An object-oriented, spatially-explicit simulation model of vegetation dynamics in a south Texas savanna

An object-oriented, spatially-explicit simulation model was developed to explore vegetation dynamics in a south Texas savanna. Object-oriented design allowed the model to be developed both as a tool to test hypotheses concerning the relative importance of changes in climate, fire and grazing regimes on vegetation dynamics, and as a simulation framework that can be readily extended and refined as ecological understanding or available data for the system changes. The La Copita Research Area 65 km west of Corpus Christi, Texas served as a case study providing parameterization data for the model. The landscape of La Copita shows 4 distinct ecological situations with respect to woody vegetation: woodlands, groves, discrete clusters and playas whose occurrences are correlated with topoedaphic factors (Archer, 1995). Because these factors are distributed heterogeneously throughout the landscape, the model included an explicit spatial environment in which plants establish and grow.; Mesquite (Prosopis glandulosa) cross-sections from La Copita were analyzed to establish age-size relationships within discrete clusters, groves and woodlands. The relationship between main stem basal diameter and age was strong and linear. Regression equations from this analysis were used to parameterize mesquite plants in the simulation.; The simulation was used to test the hypothesis that initiation of heavy, continuous grazing and fire suppression were sufficient to cause physiognomic conversion of grasslands to woodlands. Simulations with no grazing and fire at 10-year intervals resulted in steady-state savannas in which grasses dominated isolated woody assemblages. Simulations with the hypothesized conditions resulted in woodlands dominated by shrubs. Although the model produced patches with mesquite densities and size distributions significantly different from field data, simulated patches showed reasonable variation between topoedaphic conditions, and simulated patches showed patterns of increased woody plant encroachment that would be expected if heavy grazing and fire suppression are sufficient to cause the observed physiognomic conversions.