The biotic and abiotic changes associated with Brachypodium pinnatum dominance in chalk grassland in south-east England

Detailed vegetation surveys were made at three East Sussex ancient chalk grassland sites, and soil nutrient and light levels were compared within and outside dominant stands of tor grass Brachypodium pinnatum. The presence of B. pinnatum had a detrimental effect on species diversity with forbs most sensitive to its dominance, followed by cryptogams and graminoids. Within each group, low growing species were generally most vulnerable to high B, pinnatum cover but certain species, typical of more productive habitats, were able to persist. Although the general response in the three communities was similar, there were important differences, with forb diversity affected less, and coarse grasses reaching higher cover values under B. pinnatum at a previously under-grazed site than at the other two sites. The levels of soil nitrate, but not other nutrients, were significantly higher under B. pinnatum stands than under the surrounding community. This may have significant implications for the control of B. pinnatum, as elevated nitrates are likely to enhance its ability to out-compete typical chalk grassland species. (C) 1999 Elsevier Science Ltd. All rights reserved.