Modelling the effects of patch size on vegetation dynamics: Bracken [Pteridium aquilinum (L.) Kuhn] under grazing

A new spatial simulation model of vegetation dynamics, called 'VegeTate', w as applied to relationships between expansion of bracken (Pteridium aquilin um (L.) Kuhn) patches and grazing. VegeTate simulates multispecies dynamics within each cell of its grid, including localized apparent competition med iated by grazing and trampling. It can be parameterized to simulate various plant communities and species, includes seasonality and uses a hexagonal g rid. Dynamics of individual bracken patches in grassland, and of heathland initially containing sparse grass and bracken were simulated. Mixtures of g rass and bracken formed mosaics, in which locally high grass densities were associated with low standing mass and high grazing intensity, whereas high bracken densities were associated with high standing mass and low grazing intensity. Control of bracken by grazing depended on the impact of tramplin g. In simulations, small bracken patches occupying one or a few cells could be controlled by grazing, but not the expansion of large patches. In a sen sitivity analysis, this phenomenon was reproduced after substantial changes in parameter values and environmental conditions, suggesting that it shoul d be observable in the held. Interaction between patch size and grazing imp act strongly influenced the spatial dynamics of simulated vegetation. The s ize of bracken patches became bimodal, so that most of the area of bracken was within a few large patches. Both spatial and temporal vegetation dynami cs became non-linear. These results are consistent with observations that s ome bracken patches contain several genets and some genets occupy several p atches. The hexagonal grid and trampling were important factors in the simu lation results. (C) 2000 Annals of Botany Company.