A dynamic model was developed to simulate the variation of spatial species distribution patterns of tile mead-owsteppe grasslands on Songnen Plain in northeast China. Simulation was performed to study the interaction between soil alkalization and vegetation development with special consideration given Io spatial processes such as horizontal species migration and horizontal difusion of soil alkali. The coverage of five species. Calatnagrostis epigeios. Aneurolilliditon chinense. Puccinellia temiforn. Aeluropus littoralis and Suae & l coriculata. and soil alkali were selected as 6 state variables. A positive feedback mechanism embedded hi Ibe model was that when the total planl coverage is large enough, the soil undergoes de-alkalizatinn, which in turn helps improve further the plant growth condition. The de-alkalization is due to the improved soil physicaI properlies indicated by a large amount of soil non-capillary pores coexisting with a large amount of below- ground root bionlass which allows alkaline solutes to leach from surface soil to underground water by means of precipitalion. On tile other hand. when the plant coverage is too small, soil alkalizatinn takes place title to the deterioration of soil pbysical properties indicated by a small amount of plant root biomass and a large anlounl o[ capillary pores which enables evapotranspiration Io bring up alkaline solutes to tile stlrce soil. The alkalizalkm of surlace soil further hinders planl growth. The model was inlplenlented using a software tool. SPAMOD. developed al Institute ol Botany. tile Chi-nese Acadenly of Sciences. The simulation results showed that the model was in very close agreement wilh five year field observalions oll a one hectare fenccd alkaline grassland, implying that the inodeling approach used in this research is very approprlatc for grassland landscape studies: that the spatial processes reflected as horizontal species migratkm and horizontal diffilsion or soil alkali are very importam for the recovery ol A-chineltse, the major grazed species wilh rhizoma as its nlain reproduction mechanisnl: and lhat lhe vanishing rate of S. corniculata, an indicator of serious soil alkalization, is very sensitive to the variation in the soil physical pmperlies.

We use a spatially explicit landscape model to investigate the potential role of rainfall onshrub–grass transitions in the Jornada Basin of southern New Mexico during the pastcentury. In long-term simulations (1915-1998) along a 2700m transect running from a drylake bed to the foothills of a small mountain, we test two hypotheses: (i) that wetterwinters and drier summers may have facilitated shrub encroachment in grasslands, and (ii) that increases in large precipitation events may have increased soil water recharge atdeeper layers, thus favoring shrub establishment and growth. Our model simulationsgenerally support the hypothesis that wetter winters and drier summers may haveplayed a key role, but we are unable to reproduce the major shifts from grass-to shrubdominationthat occurred in this landscape during the early part of the 1900s;furthermore, the positive shrub response to wetter winters and drier summers was onlyrealized subsequent to the drought of 1951-1956, which was a relatively short 'windowof opportunity' for increased shrub establishment and growth. Our simulations alsogenerally support the hypothesis that an increase in the number of large precipitationevents may also have favored shrub establishment and growth, although these resultsare equivocal, depending upon what constitutes a large' event and the timing of suchevents. 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 waterrecharge, and (iv) subsequent water availability for the growth and reproduction ofshrubs vs. herbaceous plants at various landscape positions. Our results suggest thatonly a mechanistic understanding of these interactions, plus the role of domestic cattlegrazing, will enable us to elucidate fully the relative importance of biotic vs. abioticfactors in vegetation dynamics in this semiarid landscape.

The behavioral dependence of vegetation simulation models for spatially heterogeneous grasslands on simulationresolutionwas investigated. The dependence can be largely attributed to the non-linearity of themodels.We showedthat increasing scale or decreasing spatial resolution tended to overestimate the changing rate of an ecosystem usingour landscape simulation model for alkaline grasslands in northeast China. A technique for scaling up simulationmodels with diffusive transportation was developed in this study by means of expanding the nonlinear drivingfunctions in the model. The analysis showed that a simulation model for spatially heterogeneous landscapes mightnecessitate modification of both its mathematical structure and parameterization when applied to different scales.The scaling coefficients derived in this study were shown to be proportional to the variances or covariance of thespatially referenced variables, and can be estimated by running the model at a fine resolution for selected samplesof the coarser grid cells. The technique was applied to a grassland landscape in northeast China and the resultswere compared with five-year observations on community succession. The comparison indicated that the proposedtechnique could effectively reduce overall scaling error of the model by as much as 80%, depending on the scalingdifference between the fine and the coarse resolutions as well as the sampling scheme used.

del was built to describe the ecological processes of an alkalinized–salinized meadow steppeecosystem, including the hydrological and alkalization-salinization processes in the soil, as well as the successionand growth dynamics of the grassland communities. A numerical integration model and a water and salt balancemodel were integrated into a physically-based model, describing the dynamics of soil moisture, salt concentration,exchangeable sodium percentage (ESP) and pH. Meteorological variables and soil characteristics were the mainenvironmental factors used to estimate the growth dynamics of three herbaceous communities that were dominatedby Aneurolepidium chinense, Chloris virgata, and Suaeda glauca, respectively. Model validation showed goodagreement between the simulated results and the observed data. Simulation studies were conducted to evaluate thepotential changes in hydrological and alkalization-salinization processes, succession and growth dynamics from1991 to 1998, under five grazing intensities, namely 0%, 25%, 50%, 75% and 90% above-ground biomass removal(AGBR). The simulations show that soil moisture decreased markedly under the 50%, 75% and 90% AGBR, butincreased slightly under the 25% AGBR. The de-alkalization and de-salinization processes would be predominantunder the 0% AGBR, and the processes became a little slower under the 25% AGBR. In contrast, the 50%, 75% and90% AGBR accelerated the degradation of soil properties. The grassland was dominated by A. chinense under the0%AGBR, and by A. chinense and C. virgata under the 25% AGBR. C. virgata could grow on slightly alkalinized-salinized soil and became a dominant species after three years of 50% AGBR. The soil degraded quickly and onlyS. glauca could grow on the severe alkalinized-salinized soil if the grassland received 75% or 90% AGBR. Thegrassland grew well under the 0% AGBR, and the biomass stayed at moderate level under 25% AGBR. The 50%,75% and 90% AGBR decreased the grassland growth greatly. After accumulating the grazed biomass for each year,the 25% AGBR would provide the highest production, and the grassland production would decrease sharply withthe increasing of grazing intensities. The simulation results indicate that 25% AGBR is significant for preservingthe soil from degradation, and maintaining high grassland production.