Plant diversity in four chalk grassland sites with different aspects

Plant diversity and its components was studied on four chalk grassland sites in southeast England over a fourteen-month period. The sites were almost adjacent, and each faced one of the major compass points, N, S, E and W. Aboveground vegetation was recorded at monthly intervals by cover repetition, using a point quadrat technique. Seasonal patterns in the development of leaf area, bare ground, number of species present and diversity were similar on all sites. The N and W facing slopes and the S and E facing slopes resembled each other in several characteristics, while these two pairs of slopes differed markedly from each other. Thus, (i) there were more grasses but fewer forbs on the N and W slopes, (ii) overall diversity and forb diversity were lower on the N and W slopes but grass diversity was higher, (iii) equitability was lower on the N and W slopes, (iv) percentage similarity of begetation present was high when the N slope was compared with the W and when the S slope was compared with the E; other comparisons gave lower percentage similarity. It is proposed that the greater abundance of Carex flacca on the N and W slopes is important in causing these differences, and that it results in lower diversity on these slopes by occupying part of the habitat of the species-rich forb community which occurs on the S and E slopes. Dominance-diversity curves range from lognormal to geometric in form; some interpretation of changes they display can be made by assuming that the community is in a non-equilibrium state caused by seasonal environmental fluctuations each winter.