LONG-TERM CONTROL OF SPECIES ABUNDANCES IN A DRY GRASSLAND - A SPATIALLY EXPLICIT MODEL

The long-term spatio-temporal dynamics of a sparse dry-grassland commu nity (Thymo-Festucetum) is investigated by a spatially explicit indivi dual-based simulation model and by analytical model equations. The com munity (investigated over 15 yr in permanent plots) is characterized b y a permanently low cover (30 - 50 %), mainly of the perennial tuft gr ass Festuca cinerea. Seedling establishment and the fate of juveniles are strongly dependent on weather conditions. The simulation programme focuses on the mechanism of clonal growth of grasses and the reproduc tion of tufts by fragmentation. Questions answered by the modelling ap proach were (1) which life-history features of the species are respons ible for their persistence and for the low vegetation cover of the com munity and (2) what are the main mechanisms of species interactions. D ifferent sets of simulation runs, together with the evaluation of the analytical models, show: (1) long-term persistence of the main species is possible only by a combination of sexual and clonal reproduction; the low cover is due to low germination rate, low mortality and limite d growth of tufts; (2) the intra and interspecific control of the comm unity is performed mainly via a reduction by already established indiv iduals; (3) persistence of uncommon species relies on a diaspore buffe r in, or around, the community ('spatial mass effect').