SMALL-SCALE DISTURBANCES AND THE MAINTENANCE OF SPECIES-DIVERSITY IN MEDITERRANEAN OLD FIELDS

In three Mediterranean old fields, one, seven, and fifteen years after the last ploughing, small scale disturbances (0.25 m(2)) by contact h erbicide associated or not with litter removal and scratching of the t op 3 cm of the soil were generated in October 1988, December 1988, and March 1989. Subsequent changes in cover, species richness and composi tion were observed over the two following vegetation cycles (until May 1990), and compared to the dynamics in undisturbed samples. The ident ity of the colonizers for each disturbance date were compared to the s oil readily-germinable seed content, sampled simultaneously to the dis turbances. The species richness of the samples within the fields incre ased the first year after disturbance. Within-field heterogeneity for taxonomic composition was unchanged, but species turnover was increase d by disturbances. The dynamics of richness and composition followed c onsistent patterns in the three fields. These patterns were strongly r elated to the seasonal rainfall distribution and clearly linked to the germination timings of the species and to seed bank composition. The identity of species depending on season and treatment were independent between fields. The effects of small scale disturbances on patterns o f species establishments and replacements depended on their timings. D isrupting the dominance by autumn established gasses and legumes, they opened windows for germinations of annual forbs and for spring establ ishment of perennials. These species were otherwise present as underst ory species or in the seed bank. Results of these experiments provide insights into the mechanisms of maintenance of species diversity in th is Mediterranean old field system. Asynchronous small-scale disturbanc es appear to be effective in maintaining diversity as a result from th e interplay of differences in regeneration niches, lottery for establi shment, and the incidence of different conditions in time and space.