MEDITERRANEAN ANNUAL PASTURE DYNAMICS - IMPACT OF AUTUMN DROUGHT

1 There are wide fluctuations in the floristic composition of pastures consisting mainly of annual species in Mediterranean climatic environments. This is attributed to variations in total precipitation and rainfall distribution patterns. 2 In order to examine the effect of autumn drought on the floristic composition of Mediterranean annual pastures, soil samples with seed banks were taken from the upper and lower slopes of each of three pastures that differed in successional age. Half of each sample was subjected to a simulated autumn drought treatment while the other half received a continuous water supply (in both cases under controlled conditions). All germinations and deaths were recorded throughout the 2-month experimental period. 3 Only five species had significantly different numbers of germinations between the two watering treatments, and in four of these germination was lower in the drought-simulated treatment. Re-watering of samples from this treatment did not result in a second wave of germination. 4 Drought-induced mortality was greater in the samples with high seedling densities. A significant negative correlation was evident between seedling density and the water content of the pots at the end of the experimental drought. 5 The least drought-sensitive species were those with stem or leaf hairs and those belonging to Crassulaceae, which show a particular adaptive metabolism.