VEGETATION DYNAMICS IN AN EXPERIMENTALLY FRAGMENTED LANDSCAPE

In spatially heterogeneous habitats, plant community change may reflec t spatially localized population-level processes that are sensitive to the size of an average habitat patch. However, local species turnover can also be determined by initial conditions and large-scale processe s, in which case patch size effects may be overridden. To examine the role of patch size in directing secondary succession, we subdivided a newly abandoned agricultural field into an array of experimental patch es (32, 288, and 5000 m(2), grouped to sample equivalent portions of t he field), and have thereafter censused the resident plant and animal communities at regular intervals. Here we report results from the firs t 6 yr of studies on the changing vascular plant community in an exper imentally fragmented landscape. The general course of change in all pa tches followed a trajectory typical of old-field succession, toward in creasing dominance by longer lived and larger plant species. The same group of species that dominated at the start of the study continued to dominate after 6 yr, although in very different proportional abundanc es. Larger patches were more species rich than their smaller counterpa rts, and had a higher proportion of nonshared species, but the additio nal species were transient and low in abundance. Spatial heterogeneity in vegetation, measured as local community dissimilarity, increased i n all patches but to a lesser extent in the largest patches, where cen suses of nearby permanent quadrats indicated less divergence over time . At a population level, the strongest effect of patch size was that l ocal populations of clonal species were more prone to disappear from t he smallest patches. Nevertheless, summary measures of temporal commun ity change did not reflect significant differences in localized specie s turnover. We conclude that patch size does not markedly affect the r ate or pattern of early secondary succession, at the scales imposed in our experiment.