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 herbicide associated or not with litter removal and scratching of the top 3 cm of the soil were generated in October 1988, December 1988, and March 1989. Subsequent changes in cover, species richness and composition were observed over the two following vegetation cycles (until May 1990), and compared to the dynamics in undisturbed samples. The identity of the colonizers for each disturbance date were compared to the soil readily-germinable seed content, sampled simultaneously to the disturbances. The species richness of the samples within the fields increased the first year after disturbance. Within-field heterogeneity for taxonomic composition was unchanged, but species turnover was increased by disturbances. The dynamics of richness and composition followed consistent patterns in the three fields. These patterns were strongly related 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 of species establishments and replacements depended on their timings. Disrupting the dominance by autumn established gasses and legumes, they opened windows for germinations of annual forbs and for spring establishment of perennials. These species were otherwise present as understory species or in the seed bank. Results of these experiments provide insights into the mechanisms of maintenance of species diversity in this Mediterranean old field system. Asynchronous small-scale disturbances appear to be effective in maintaining diversity as a result from the interplay of differences in regeneration niches, lottery for establishment, and the incidence of different conditions in time and space.