 concentration in the regional climate system of West Africa using a zonally symmetric, synchronously coupled biosphere-atmosphere model. We first document the response of precipitation and vegetation to incremental changes of CO2 concentration; the impact of CO2 concentration on the variability of the regional biosphere-atmosphere system is then addressed using the second half of the twentieth century as an example. An increase of CO2 concentration causes the regional biosphere-atmosphere system to become wetter and greener, with the radiative effect of CO2 and improved plant-water relation dominant in the Sahelian grassland region and the direct enhancement of leaf carbon assimilation dominant in the tree-covered region to the south. Driven by the observed sea surface temperature (SST) of the tropical Atlantic Ocean during the period 1950,97 and with CO2 concentration prescribed at a pre-industrial level 300ppmv, the model simulates a persistent Sahel drought during the period of 1960s,1990s. The simulated drought takes place in the form of a transition of the coupled biosphere-atmosphere system from a wet/green regime in the 1950s to a dry/barren regime after the 1960s. This climate transition is triggered by SST forcing and materialized through vegetation-climate interactions. The same SST forcing does not produce such a persistent drought when a constant modern CO2 concentration of 350ppmv is specified, indicating that the biosphere-atmosphere system at higher CO2 level is more resilient to drought-inducing external forcings. This finding suggests that the regional climate in Sahel, which tends to alternate between dry and wet spells, may experience longer (or more frequent) wet episodes and shorter (or less frequent) dry episodes in the future than in the past. Our study has significant implications regarding the impact of climate change on regional socio-economic development. [source]

Vegetation dynamics on rangelands: a critique of the current paradigms

JOURNAL OF APPLIED ECOLOGY, Issue 4 2003
D. D. Briske
Summary 1Rangeland ecologists have been debating the validity of two current paradigms for the evaluation of vegetation dynamics on rangelands. This debate frequently contrasts the conventional model of continuous and reversible vegetation dynamics (range model) with a more contemporary model that can accommodate discontinuous and non-reversible vegetation change (state-and-transition model). 2The range and the state-and-transition models are conceptually related to the equilibrium and non-equilibrium paradigms within ecology, respectively. The methodological dichotomy that has developed between the range and the state-and-transition models has fostered the perception that these two ecological paradigms are mutually exclusive. We challenge this perception and contend that both methodologies and their corresponding paradigms are non-exclusive. 3Equilibrium and non-equilibrium ecosystems are not distinguished on the basis of unique processes or functions, but rather by the evaluation of system dynamics at various temporal and spatial scales. Consequently, ecosystems may express both equilibrium and non-equilibrium dynamics. This confirms early interpretations that ecosystems are distributed along a continuum from equilibrium to non-equilibrium states. 4Although both equilibrium and non-equilibrium dynamics occur in numerous ecosystems, the empirical evidence is frequently confounded by (i) uncertainty regarding the appropriate evidence necessary to distinguish between paradigms; (ii) disproportionate responses among vegetation attributes to climate and grazing; (iii) comparisons among systems with varying degrees of managerial involvement; and (iv) the evaluation of vegetation dynamics at various spatial and temporal scales. 5Synthesis and applications. This critique supports the conclusion that a paradigm shift has not taken place in rangeland ecology, but rather, the debate has forced a more comprehensive interpretation of vegetation dynamics along the entirety of the equilibrium,non-equilibrium continuum. Therefore, the rangeland debate should be redirected from the dichotomy between paradigms to one of paradigm integration. [source]

Vegetation dynamics in western Uganda during the last 1000 years: climate change or human induced environmental degradation?

AFRICAN JOURNAL OF ECOLOGY, Issue 2009
Julius B. Lejju
Abstract A multi-proxy analysis of microfossils from sedimentary records, together with evidence from historical and archaeological data, has provided evidence of vegetation dynamics and human environment interactions in western Uganda for the last 1000 years. Pollen, fungal spores and phytoliths extracted from sediment cores obtained from a papyrus swamp at Munsa archaeological site indicate a relatively wet and forested environment in western Uganda prior to ca 1000 yr bp (cal 977,1159 ad). A subsequent decline in forest vegetation occurred from ca 920 yr bp (cal 1027,1207 ad). However, the deforestation period occurred during a wet period as registered in the River Nile water records, suggesting a human induced deforestation at Munsa rather than reduced precipitation. Increased numbers of herbivores, presumably domesticated cattle, postdeforestation are evidenced by the presence of dung fungal spores and broad accord with the archaeological evidence for initial occupation of the site at Munsa and the establishment of a mixed economy based on crops, cattle and iron working between 1000 and 1200 ad. From ca 200 yr bp (cal 1647,1952 ad), forest recovery occurred at Munsa site and appears to reflect abandonment of the site, as suggested by archaeological evidence, possibly following a period of prolonged drought and famine between 1600 and 1800 ad, as recounted in the oral rich traditions of western Uganda and also reflected by low water levels of River Nile. [source]

Vegetation dynamics of predator-free land-bridge islands

JOURNAL OF ECOLOGY, Issue 2 2006
JOHN TERBORGH
Summary 1We tested the ,green world' hypothesis of Hairston, Smith and Slobodkin by monitoring vegetation change on recently created predator-free land-bridge islands in a huge hydroelectric impoundment, Lago Guri, in the State of Bolivar, Venezuela. 2Our results affirm the green world hypothesis and expose the operation of a strong top-down trophic cascade that negatively impacted nearly every plant species present, implying that community stability is maintained through the action of predators. 3To test the hypothesis, we monitored vegetation on nine predator-free islands and compared demographic parameters to those observed at control sites supporting complete or nearly complete suites of predators. 4Herbivore abundance was high on ,small' (, 0.5, < 2 ha) islands, moderate on ,medium' islands (> 3, < 15 ha) and low on the ,large' landmasses that served for reference. 5Small sapling densities on small islands were only 37% of controls in 1997 (after 11 years of isolation), and when recensused in 2002, had fallen to 25% of controls. High mortality and, especially, low recruitment contributed to the decline in sapling cohorts. 6Sapling decline occurred earlier on small islands, although recruitment failure had become equally pronounced on medium islands by the end of the monitoring period. 7Several mechanisms could potentially account for suppressed sapling recruitment, but the weight of evidence points to herbivory on seedlings and small saplings by leaf-cutter ants (Atta spp. and Acromyrmex sp.). Exposure to prevailing trade winds (windward vs. leeward slopes of islands) had no detectable effect on the density or diversity of seedlings or saplings. [source]

Vegetation dynamics during the Younger Dryas-Holocene transition in the extreme northern taiga zone, northeastern European Russia

BOREAS, Issue 2 2006
MINNA VÄLIRANTA
Vegetation dynamics during the Younger Dryas-Holocene transition in the extreme northern taiga zone of the Usa basin, northeastern European Russia, were reconstructed using plant macrofossil and pollen evidence from a sediment core from Lake Llet-Ti. The pollen stratigraphy during the Younger Dryas (about 12 500,11 500 cal. yr BP) is characterized by pollen types indicative of treeless arctic vegetation, whereas the macrofossil evidence shows the occurrence of scattered spruce and birch trees around the lake. The Younger Dryas-early Holocene transition is characterized by a rapid increase in vegetation density, including an increase in the birch population, followed by the expansion of the spruce population at about 10 000 cal. yr BP. Dense spruce-birch forest dominated until 5000 cal. yr BP. Our results contribute to the debate about the Lateglacial environments in northern Russia, and illustrate the importance of plant macrofossil records in Lateglacial vegetation reconstructions. [source]

Vegetation and topographic controls on sediment deposition and storage on gully beds in a degraded mountain area

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 6 2009
Armando Molina
Abstract Active gully systems developed on highly weathered or loose parent material are an important source of runoff and sediment production in degraded areas. However, a decrease of land pressure may lead to a return of a partial vegetation cover, whereby gully beds are preferred recolonization spots. Although the current knowledge on the role of vegetation on reducing sediment production on slopes is well developed, few studies exist on the significance of restoring sediment transport pathways on the total sediment budget of degraded mountainous catchments. This study in the Ecuadorian Andes evaluates the potential of vegetation to stabilize active gully systems by trapping and retaining eroded sediment in the gully bed, and analyses the significance of vegetation restoration in the gully bed in reducing sediment export from degraded catchments. Field measurements on 138 gully segments located in 13 ephemeral steep gullies with different ground vegetation cover indicate that gully bed vegetation is the most important factor in promoting short-term (1,15 years) sediment deposition and gully stabilization. In well-vegetated gully systems ( , 30% of ground vegetation cover), 0.035 m3 m,1 of sediment is deposited yearly in the gully bed. Almost 50 per cent of the observed variance in sediment deposition volumes can be explained by the mean ground vegetation cover of the gully bed. The presence of vegetation in gully beds gives rise to the formation of vegetated buffer zones, which enhance short-term sediment trapping even in active gully systems in mountainous environments. Vegetation buffer zones are shown to modify the connectivity of sediment fluxes, as they reduce the transport efficiency of gully systems. First calculations on data on sediment deposition patterns in our study area show that gully bed deposition in response to gully bed revegetation can represent more than 25 per cent of the volume of sediment generated within the catchment. Our findings indicate that relatively small changes in landscape connectivity have the potential to create strong (positive) feedback loops between erosion and vegetation dynamics. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Landscape issues in plant ecology

ECOGRAPHY, Issue 2 2002
Sylvie De Blois
In the last decade, we have seen the emergence and consolidation of a conceptual framework that recognizes the landscape as an ecological unit of interest. Plant ecologists have long emphasized landscape-scale issues, but there has been no recent attempt to define how landscape concepts are now integrated in vegetation studies. To help define common research paradigms in both landscape and plant ecology, we discuss issues related to three main landscape concepts in vegetation researches, reviewing theoretical influences and emphasizing recent developments. We first focus on environmental relationships, documenting how vegetation patterns emerge from the influence of local abiotic conditions. The landscape is the physical environment. Disturbances are then considered, with a particular attention to human-driven processes that often overrule natural dynamics. The landscape is a dynamic space. As environmental and historical processes generate heterogeneous patterns, we finally move on to stress current evidence relating spatial structure and vegetation dynamics. This relates to the concept of a landscape as a patch-corridor-matrix mosaic. Future challenges involve: 1) the capacity to evaluate the relative importance of multiple controlling processes at broad spatial scale; 2) better assessment of the real importance of the spatial configuration of landscape elements for plant species and finally; 3) the integration of natural and cultural processes and the recognition of their interdependence in relation to vegetation management issues in human landscapes. [source]

Local hydrologic effects of introducing non-native vegetation in a tropical catchment

ECOHYDROLOGY, Issue 1 2008
Maite Guardiola-Claramonte
Abstract This study investigates the hydrologic implications of land use conversion from native vegetation to rubber (Hevea brasiliensis) in Southeast Asia. The experimental catchment, Nam Ken (69 km2), is located in Xishuangbanna Prefecture (22°N, 101°E), in the south of Yunnan province, in southwestern China. During 2005 and 2006, we collected hourly records of 2 m deep soil moisture profiles in rubber and three native land-covers (tea, secondary forest and grassland), and measured surface radiation above the tea and rubber canopies. Observations show that root water uptake of rubber during the dry season is controlled by day-length, whereas water demand of the native vegetation starts with the arrival of the first monsoon rainfall. The different dynamics of root water uptake in rubber result in distinct depletion of soil moisture in deeper layers. Traditional evapotranspiration and soil moisture models are unable to simulate this specific behaviour. Therefore, a different conceptual model, taking in account vegetation dynamics, is needed to predict hydrologic changes due to land use conversion in the area. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Flooding, soil seed bank dynamics and vegetation resilience of a hydrologically variable desert floodplain

FRESHWATER BIOLOGY, Issue 2 2006
SAMANTHA J. CAPON
Summary 1. This paper explores soil seed bank composition and its contribution to the vegetation dynamics of a hydrologically variable desert floodplain in central Australia: the Cooper Creek floodplain. We investigated patterns in soil seed bank composition both temporally, in response to flooding (and drying), and spatially, with relation to flood frequency. Correlations between extant vegetation and soil seed bank composition are explored with respect to flooding. 2. A large and diverse germinable soil seed bank was detected comprising predominantly annual monocot and annual forb species. Soil seed bank composition did not change significantly in response to a major flood event but some spatial patterns were detected along a broad flood frequency gradient. Soil seed bank samples from frequently flooded sites had higher total germinable seed abundance and a greater abundance of annual monocots than less frequently flooded sites. In contrast, germinable seeds of perennial species belonging to the Poaceae family were most abundant in soil seed bank samples from rarely flooded sites. 3. Similarity between the composition of the soil seed bank and extant vegetation increased following flooding and was greatest in more frequently flooded areas of the floodplain, reflecting the establishment of annual species. The results indicate that persistent soil seed banks enable vegetation in this arid floodplain to respond to unpredictable patterns of flooding and drying. [source]

Effects of storm frequency on dune vegetation

GLOBAL CHANGE BIOLOGY, Issue 10 2010
ELISE S. GORNISH
Abstract In the Gulf of Mexico, barrier islands absorb the majority of wind and wave action from storms, which modifies their dune morphology and vegetation dynamics. Storm frequency is predicted to increase as a result of climate change, yet the effects of this change on coastal ecosystems remain poorly understood. Using estimates of plant growth in storm and nonstorm years from long-term census data describing the dynamics of dune vegetation on St. George Island, FL, we built a first-order model that predicts how dune communities will respond to a change in storm frequency. It predicts that an increasing frequency of storms will result in a change in the vegetation across the dunes. The fore- and interdune communities are predicted to become more similar to one another through the dominance of a small number of common storm-resilient species. Alternatively, the backdune community is predicted to become more distinct through an increase in rare species that represent primary succession. Finally, the model predicts that many species will not respond to an increase in the number of storms per year in the same manner in which they respond to current storm frequency. This model is beneficial both for the development of more complex approaches to predicting effects of climate change and for informing preventative management techniques. [source]

Feedbacks between phosphorus deposition and canopy cover: The emergence of multiple stable states in tropical dry forests

GLOBAL CHANGE BIOLOGY, Issue 1 2008
MARCIA DeLONGE
Abstract Dry forests represent a large percentage of tropical forests and are vulnerable to both anthropogenic and natural disturbances, yet important aspects of their sensitivity to disruption remain poorly understood. It is particularly unclear how changes in land-use or tropical storm patterns may affect the resiliency of phosphorus (P)-limited neotropical forests. In these systems, vegetation is sustained in the long-term by atmospheric P-inputs through rainfall, dust, or fog. Past research supports the idea that dust and fog deposition are dependent on canopy density (e.g. leaf area index). Thus, the canopy may function as a ,trap' for P, enabling a positive feedback between vegetation and P-deposition. We developed a conceptual model to investigate how Neotropical vegetation may respond to reduced P-deposition due to canopy losses. The model suggests that a canopy-deposition feedback may induce bistable vegetation dynamics; under some conditions, forests may be unable to naturally recover from relatively small disturbances. [source]

Simulating vegetation processes along the Kalahari transect

GLOBAL CHANGE BIOLOGY, Issue 3 2004
F. I. Woodward
Abstract The Sheffield Dynamic Global Vegetation Model has simulated the structure and net carbon exchange of vegetation at five sites along the Kalahari transect where there is a strong gradient in precipitation from 299 to 918 mm yr,1. There has been a decline in precipitation of 8 mm yr,1 along the whole of the transect since about 1970. Simulations of vegetation dynamics and structure indicate that this decline has exerted a notable effect on the vegetation, with reductions in woody plant cover at the dry end of the transect and reductions in tree density at the wetter end. These changes were driven primarily by reductions in the net primary production and increased rates of mortality, with rather small impacts of fire. [source]

Historical shrub,grass transitions in the northern Chihuahuan Desert: modeling the effects of shifting rainfall seasonality and event size over a landscape gradient

GLOBAL CHANGE BIOLOGY, Issue 10 2003
Qiong Gao
Abstract We use a spatially explicit landscape model to investigate the potential role of rainfall on shrub,grass transitions in the Jornada Basin of southern New Mexico during the past century. In long-term simulations (1915,1998) along a 2700 m transect running from a dry lake bed to the foothills of a small mountain, we test two hypotheses: (i) that wetter winters and drier summers may have facilitated shrub encroachment in grasslands, and (ii) that increases in large precipitation events may have increased soil water recharge at deeper layers, thus favoring shrub establishment and growth. Our model simulations generally support the hypothesis that wetter winters and drier summers may have played a key role, but we are unable to reproduce the major shifts from grass- to shrub-domination that occurred in this landscape during the early part of the 1900s; furthermore, the positive shrub response to wetter winters and drier summers was only realized subsequent to the drought of 1951,1956, which was a relatively short ,window of opportunity' for increased shrub establishment and growth. Our simulations also generally support the hypothesis that an increase in the number of large precipitation events may also have favored shrub establishment and growth, although these results are equivocal, depending upon what constitutes a ,large' event and the timing of such events. We found complex interactions among (i) the amount/seasonality of rainfall, (ii) its redistribution in the landscape via run-on and runoff, (iii) the depth of the soil water recharge, and (iv) subsequent water availability for the growth and reproduction of shrubs vs. herbaceous plants at various landscape positions. Our results suggest that only a mechanistic understanding of these interactions, plus the role of domestic cattle grazing, will enable us to elucidate fully the relative importance of biotic vs. abiotic factors in vegetation dynamics in this semiarid landscape. [source]

Impact of CO2 concentration changes on the biosphere-atmosphere system of West Africa

Simulating climate change impacts on fire frequency and vegetation dynamics in a Mediterranean-type ecosystem

GLOBAL CHANGE BIOLOGY, Issue 5 2002
Florent Mouillot
Abstract The impacts of climate change on Mediterranean-type ecosystems may result from complex interactions between direct effects on water stress and subsequent modifications in flammability and fire regime leading to changes in standing biomass and plant species composition. We analysed these interrelations through a simulation approach combining scenarios of climate change developed from GCM results and a multispecies functional model for vegetation dynamics, SIERRA. A fire risk procedure based on weekly estimates of vegetation water stress has been implemented. Using climate data from 1960 to 1997, simulations of a typical maquis woodland community have been performed as baseline and compared with two climate scenarios: a change in the rainfall regime alone, and changes in both rainfall and air temperature. Climate changes are defined by an increase in temperature, particularly in summer, and a change in the rainfall pattern leading to a decrease in low rainfall events, and an increase in intense rainfall events. The results illustrate the lack of drastic changes in the succession process, but highlight modifications in the water budget and in the length of the drought periods. Water stress lower than expected regarding statistics on the current climate is simulated, emphasizing a long-term new equilibrium of vegetation to summer drought but with a higher sensibility to rare events. Regarding fire frequency, climate changes tend to decrease the time interval between two successive fires from 20 to 16 years for the maquis shrubland and from 72 to 62 years in the forested stages. This increase in fire frequency leads to shrub-dominated landscapes, which accentuates the yield of water by additional deep drainage and runoff. [source]

An introduction to the European Terrestrial Ecosystem Modelling Activity

GLOBAL ECOLOGY, Issue 6 2001
Martin T. Sykes
Abstract The objective of the European Terrestrial Ecosystem Modelling Activity (ETEMA) was to address some of the major challenges in developing generalized models to examine responses of natural and seminatural ecosystems to environmental change at the regional to European scale. The approach described herein was to break down the totality of ecosystem functioning into its key components, each with its characteristic spatial and temporal scales. A conceptual framework was developed describing the configuration of these components as modules within a generalized simulation model. The framework describes the key inputs, outputs and state variables, their spatial and temporal contexts, and information flows between modules. The ,backbone' of the model is a system of nested timing loops corresponding to the disparate time scales at which different ecosystem processes occur. The framework is a theoretical construct into which ecosystem models at levels of complexity ranging from the very general to the highly detailed can be mapped, and thus provides a guide for development of models for novel, particularly regional-scale, applications. A number of subsystem studies of the major components of ecosystem functioning, i.e. modules of the conceptual framework, are briefly introduced herein. The general aim of the subsystem studies was to identify the key alternative formulations (as opposed to minor variants) and test these against observational data. The various subsystem studies concern planetary boundary layer,ecosystem interactions, ecosystem CO2 and H2O fluxes, vegetation physiology and phenology, biogeography and vegetation dynamics, detritus and SOM dynamics, soil moisture and human and natural disturbances and, as individual papers, they complete this special ETEMA issue. [source]

Medium-term vegetation dynamics and their association with edaphic conditions in two Hungarian saline grassland communities

GRASSLAND SCIENCE, Issue 1 2010
Tibor Tóth
Abstract Medium-term (5.5 years) changes in the cover of major species in "Artemisia saline puszta" (Ass) and "Pannonic Puccinellia limosa hollow" (PPlh) grassland communities in the Kiskunság region, Hungary, were monitored and analyzed in relation to abiotic factors (e.g. air temperature, precipitation, soil moisture, salinity and alkalinity). Soil salinity varied considerably, indicating leaching and desalinization of surface layers as the most typical process occurring in the region. Yearly average covers of Artemisia santonicum and Plantago maritima were negatively and positively related to surface soil salinity, respectively, in accordance with their salt tolerance. Multiple regression analysis showed soil pH and salinity to be the most important factors determining yearly average cover of plants at Ass. Increasing pH increased the cover of A. santonicum and P. maritima, but decreased the cover of Podospermum canum. Increasing salinity decreased the cover of A. santonicum and P. canum. At PPlh, pH of groundwater had a positive effect and the lakewater level had a negative effect on the cover of Puccinellia limosa. The results provide information on the ongoing changes in the soil properties and the resulting changes in plant composition in these Hungarian salt-affected grasslands. [source]

Validation of ERS scatterometer-derived soil moisture data in the central part of the Duero Basin, Spain

HYDROLOGICAL PROCESSES, Issue 8 2005
Antonio Ceballos
Abstract The objective of this study was to validate the soil moisture data derived from coarse-resolution active microwave data (50 km) from the ERS scatterometer. The retrieval technique is based on a change detection method coupled with a data-based modelling approach to account for seasonal vegetation dynamics. The technique is able to derive information about the soil moisture content corresponding to the degree of saturation of the topmost soil layer (,5 cm). To estimate profile soil moisture contents down to 100 cm depth from the scatterometer data, a simple two-layer water balance model is used, which generates a red noise-like soil moisture spectrum. The retrieval technique had been successfully applied in the Ukraine in a previous study. In this paper, the performance of the model in a semi-arid Mediterranean environment characterized by low annual precipitation (400 mm), hot dry summers and sandy soils is investigated. To this end, field measurements from the REMEDHUS soil moisture station network in the semi-arid parts of the Duero Basin (Spain) were used. The results reveal a significant coefficient of determination (R2 = 0·75) for the averaged 0,100 cm soil moisture profile and a root mean square error (RMSE) of 2·2 vol%. The spatial arrangement of the REMEDHUS soil moisture stations also allowed us to study the influence of the small-scale variability of soil moisture within the ERS scatterometer footprint. The results show that the small-scale variability in the study area is modest and can be explained in terms of texture fraction distribution in the soil profiles. Copyright © 2004 John Wiley & Sons, Ltd. [source]

The North Atlantic Oscillation and European vegetation dynamics

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 14 2008
Célia Gouveia
Abstract The relationship between vegetation greenness and the North Atlantic Oscillation (NAO) is assessed over Europe. The study covers the 21-year period from 1982 to 2002 and is based on monthly composites of the Normalised Difference Vegetation Index (NDVI) and Brightness Temperature from the Global Inventory Monitoring and Modelling System (GIMMS) as well as on monthly precipitation from the Global Precipitation Climatology Centre (GPCC). A systematic analysis is first performed of point correlation fields over the 21-year period between the winter NAO index and spring and summer NDVI, followed by an assessment of the vegetation response to precipitation and temperature conditions in winter, over two contrasting regions, namely the Iberian Peninsula and Northeastern Europe. Finally, the impact of NAO on vegetation dynamics over the two regions is evaluated by studying the corresponding annual cycles of NDVI and comparing their behaviour for years associated with opposite NAO phases. Over the Iberian Peninsula there is strong evidence that positive (negative) values of winter NAO induce low (high) vegetation activity in the following spring and summer seasons. This feature is mainly associated with the impact of NAO on winter precipitation, together with the strong dependence of spring and summer NDVI on water availability during the previous winter. Northeastern Europe shows a different behaviour, with positive (negative) values of winter NAO inducing high (low) values of NDVI in spring, but low (high) values of NDVI in summer. This behaviour mainly results from the strong impact of NAO on winter temperature, associated with the critical dependence of vegetation growth on the combined effect of warm conditions and water availability during the winter season. Copyright © 2008 Royal Meteorological Society [source]

Vegetation dynamics on rangelands: a critique of the current paradigms

Interactive controls of herbivory and fluvial dynamics on landscape vegetation patterns on the Tanana River floodplain, interior Alaska

JOURNAL OF BIOGEOGRAPHY, Issue 9 2007
Lem G. Butler
Abstract Aim, We examined the interactive effects of mammalian herbivory and fluvial dynamics on vegetation dynamics and composition along the Tanana River in interior Alaska. Location, Model parameters were obtained from field studies along the Tanana River, Alaska between Fairbanks (64°50.50, N, 147°43.30, W) and Manley Hot Springs (65°0.0, N, 150°36.0, W). Methods, We used a spatially explicit model of landscape dynamics (ALFRESCO) to simulate vegetation changes on a 1-year time-step. The model was run for 250 years and was replicated 100 times. Results, Increases in herbivory decreased the proportion of early successional vegetation and increased the proportion of late successional vegetation on the simulated landscape. Erosion and accretion worked as antagonists to herbivory, increasing the amount of early successional vegetation and decreasing the amount of late successional vegetation. However, the interactive effects of herbivory and erosion/accretion were especially important in determining system response, particularly in early seral vegetation types. High erosion rates, when coupled with low herbivory, greatly increased the proportion of willow on the landscape. When coupled with high herbivory, however, they greatly increased the proportion of alder on the landscape. At low levels of herbivory, alder abundance peaked at intermediate levels of erosion/accretion. Main conclusions, Neither erosion/accretion nor herbivory produced consistent landscape patterns that could be predicted independently of the other. These findings underscore the importance of the interactive effects of biotic and abiotic disturbances in shaping large-scale landscape vegetation patterns in boreal floodplain ecosystems , systems traditionally thought to be driven primarily by abiotic disturbance alone. [source]

Vegetation dynamics in western Uganda during the last 1000 years: climate change or human induced environmental degradation?

Exploring climatic and biotic controls on Holocene vegetation change in Fennoscandia

JOURNAL OF ECOLOGY, Issue 2 2008
Paul A. Miller
Summary 1We investigated the potential drivers of Holocene vegetation changes recorded at four Scandinavian pollen sites, two in Sweden and two in Finland, at a time when they were largely free of anthropogenic influence. 2We used the generalized dynamic vegetation model LPJ-GUESS forced with climate anomaly output from an atmospheric general circulation model to simulate tree species dynamics from 10 000 years ago to the present. The model results were compared to high-resolution pollen accumulation rates gathered at the sites. 3Our results indicate that both the observed northern distributional limits of temperate trees, and the limits of Pinus sylvestris and Alnus incana at the tree line, are a result of millennial variations in summer and winter temperatures. The simulation of several distinct trends in species occurrence observed in the pollen record indicates that a time lag due to the slow spreading of species need not be invoked for most species. 4Sensitivity studies indicate that competition, natural disturbance and the magnitude of interannual variability play key roles in determining the biomass, establishment and even the presence of species near their bioclimatic limits. However, neither disturbance due to fire nor limits on establishment due to drought were likely to have been major determinants of the observed trends on the timescales considered. 5We were unable to limit the modelled occurrence of Picea abies at the study sites to the periods at which it was observed in the pollen records, indicating that we have still not completely understood the driving or limiting factors for Holocene changes in Picea abies abundance. 6Synthesis. This study shows that by combining quantitative vegetation reconstructions with a modern, process-based dynamic vegetation model, we may gain new insights into the potential biotic and abiotic drivers of Holocene vegetation dynamics, and their relative importance. This knowledge will be crucial in enabling us to assess more confidently the response of northern European vegetation to future climate change. [source]

Plant regeneration directs changes in grassland composition after extreme drought: a 13-year study in southern Switzerland

JOURNAL OF ECOLOGY, Issue 4 2004
A. STAMPFLI
Summary 1The cover of plant species was recorded annually from 1988 to 2000 in nine spatially replicated plots in a species-rich, semi-natural meadow at Negrentino (southern Alps). This period showed large climatic variation and included the centennial maximum and minimum frequency of days with , 10 mm of rain. 2Changes in species composition were compared between three 4-year intervals characterized by increasingly dry weather (1988,91), a preceding extreme drought (1992,95), and increasingly wet weather (1997,2000). Redundancy analysis and anova with repeated spatial replicates were used to find trends in vegetation data across time. 3Recruitment capacity, the potential for fast clonal growth and seasonal expansion rate were determined for abundant taxa and tested in general linear models (GLM) as predictors for rates of change in relative cover of species across the climatically defined 4-year intervals. 4Relative cover of the major growth forms present, graminoids and forbs, changed more in the period following extreme drought than at other times. Recruitment capacity was the only predictor of species' rates of change. 5Following perturbation, re-colonization was the primary driver of vegetation dynamics. The dominant grasses, which lacked high recruitment from seed, therefore decreased in relative abundance. This effect persisted until the end of the study and may represent a lasting response to an extreme climatic event. [source]

Late-glacial and Holocene climatic effects on fire and vegetation dynamics at the prairie,forest ecotone in south-central Minnesota

JOURNAL OF ECOLOGY, Issue 5 2003
Philip Camill
Summary 1Treeline ecotones, such as the prairie,forest boundary, represent climatically sensitive regions where the relative abundance of vegetation types is controlled by complex interactions between climate and local factors. Responses of vegetation and fire to climate change may be tightly linked as a result of strong feedbacks among fuel production, vegetation structure and fire frequency/severity, but the importance of these feedbacks for controlling the stability of this ecotone is unclear. 2In this study, we examined the prairie,forest ecotone in south-central Minnesota using two lake sediment cores to reconstruct independent records of climate, vegetation and fire over the past 12 500 years. Using pollen, charcoal, sediment magnetic analyses and LOI properties, we investigated whether fires were controlled directly by climate or indirectly by fuel production. 3Sediment magnetic and LOI data suggest four broad climatic periods occurring c. 11 350,8250 BP (cool/humid), c. 8250,4250 BP (warm/dry), c. 4250,2450 BP (warm/humid), and c. 2450,0 BP (cool/humid), indicating that, since the mid-Holocene, climate has shifted towards wetter conditions favouring greater in-lake production and fuel production on the landscape. 4The area surrounding both lakes was characterized by boreal forest c. 12 500,10 000 BP, changing to an Ulmus-Ostrya forest c. 10 000,9000 BP, changing to a community dominated by prairie (Poaceae-Ambrosia-Artemisia) and deciduous forest taxa c. 8000,4250 BP, and finally shifting to a Quercus -dominated woodland/savanna beginning c. 4250,3000 BP. 5Charcoal influx increased from an average of 0.11,0.62 mm2 cm,2 year,1 during the early Holocene forest period (c. 11 350,8250 BP) to 1.71,3.36 mm2 cm,2 year,1 during the period of prairie expansion (c. 8250,4250 BP) and again increased to 4.18,4.90 mm2 cm,2 year,1 at the start of the woodland/savanna period (c. 4250 BP). 6As a result of the influence of climate on community composition and fuel productivity, changes in fire severity may be the result and not the cause of shifts in vegetation. [source]

Persistence of coastal spruce refugia during the Holocene in northern New England, USA, detected by stand-scale pollen stratigraphies

JOURNAL OF ECOLOGY, Issue 2 2002
Molly Schauffler
Summary 1 Pollen data from wet, forested hollows in five spruce (Picea) stands on the eastern coast of Maine, USA, reveal that spruce has been well-established (spruce pollen > 6%) for at least 5000 years at four of the sites (Isle au Haut, Schoodic Peninsula, and Roque Island). Spruce became dominant in the fifth stand (Blackwoods, Mount Desert Island) only in the last 2000 years. This is in contrast to pollen stratigraphies from two inland forest hollows and from inland lakes that indicate a significant region-wide increase in the abundance of spruce only 1000 years ago. 2 All five coastal pollen stratigraphies suggest that conditions along the east coast of Maine became cooler and moister sometime between 6000 and 5000 years ago. Mid-Holocene changes in vegetation and sediment accumulation correspond with the timing of rapid increases in tidal amplitude and diurnal mixing of cold water in the Gulf of Maine, suggestive that these resulted in increased marine effects on the local climate at a time that was generally warmer than present. 3 Two inland forest-hollow stratigraphies do not show evidence of mid-Holocene cooling. Coastal effects therefore persisted for several thousand years despite regional climate changes. 4 The pollen data suggest that refugia along the coast (and probably in isolated sites inland), may have played a critical role in allowing the rapid regional expansion of spruce around 1000 years ago. The steep increases in the abundance of spruce pollen in all forest-hollow and lake pollen stratigraphies in northern New England at that time corroborate other evidence of a region-wide shift to cooler and moister conditions. 5 Pollen stratigraphies from small forested hollows provide a means to examine local vegetation dynamics and interpret those dynamics in the context of regional signals. [source]

Pollen-based biome reconstructions for Colombia at 3000, 6000, 9000, 12 000, 15 000 and 18 000 14C yr ago: Late Quaternary tropical vegetation dynamics

JOURNAL OF QUATERNARY SCIENCE, Issue 2 2002
Robert Marchant
Abstract Colombian biomes are reconstructed at 45 sites from the modern period extending to the Last Glacial Maximum (LGM). The basis for our reconstruction is pollen data assigned to plant functional types and biomes at six 3000-yr intervals. A reconstruction of modern biomes is used to check the treatment of the modern pollen data set against a map of potential vegetation. This allows the biomes reconstructed at past periods to be assessed relative to the modern situation. This process also provides a check on the a priori assignment of pollen taxa to plant functional types and biomes. For the majority of the sites, the pollen data accurately reflect the potential vegetation, even though much of the original vegetation has been transformed by agricultural practices. At 18 000 14C yr BP, a generally cool and dry environment is reflected in biome, assignments of cold mixed forests, cool evergreen forests and cool grassland,shrub; the latter extending to lower altitudes than presently recorded. This signal is strongly recorded at 15 000 and 12 000 14C yr BP, the vegetation at these times also reflecting a relatively cool and dry environment. At 9000 14C yr BP there is a shift to biomes thought to result from slightly cooler environmental conditions. This trend is reversed by 6000 14C yr BP; most sites, within a range of different environmental settings, recording a shift to more xeric biome types. There is an expansion of steppe and cool mixed-forest biomes, replacing tropical dry forest and cool grassland,shrub biomes, respectively. These changes in biome assignments from the modern situation can be interpreted as a biotic response to mid-Holocene climatic aridity. At 3000 14C yr BP the shift is mainly to biomes characteristic of slightly more mesic environmental conditions. There are a number of sites that do not change biome assignment relative to the modern reconstruction, although the affinities that these sites have to a specific biome do change. These ,anomalies' are interpreted on a site-by-site basis. Spatially constant, but differential response of the vegetation to climatic shifts are related to changes in moisture sources and the importance of edaphic controls on the vegetation. The Late Quaternary reconstruction of large-scale vegetation dynamics in Colombia allows an understanding of the environmental controls on these to be developed. In particular, shifts in the character of the main climatic systems that influence Colombian vegetation are described. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Temporal dynamics of marginal steppic vegetation over a 26-year period of substantial environmental change

JOURNAL OF VEGETATION SCIENCE, Issue 2 2009
Silvia Matesanz
Abstract Questions: (1) Is climate a strong driver of vegetation dynamics, including interannual variation, in a range margin steppic community? (2) Are there long-term trends in cover and species richness in this community, and are these consistent across species groups and species within groups? (3) Can long-term trends in plant community data be related to variation in local climate over the last three decades? Location: A range margin steppic grassland community in central Germany. Methods: Cover, number and size of all individuals of all plant species present in three permanent 1-m2 plots were recorded in spring for 26 years (1980,2005). Climatic data for the study area were used to determine the best climatic predictor for each plant community, functional group and species variable (annual data and interannual variation) using best subsets regression. Results: April and autumn temperature showed the highest correlation with total cover and species richness and with interannual variations of cover and richness. However, key climate drivers differed between the five most abundant species. Similarly, total cover and number and cover of perennials significantly decreased over time, while no trend was found for the cover and number of annuals. However, within functional groups there were also contrasting species-specific responses. Long-term temperature increases and high interannual variability in both temperature and precipitation were strongly related to long-term trends and interannual variations in plant community data. Conclusions: Temporal trends in vegetation were strongly associated with temporal trends in climate at the study site, with key roles for autumn and spring temperature and precipitation. Dynamics of functional groups and species within groups and their relationships to changes in temperature and precipitation reveal complex long-term and interannual patterns that cannot be inferred from short-term studies with only one or a few individual species. Our results also highlight that responses detected at the functional group level may mask contrasting responses within functional groups. We discuss the implications of these findings for attempts to predict the future response of biodiversity to climate change. [source]

Tolerance of Pinus taeda and Pinus serotina to low salinity and flooding: Implications for equilibrium vegetation dynamics

JOURNAL OF VEGETATION SCIENCE, Issue 1 2008
Benjamin Poulter
Abstract Questions: 1. Do pine seedlings in estuarine environments display discrete or continuous ranges of physiological tolerance to flooding and salinity? 2. What is the tolerance of Pinus taeda and P. serotina to low salinity and varying hydrologic conditions? 3. Are the assumptions for ecological equilibrium met for modeling plant community migration in response to sea-level rise? Location: Albemarle Peninsula, North Carolina, USA. Methods: In situ observations were made to quantify natural pine regeneration and grass cover along a salinity stress gradient (from marsh, dying or dead forest, to healthy forest). A full-factorial greenhouse experiment was set up to investigate mortality and carbon allocation of Pinus taeda and P. serotina to low-salinity conditions and two hydrology treatments over 6 months. Treatments consisted of freshwater and two salinity levels (4 ppt and 8 ppt) under either permanently flooded or periodically flushed hydrologic conditions. Results: Natural pine regeneration was common (5,12 seedlings per m2) in moderate to well-drained soils where salinity concentrations were below ca. 3.5 ppt. Pine regeneration was generally absent in flooded soils, and cumulative mortality was 100% for 4 and 8 ppt salinity levels under flooded conditions in the greenhouse study. Under weekly flushing conditions, mortality was not significantly different between 0 and 4 ppt, confirming field observations. Biomass accumulation was higher for P. taeda, but for both pine species, the root to shoot ratio was suppressed under the 8 ppt drained treatment, reflecting increased below-ground stress. Conclusions: While Pinus taeda and P. serotina are commonly found in estuarine ecosystems, these species display a range of physiological tolerance to low-salinity conditions. Our results suggest that the rate of forest migration may lag relative to gradual sea-level rise and concomitant alterations in hydrology and salinity. Current bioclimate or landscape simulation models assume discrete thresholds in the range of plant tolerance to stress, especially in coastal environments, and consequently, they may overestimate the rate, extent, and timing of plant community response to sea-level rise. [source]

Quantifying patterns and controls of mire vegetation succession in a southern boreal bog in Finland using partial ordinations

JOURNAL OF VEGETATION SCIENCE, Issue 6 2007
E.-S. Tuittila
Abstract Question: How do we distinguish between concurrent allogenic and autogenic forcings behind changing patterns in plant community structures during mire development? Location: Lakkasuo raised bog, southern Finland. Methods: Two radiometrically dated peat profiles were studied using high resolution plant macrofossil analysis. A combination of partial direct and indirect gradient analyses (CCA and DCA) was applied to quantify the role of different drivers of vegetation changes. Results: Autogenic hydroseral succession explained 16% of the compositional variation in the vegetation. Disturbance successions initiated by fire explained 15% of the variation in the hummock, but only 9% in the wetter lawn. The early post-disturbance successional stages were characterized by Eriophorum vaginatum. After partialling out the effects of peat depth and time since fire, a moisture gradient explained 29% of variation in the hummock core and 26% in the lawn. The analyses also indicated alternation between species with a similar niche. This interaction gradient explained 26% and 31% of the compositional variation in the hummock and lawn, respectively. The similar order of species replacement from both cores supported the existence of general directional succession in mire vegetation, both during the mire development and after fire events. The autogenic succession was slow and gradual while the disturbance successions were episodic and fast. Conclusion: Our results support the paradigm of the complex nature of mire vegetation dynamics where several interlinked agents have simultaneous effects. The approach of combining partial ordinations developed here appeared to be a useful tool to assess the role of different environmental factors in controlling the vegetation succession. [source]