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Annals of Botany 85 (Supplement B): 63-76, 2000
© 2000 Annals of Botany Company

Modelling the Effects of Patch Size on Vegetation Dynamics: Bracken [Pteridium aquilinum (L.) Kuhn] under Grazing

Colin P. D. Birch 1, Nicolas Vuichard 2, and Ben R. Werkman 3

1 Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK
2 ISIMA, Campus des Cezeaux, BP 125, 63173 Aubiere Cedex, France
3 Climate Impacts Research Centre, Abisko Scientific Research Station, Box 62, SE-981 07 Abisko, Sweden

Fax +44(0)1224 311556, c.birch{at}mluri.sari.ac.uk

A new spatial simulation model of vegetation dynamics, called ‘VegeTate’, was applied to relationships between expansion of bracken (Pteridium aquilinum (L.) Kuhn) patches and grazing. VegeTate simulates multispecies dynamics within each cell of its grid, including localized apparent competition mediated by grazing and trampling. It can be parameterized to simulate various plant communities and species, includes seasonality and uses a hexagonal grid. Dynamics of individual bracken patches in grassland, and of heathland initially containing sparse grass and bracken were simulated. Mixtures of grass 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 trampling. 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 sensitivity analysis, this phenomenon was reproduced after substantial changes in parameter values and environmental conditions, suggesting that it should be observable in the field. Interaction between patch size and grazing impact strongly influenced the spatial dynamics of simulated vegetation. The size of bracken patches became bimodal, so that most of the area of bracken was within a few large patches. Both spatial and temporal vegetation dynamics became non-linear. These results are consistent with observations that some bracken patches contain several genets and some genets occupy several patches. The hexagonal grid and trampling were important factors in the simulation results.

Bracken, Calluna vulgaris, grazing, heathland, hexagonal grid, indirect competition, invasion, mosaic, spatial simulation, Pteridium aquilinum, trampling

Submitted on July 29, 1999
Revised on October 20, 1999
Accepted on November 24, 1999


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