Fine-scale spatial population patterns and mobility of winter-annual herbs in a dry grassland

Winter annuals were mapped in a series of permanent plots located in gaps in an Austrian dry grassland over a period of 3 yr. Great differences in abundance were found among nine winter annuals - Arabis auriculata, Arenaria leptoclados, Cerastium glutinosum, Erophila spathulata, Holosteum umbellatum, Hornungia petiaca, Saxifraga tridactylites, Thlaspi perfoliatum and Veronica praecox - occurring in the gaps. The absolute frequency of species varied considerably from year to year, while their relative proportions remained nearly constant in the plots over the study period. High spatial dissociation (low level of spatial coincidence) between populations was observed; this was ascribed to boleochory, known to generate monospecific patches. No trend in self-replacement of a species was found. This means that occupying a place in one year does not necessarily mean the same place will be occupied in following years. The probability of a species replacing another was found to be low. The space could be inhabited by any species or remain unoccupied. We suggest that the distribution patterns of species are controlled by dispersal and preference for microhabitats, at least at the scale of 20 cm x 20 cm. It was shown by a permutation method based on cumulative frequencies, that at smaller scales the habitat differentiation vanishes. We suggest that, up to a typical dispersal range of the winter annuals, the Carousel model can be applied due to the lack of coincidence (and presumably also interactions) of individuals. Nevertheless, time may eliminate the habitat differentiation at any scale. A design of an improved data-sampling and a handling procedure was developed.