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 community (Thymo-Festucetum) is investigated by a spatially explicit individual-based simulation model and by analytical model equations. The community (investigated over 15 yr in permanent plots) is characterized by a permanently low cover (30 - 50 %), mainly of the perennial tuft grass 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 reproduction of tufts by fragmentation. Questions answered by the modelling approach were (1) which life-history features of the species are responsible for their persistence and for the low vegetation cover of the community and (2) what are the main mechanisms of species interactions. Different 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 limited growth of tufts; (2) the intra and interspecific control of the community is performed mainly via a reduction by already established individuals; (3) persistence of uncommon species relies on a diaspore buffer in, or around, the community ('spatial mass effect').