Using a biogeochemistry model in simulating forests productivity responses to climatic change and [CO2] increase: example of Pinus halepensis in Provence (south-east France)

Tree-ring chronologies provide long-term records of growth in natural environmental conditions and may be used to evaluate the impacts of climatic change and [CO₂] increase on forest productivity. This study focuses on 21 Pinus halepensis forest stands in calcareous Provence (in the south-east France). Changes in productivity are simulated using the global biogeochemistry model BIOME3, that we have adapted to run with chronological data. Tree-ring data (width and density) were used to estimate, for each stand, an observed series of changes in productivity. Simulated and observed productivity changes are then compared to validate the chronological biogeochemistry model BIOME3C. Variations in productivity were well reconstructed at 15 sites. After this validation, BIOME3C was used to simulate forest productivity changes for a 2×CO₂ scenario. The 2×CO₂ climate used as input was obtained using results from Météo-France’s ARPEGE atmospheric general circulation model (AGCM), downscaled to local meteorological stations. Productivity increases moderately for all stands (from 17 to 24%) when climatic changes alone were taken into account. The main factor responsible for this increase is a reduction in summer drought severity. Productivity increases highly for all stands (from 72 to 86%) when the physiological fertilising effect of the [CO₂] increase is considered separately. When both climatic changes and the [CO₂] increase were taken into account, productivity increases highly, from 107% (for Moustier) to 141% (for La Ciotat). The direct fertilising effect of [CO₂] increase has a greater influence on the forest stands productivity than the indirect climatic changes effect. These results also exhibit the importance of the synergy between the effects of climate change and [CO₂] increase, as the increase in productivity resulting from the combined effects are more than the sum of the individual CO₂ and climate effects. Although the detected effects of global change during the 20th century were slight, acceleration of these changes is likely to lead to great changes in the future productivity of P. halepensis forests.     

Revisiting Kappa to account for change in the accuracy assessment of land-use change models

Land-use change models are typically calibrated to reproduce known historic changes. Calibration results can then be assessed by comparing two datasets: the simulated land-use map and the actual land-use map at the same time. A common method for this is the Kappa statistic, which expresses the agreement between two categorical datasets corrected for the expected agreement. This expected agreement is based on a stochastic model of random allocation given the distribution of class sizes. However, when a model starts from an initial land-use map and makes changes to it, that stochastic model does not pose a meaningful reference level. This paper introduces K_Simulation, a statistic that is identical in form to the Kappa statistic but instead applies a more appropriate stochastic model of random allocation of class transitions relative to the initial map. The new method is illustrated on a simple example and then the results of the Kappa statistic and K_Simulation are compared using the results of a land-use model. It is found that only K_Simulation truly tests models in their capacity to explain land-use changes over time, and unlike Kappa it does not inflate results for simulations where little change takes place over time.     

Simulation of wetlands forest vegetation dynamics

A computer program, SWAMP, was designed to simulate the effects of flood frequency and depth to water table on southern wetlands forest vegetation dynamics. By incorporating these hydrologic characteristics into the model, forest vegetation and vegetation dynamics can be simulated. The model, based on data from the White River National Wildlife Refuge near De Witt, Arkansas, “grows” individual trees on a 20 x 20-m plot taking into account effects on the tree growth of flooding, depth to water table, shade tolerance, overtopping and crowding, and probability of death and reproduction. A potential application of the model is illustrated with simulations of tree fruit production following flood-control implementation and lumbering.     

An inventory of tree species in Europe—An essential data input for air pollution modelling

We present a detailed tree species inventory covering Europe, parts of Africa and parts of Asia. The inventory contains 39 groups of species that are important for biogenic VOCs or pollen emission calculations. For example: oak (Quercus), poplar (Populus), pines (Pinus), spruce (Picea), birch (Betula) and alder (Alnus). The inventory is based on national forest inventories and national statistics and gives tree species distribution in percentage within broadleaved as well as conifer forests. The inventory includes data from 799 regions and is redistributed into the 50 km × 50 km EMEP grid. The inventory is therefore prepared for easy implementation into atmospheric transport models by providing an extension to already applied land use data such as the Corine Land Cover (CLC2000) or Global Land Cover (GLC2000). The gridded version of the data set will be available on the webpage http://www.dmu.dk/International/Air/Models/Background/Trees/.     

Multiple sources of predictive uncertainty in modeled estimates of net ecosystem CO2 exchange

Net ecosystem CO₂ exchange (NEE) is typically measured directly by eddy covariance towers or is estimated by ecosystem process models, yet comparisons between the data obtained by these two methods can show poor correspondence. There are three potential explanations for this discrepancy. First, estimates of NEE as measured by the eddy-covariance technique are laden with uncertainty and can potentially provide a poor baseline for models to be tested against. Second, there could be fundamental problems in model structure that prevent an accurate simulation of NEE. Third, ecosystem process models are dependent on ecophysiological parameter sets derived from field measurements in which a single parameter for a given species can vary considerably. The latter problem suggests that with such broad variation among multiple inputs, any ecosystem modeling scheme must account for the possibility that many combinations of apparently feasible parameter values might not allow the model to emulate the observed NEE dynamics of a terrestrial ecosystem, as well as the possibility that there may be many parameter sets within a particular model structure that can successfully reproduce the observed data. We examined the extent to which these three issues influence estimates of NEE in a widely used ecosystem process model, Biome-BGC, by adapting the generalized likelihood uncertainty estimation (GLUE) methodology. This procedure involved 400,000 model runs, each with randomly generated parameter values from a uniform distribution based on published parameter ranges, resulting in estimates of NEE that were compared to daily NEE data from young and mature Ponderosa pine stands at Metolius, Oregon. Of the 400,000 simulations run with different parameter sets for each age class (800,000 total), over 99% of the simulations underestimated the magnitude of net ecosystem CO₂ exchange, with only 4.07% and 0.045% of all simulations providing satisfactory simulations of the field data for the young and mature stands, even when uncertainties in eddy-covariance measurements are accounted for. Results indicate fundamental shortcomings in the ability of this model to produce realistic carbon flux data over the course of forest development, and we suspect that much of the mismatch derives from an inability to realistically model ecosystem respiration. However, difficulties in estimating historic climate data are also a cause for model-data mismatch, particularly in a highly ecotonal region such as central Oregon. This latter difficulty may be less prevalent in other ecosystems, but it nonetheless highlights a challenge in trying to develop a dynamic representation of the terrestrial biosphere.     

PIXGRO: A model for simulating the ecosystem CO2 exchange and growth of spring barley

A model, PIXGRO, developed by coupling a canopy flux sub-model (PROXEL_NEE; PROcess-based piXEL Net Ecosystem CO₂ Exchange) to a vegetation structure submodel (CGRO), for simulating both net ecosystem CO₂ exchange (NEE) and growth of spring barley is described. PIXGRO is an extension of the stand-level CO₂ and H₂O-flux model PROXEL_NEE, that simulates the NEE on a process basis, but goes further to include the dry matter production, partitioning, and crop development for spring barley. Dry matter partitioned to the leaf was converted to leaf area index (LAI) using relationships for the specific leaf area (SLA). The canopy flux component, PROXEL_NEE was calibrated using information from the literature on C3 plants and was tested using CO₂ flux data from an eddy-covariance (EC) method in Finland with long-term observations. The growth component (CGRO) was calibrated using data from the literature on spring barley as well as data from the Finland site. It was then validated against field data from two sites in Germany and partly via the use of MODIS remotely sensed LAI from the Finland site.Both the diurnal and the seasonal patterns of gross CO₂ uptake were very well simulated (R² = 0.92). A slight seasonal bias may be attributed to leaf ageing. Crop growth was also well simulated; simulated dry matter agreed with field observed data from Germany (R² = 0.90). For LAI, the agreement between the simulated and observed was good (R² = 0.80), giving an indication that functions describing the conversion of fixed CO₂ to dry matter and the subsequent partitioning leaf dry matter and LAI simulation were robust and provided reliable estimates.The MODIS LAI at a resolution of 1000 m agreed poorly (R² = 0.45) with the PIXGRO simulated LAI and the observed LAI at the Finland site in 2001. We attributed this to the coarse resolution of the image and/or the small size of the barley field (about 17 ha or 0.25 km²) at the Finland site. By deriving a regression relation between the observed LAI and NDVI from a higher resolution MODIS (500 m resolution), the MODIS-recalculated LAI agreed better with the PIXGRO-simulated LAI (R² = 0.86).PIXGRO provides a prototype model bridging the disciplines of plant physiology, crop modeling and remote sensing, for use in a spatial context in evaluating carbon balances and plant growth at stand level, landscape, regional, and with some care, continental scales. Since almost 50% of the European land surface is covered by crops, such a model is needed for the dynamic estimation of LAI and NEE of croplands.     

Development and optimization of an Agro-BGC ecosystem model for C4 perennial grasses

Extrapolating simulations of bioenergy crop agro-ecosystems beyond data-rich sites requires biophysically accurate ecosystem models and careful estimation of model parameters not available in the literature. To increase biophysical accuracy we added C₄ perennial grass functionality and agricultural practices to the Biome-BGC (BioGeochemical Cycles) ecosystem model. This new model, Agro-BGC, includes enzyme-driven C₄ photosynthesis, individual live and dead leaf, stem, and root carbon and nitrogen pools, separate senescence and litter fall processes, fruit growth, optional annual seeding, flood irrigation, a growing degree day phenology with a killing frost option, and a disturbance handler that simulates nitrogen fertilization, harvest, fire, and incremental irrigation. To obtain spatially generalizable vegetation parameters we used a numerical method to optimize five unavailable parameters for Panicum virgatum (switchgrass) using biomass yield data from three sites: Mead, Nebraska, Rockspring, Pennsylvania, and Mandan, North Dakota. We then verified simulated switchgrass yields at three independent sites in Illinois (IL). Agro-BGC is more accurate than Biome-BGC in representing the physiology and dynamics of C₄ grass and management practices associated with agro-ecosystems. The simulated two-year average mature yields with single-site Rockspring optimization have Root Mean Square Errors (RMSE) of 70, 152, and 162 and biases of 43, −87, 156 g carbon m⁻² for Shabbona, Urbana, and Simpson IL, respectively. The simulated annual yields in June, August, October, December, and February have RMSEs of 114, 390, and 185 and biases of −19, −258, and 147 g carbon m⁻² for Shabbona, Urbana, and Simpson IL, respectively. These RMSE and bias values are all within the largest 90% confidence interval around respective IL site measurements. Twenty-four of twenty-six simulated annual yields with Rockspring optimization are within 95% confidence intervals of Illinois site measurements during the mature fourth and fifth years of growth. Ten of eleven simulated two-year average mature yields with Rockspring optimization are within 65% confidence intervals of Illinois site measurements and the eleventh is within the 95% confidence interval. Rockspring optimized Agro-BGC achieves accuracies comparable to those of two previously published models: Agricultural Land Management Alternatives with Numerical Assessment Criteria (ALMANAC) and Integrated Farm System Model (IFSM). Agro-BGC suffers from static vegetation parameters that can change seasonally and as plants age. Using mature plant data for optimization mitigates this deficiency. Our results suggest that a multi-site optimization scheme using mature plant data from more sites would be adequate for generating spatially generalizable vegetation parameters for simulating mature bioenergy crop agro-ecosystems with Agro-BGC.     

Vegetation competition model for water and light limitation. I: Model description, one-dimensional competition and the influence of groundwater

Vegetation growth models often concentrate on the interaction of vegetation with soil moisture but usually omit the influence of groundwater. However the proximity of groundwater can have a profound effect on vegetation growth, because it strongly influences the spatial and temporal distribution of soil moisture and therefore water and oxygen stress of vegetation. In two papers we describe the behavior of a coupled vegetation-groundwater-soil water model including the competition for water and light. In this first paper we describe the vegetation model, compare the model to measured flux data and show the influence of water and light competition in one dimension. In the second paper we focus on the influence of lateral groundwater flow and spatial patterns along a hillslope. The vegetation model is based on a biophysical representation of the soil-plant-atmosphere continuum. Transpiration and stomatal conductance depend both on atmospheric forcing and soil moisture content. Carbon assimilation depends on environmental conditions, stomatal conductance and biochemical processes. Light competition is driven by tree height and water competition is driven by root water uptake and its water and oxygen stress reaction. The modeled and measured H₂O and CO₂ fluxes compare well to observations on both a diurnal and a yearly timescale. Using an upscaling procedure long simulation runs were performed. These show the importance of light competition in temperate forests: once a tree is established under slightly unfavorable soil moisture conditions it can not be outcompeted by smaller trees with better soil moisture uptake capabilities, both in dry as in wet conditions. Performing the long simulation runs with a background mortality rate reproduces realistic densities of wet and dry adapted tree species along a wet to dry gradient. These simulations show that the influence of groundwater is apparent for a large range of groundwater depths, by both capillary rise and water logging. They also show that species composition and biomass have a larger influence on the water balance in eco-hydrological systems than soil and groundwater alone.     

Knowing fire incidence through fuel phenology: A remotely sensed approach

Fire is a basic ecological factor that contributes to determine vegetation diversity and dynamics in time and space. Fuel characteristics play an essential role in fire ignition and propagation; at the landscape scale fuel availability and flammability are closely related to the vegetation phenology that directly affects wildfire pattern in time and space. In this view, the annual normalized difference vegetation index (NDVI) profiles derived from high temporal resolution satellites, like SPOT Vegetation, represent an effective tool for monitoring the coarse-scale vegetation seasonal timing. The objective of this study thus consists in quantifying the explanatory power of multitemporal NDVI profiles on the fire regime characteristics of the potential natural vegetation (PNV) types of Sardinia (Italy) over a 5-year period (2000-2004). The results obtained show a good association between the NDVI temporal dynamics of the PNV of Sardinia and the corresponding fire regime characteristics, emphasizing the role of the bioclimatic timing of the vegetation in controlling the coarse-scale wildfire spatio-temporal distribution of Sardinia. By providing a sound phytogeographical framework for describing different wildfire regimes, PNV maps can thus be considered helpful cartographic documents for fire management strategies at the landscape scale.     

Modeling the vegetation-atmosphere carbon dioxide and water vapor interactions along a controlled CO2 gradient

Ecosystem functioning is intimately linked to its physical environment by complex two-way interactions. These two-way interactions arise because vegetation both responds to the external environment and actively regulates its micro-environment. By altering stomatal aperture, and therefore the transpiration rate, plants modify soil moisture and atmospheric humidity and these same physical variables, in return, modify stomatal conductance. Relationships between biotic and abiotic components are particularly strong in closed, managed environments such as greenhouses and growth chambers, which are used extensively to investigate ecosystem responses to climatic drivers. Model-assisted designs that account for the physiological dynamics governing two-way interactions between biotic and abiotic components are absent from many ecological studies. Here, a general model of the vegetation-atmosphere system in closed environments is proposed. The model accounts for the linked carbon-water physiology, the turbulent transport processes, and the energy and radiative transfer within the vegetation. Leaf gas exchange is modeled using a carbon gain optimization approach that is coupled to leaf energy balance. The turbulent transport within the canopy is modeled in two-dimensions using first-order closure principles. The model is applied to the Lysimeter CO₂ Gradient (LYCOG) facility, wherein a continuous gradient of atmospheric CO₂ is maintained on grassland assemblages using an elongated chamber where the micro-climate is regulated by variation in air flow rates. The model is employed to investigate how species composition, climatic conditions, and the imposed air flow rate affect the CO₂ concentration gradient within the LYCOG and the canopy micro-climate. The sensitivity of the model to key physiological and climatic parameters allows it to be used not only to manage current experiments, but also to formulate novel ecological hypotheses (e.g., by modeling climatic regimes not currently employed in LYCOG) and suggest alternative experimental designs and operational strategies for such facilities.     

Creation and preservation of vegetation patterns by grazing

Structural patterns of tall stands (“tussock”) and short stands (“lawn”) are observed in grazed vegetation throughout the world. Such structural vegetation diversity influences plant and animal diversity. A possible mechanism for the creation and preservation of such patterns is a positive feedback between grazing and plant palatability. Although some theoretical studies have addressed this point in a non-spatial setting, the spatial consequences of this feedback mechanism on the stability and spatial characteristics of vegetation structure patterns have not been studied.We addressed this issue by analyzing a spatially explicit individual-based plant-grazer simulation model, based on published empirical relations and the assumption of optimal foraging.In the model, the selection by the grazer of short stands (that have a higher energy content and are more palatable) is affected by traveling costs and the spatial organization of swards. Nevertheless, the most selected biomass in this type of short stands was the optimal biomass predicted by cropping and digestion constraints. As a result of the optimal foraging strategy, the grazers displayed Lévy-flight traveling behavior during the simulations with characteristic exponent μ ≈ 2.Patterns of short and tall stands created by grazing were preserved for at least a decade. Even in seasonal habitat, the spatial organization of the patterns remained relatively constant, despite fluctuations in the area of short stands. Heterogeneity of initial vegetation increased heterogeneity of the grazing-induced pattern, but did not affect its stability.The area of short stands that was preserved by grazing scaled with the herbivore mass to the power 0.4 and with the carrying capacity of the vegetation to the power −0.75.Patterns of tall and short stands can be created and perpetuated by optimally grazing ruminants, irrespective of possible underlying soil patterns. The simulations generate predictions for the stability and spatial characteristics of such structural vegetation patterns.     

Evaluation of numerical simulations of CO2 transport in a city block with field measurements

Studying urban air-transport phenomena is highly complex, because of the heterogenous flow patterns that can arise. The main reason for these is the variable topology of urban areas, however, there is a large number of influencing variables such as meteorological conditions (e.g., wind situation, temperature) and anthropogenic factors such as traffic emissions. During a one-year CO₂ measurement campaign in the city of Basel, Switzerland, steep CO₂ gradients were measured around a large building. The concentration differences showed a strong dependency on the local flow regimes. Analysis of the field data alone did not provide a complete explanation for the mechanisms underlying the observed phenomena. The key numerical parameters were defined and the influence of turbulent kinetic energy dependency on the time interval for the Reynolds decomposition was studied. A Reynolds-Average Navier-Stokes Computational Fluid Dynamics (CFD) approach was applied in the study area and the CO₂ concentrations were simulated for six significant meteorological situations and compared to the measured data. Two flow regimes dependent on the wind situation, which either enhanced or suppressed the concentration of CO₂ in the street canyon, were identified. The enhancement of CO₂ in the street canyon led to a large difference in CO₂ concentration between the backyard- and street-sides of a building forming the one wall of the canyon. The specific characteristics of the flow patterns led to the identification of the processes determining the observed differences in CO₂ concentrations. The combined analysis of measurement and modeling showed the importance of reliable field measurements and CFD simulations with a high spatial resolution to assess transport mechanisms in urban areas.     

Evaluating the effects of future climate change and elevated CO2 on the water use efficiency in terrestrial ecosystems of China

Water use efficiency (WUE) is an important variable used in climate change and hydrological studies in relation to how it links ecosystem carbon cycles and hydrological cycles together. However, obtaining reliable WUE results based on site-level flux data remains a great challenge when scaling up to larger regional zones. Biophysical, process-based ecosystem models are powerful tools to study WUE at large spatial and temporal scales. The Integrated BIosphere Simulator (IBIS) was used to evaluate the effects of climate change and elevated CO₂ concentrations on ecosystem-level WUE (defined as the ratio of gross primary production (GPP) to evapotranspiration (ET)) in relation to terrestrial ecosystems in China for 2009-2099. Climate scenario data (IPCC SRES A2 and SRES B1) generated from the Third Generation Coupled Global Climate Model (CGCM3) was used in the simulations. Seven simulations were implemented according to the assemblage of different elevated CO₂ concentrations scenarios and different climate change scenarios. Analysis suggests that (1) further elevated CO₂ concentrations will significantly enhance the WUE over China by the end of the twenty-first century, especially in forest areas; (2) effects of climate change on WUE will vary for different geographical regions in China with negative effects occurring primarily in southern regions and positive effects occurring primarily in high latitude and altitude regions (Tibetan Plateau); (3) WUE will maintain the current levels for 2009-2099 under the constant climate scenario (i.e. using mean climate condition of 1951-2006 and CO₂ concentrations of the 2008 level); and (4) WUE will decrease with the increase of water resource restriction (expressed as evaporation ratio) among different ecosystems.     

Characteristics of flow structure of free-surface flow in a partly obstructed open channel with vegetation patch

Free-surface flows over patchy vegetation are common in aquatic environments. In this study, the hydrodynamics of free-surface flow in a rectangular channel with a bed of rigid vegetation-like cylinders occupying half of the channel bed was investigated and interpreted by means of laboratory experiments and numerical simulations. The channel configurations have low width-to-depth aspect ratio (1.235 and 2.153). Experimental results show that the adjustment length for the flow to be fully developed through the vegetation patch in the present study is shorter than observed for large-aspect-ratio channels in other studies. Outside the lateral edge of the vegetation patch, negative velocity gradient (\(\partial \overline{u}/\partial z < 0\)) and a local velocity maximum are observed in the vertical profile of the longitudinal velocity in the near-bed region, corresponding to the negative Reynolds stress (\(- \overline{{u^{\prime}w^{\prime}}} < 0\)) at the same location. Assuming coherent vortices to be the dominant factor influencing the mean flow field, an improved Spalart–Allmaras turbulence model is developed. The model improvement is based on an enhanced turbulence length scale accounting for coherent vortices due to the effect of the porous vegetation canopy and channel bed. This particular flow characteristic is more profound in the case of high vegetation density due to the stronger momentum exchange of horizontal coherent vortices. Numerical simulations confirmed the local maximum velocity and negative gradient in the velocity profile due to the presence of vegetation and bed friction. This in turn supports the physical interpretation of the flow processes in the partly obstructed channel with vegetation patch. In addition, the vertical profile of the longitudinal velocity can also be explained by the vertical behavior of the horizontal coherent vortices based on a theoretical argument.     

SEIB–DGVM: A new Dynamic Global Vegetation Model using a spatially explicit individual-based approach

We report the development of a new spatially explicit individual-based Dynamic Global Vegetation Model (SEIB–DGVM), the first DGVM that can simulate the local interactions among individual trees within a spatially explicit virtual forest. In the model, a sample plot is placed at each grid box, and then the growth, competition, and decay of each individual tree within each plot is calculated by considering the environmental conditions for that tree as it relates to the trees that surround it. Based on these parameters only, the model simulated time lags between climate change and vegetation change. This time lags elongated when original biome was forest, because existing trees prevent newly establish trees from receiving enough sunlight and space to quickly replace the original vegetation. This time lags also elongated when horizontal heterogeneity of sunlight distribution was ignored, indicating the potential importance of horizontal heterogeneity for predicting transitional behavior of vegetation under changing climate. On a local scale, the model reproduced climate zone-specific patterns of succession, carbon dynamics, and water flux, although on a global scale, simulations were not always in agreement with observations. Because the SEIB–DGVM was formulated to the scale at which field biologists work, the measurements of relevant parameters and data comparisons are relatively straightforward, and the model should enable more robust modeling of terrestrial ecosystems.     

Application of a three-dimensional model for assessing effects of small clear-cuttings on radiation and soil temperature

A three-dimensional model Mixfor-3D of soil–vegetation–atmosphere transfer (SVAT) was developed and applied to estimate possible effects of tree clear-cutting on radiation and soil temperature regimes of a forest ecosystem. The Mixfor-3D model consists of several closely coupled 3D sub-models describing: forest stand structure; radiative transfer in a forest canopy; turbulent transfer of sensible heat, H₂O and CO₂ between ground surface and the atmospheric surface layer; evapotranspiration of ground surface vegetation and soil; heat and moisture transfer in soil. The model operates with the horizontal grid resolution, 2 m × 2 m; vertical resolution, 1 m and primary time step, 1 h.     

14C of grasses as an indicator of fossil fuel CO2 pollution

Measuring the amount of fossil fuel carbon stored in the vegetation is now crucial to understand the mechanisms ruling climate changes. In this respect, highly polluted areas such as major towns represent “natural” laboratories because fossil fuel CO₂ (¹⁴C-free) is isotopically distinct from mean atmospheric CO₂ (¹⁴C-labeled). Here, a¹⁴C study of urban grasses near a major highway in Paris, France, shows that plants store up to 13% of fossil fuel carbon. The ¹⁴C composition of urban grasses is thus a novel parameter to assess the fossil fuel CO₂ pollution.     

Changes in landscape and vegetation of coastal dunes in northwest Europe: a review

In coastal dunes, landscape changes are a rule, rather than an exception. This paper gives an overview of changes in landscape and vegetation with a focus on the past century. The history of dunes is characterised by phases of sand drift, alternated with geomorphological stability. The historical development of dune woodland during these stable phases has been documented for sites all over Europe. Vegetation reconstructions of historical open dune habitats however is very difficult due to limited preservation of fossil remains. People have drastically altered coastal dune landscapes through centuries of exploitation and more recently development of the coast. Historical land use has generally pushed vegetation back into a semi-natural state. During roughly the past century a tendency of increasing fixation and succession is observed on coastal dunes throughout northwest Europe. Six causes of change are discussed. 1) Changes in land use, mainly abandonment of agricultural practices, have led to the development of late successional stages such as scrub and woodland. 2) Crashing rabbit populations due to myxomatosis in the 1950s caused vigorous grass growth and probably stimulated scrub development. 3) A general tendency of landscape fixation is observed due to both natural and anthropogenic factors. 4) Eutrophication, mainly due to atmospheric nitrogen deposition is clearly linked to grass encroachment on acidic but also on some calcareous dunes. 5) The impact of climate change on vegetation is still unclear but probably lengthening of growing season and maybe enhanced CO₂ concentrations have led to an acceleration of succession. 6) A general anthropogenisation of the landscape occurs with rapid spread of non-native species as an important consequence. The reconstruction of a natural reference landscape is considered largely unattainable because of irreversible changes and the long tradition of human impact, in many cases since the development of the dunes. Two contradictory elements need reconciliation. First, the general acceleration of succession and scrub and woodland development in particular is partly caused by a decreased anthropogenic interference in the landscape and deserves more appreciation. Second, most biodiversity values are largely linked to open, early succession dune habitats and are threatened by the same tendency. Apart from internal nature management, in which grazing plays an important part, re-mobilisation of stable, senescent dunes is an important challenge for dune management.     

Relevance of rangeland degradation in semiarid northeastern South Africa to the nonequilibrium theory.

According to the nonequilibrium theory, livestock grazing has a limited effect on long-term vegetation productivity of semiarid rangelands, which is largely determined by rainfall. The communal lands in northeastern South Africa contain extensive degraded areas which have been mapped by the National Land Cover (NLC) program. Much evidence suggests that long-term heavy grazing is the cause of this degradation. In order to test for the prevalence of nonequilibrium dynamics, we determined the relative effects of rainfall- and grazing-induced degradation on vegetation productivity. The vegetation production in the NLC degraded areas was estimated using growth-season sums of the Normalized Difference Vegetation Index (sigmaNDVI), calculated using data from both the Advanced Very High Resolution Radiometer (AVHRR) (1985-2003) and Moderate-resolution Imaging Spectroradiometer (MODIS) (2000-2005). On average, rainfall and degradation accounted for 38% and 20% of the AVHRR sigmaNDVI variance and 50% and 33% of the MODIS sigmaNDVI variance, respectively. Thus, degradation had a significant influence on long-term vegetation productivity, and therefore the rangelands did not behave according to the nonequilibrium model, in which grazing is predicted to have a negligible long-term impact.     

Modeling the longitudinal profiles of streamwise velocity in an open channel with a model patch of vegetation

This paper proposes a model for predicting the longitudinal profiles of streamwise velocities in an open channel with a model patch of vegetation. The governing equation was derived from the momentum equation and flow continuity equation. The model can estimate the longitudinal profiles of velocities both inside and outside a vegetation patch. Laboratory experiments indicate that the longitudinal profiles of velocities inside a patch and in the adjacent bare channel have the same adjustment distance in the longitudinal direction, but the profiles have different trends because the vegetation drag drives the flow from the patch to the adjacent bare channel. The model considers different dimensionless parameters in two flow adjustment regions upstream of and inside the patch. Sixteen sets of experimental data from different sources are used to verify the model. The model is capable of modeling the longitudinal profiles of velocities inside and outside patches of cylinders or cylinder-like plants. Compared to a previous model, the current model improves the modeling accuracy of longitudinal profiles of velocities.