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