Control of Relative Abundance of Perennials in Chalk Grassland in Southern England. II. Vertical Canopy Structure

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Abstract
(1) The vertical distribution of leaves was recorded using a points quadrat, at intervals throughout 1981 and 1982 from grazed chalk grasslands in Sussex and Wiltshire; the data were analysed to describe the overtoppig relationships between leaves of different species. (2) In most cases there was a positive correlation between the relative height of rosette leaves of each species (''relative leaf positon'') and their relative abundance. (3) Despite the short turf of these chalk grasslands (<10 cm excluding inflorescence stalks) there is pronounced vertical structure; the tallest species (often graminoids) tend to be the most abundant, and the shortest species (often dicotyledons) the least abundant. (4) It is suggested that relative abundance is partly determined by competition for ligh and that there is a steady-state relationships between species with taller and with shorter rosette leaves. The balance between species is determined by grazing with taller species favoured between grazing events and the shorter species at the time of grazing. (5) In grazed chalk grasslands in southern England, the balance falls in favour of species with taller and widely spreading growth-forms because grazing intensity is low and there is often a long time between successive grazing periods. (6) The species with shorter growth-forms persists, and the species with taller growth-forms do not come to ecological dominance, because of chronic, differential defoliation.