SEASONAL DYNAMICS OF MEDITERRANEAN PASTURE SEED BANKS ALONG ENVIRONMENTAL GRADIENTS

The size and dynamics of seed banks were studied in grazed and ungraze d Mediterranean pastures at different altitudes and topography positio ns. The soil samples were collected in autumn and spring and the seed banks composition was determined by greenhouse germination over a 9-mo nth period. The percentage of bare ground and the presence of new seed lings were recorded monthly from October to July in the field. A fall in seed density and species richness in the banks and a tendency for s eeds to remain in the banks were linked to a rise in altitude. Germina tion in lower pastures mainly occurred in October in the numerous gaps left by the summer drought. At higher altitudes, the scarcity of gaps and the harsh climate led to an autumn-spring segregation of germinat ion. On a local scale, the low slope positions and the ungrazed plots had a. larger number of persistent seed bank species and a lower perce ntage of bare ground where seeds could germinate than their respective plots in the upper positions and grazed plots. A higher seed density in ungrazed than grazed plots was only detected in the three highest p lots. No seed bank species richness trend was detected. In populations of the same species in different types of environments, the seasonal variation of seed numbers was environment-dependent for the majority o f the species. In general, perennial grassland and its related low gap s availability appear to favour persistent seed banks.