FLOODING, TOPOGRAPHY, AND SUCCESSIONAL AGE AS DETERMINANTS OF SPECIES-DIVERSITY IN OLD FIELD VEGETATION

We studied the spatial pattern of plant richness and diversity along t opographic gradients in two pairs of plots with 3 and 4 versus 9 and 1 0 years of abandonment from cultivation, in the Inland Pampa of Argent ina, Vegetation in each plot was sampled for species cover every 10 m along a transect running parallel to the maximum local slope. Observat ions began after 2 years of unusually severe floods and continued for over 6 years. Community richness and diversity both increased with top ographic height only the first year after flooding, in young and old s uccessional plots. Richness and diversity decreased over time in upper and intermediate topographic positions, converging toward lesser valu es observed in lower positions. A transient peak in diversity occurred in lower topographic positions several years after flood disturbance. Species richness was similarly constrained by flooding stress and suc cessional development. Thus, maximum diversity occurred at an early st age of succession in upper, infrequently flooded sites. Spatial and te mporal patterns of plant diversify in this successional system did not generally conform with predictions from intermediate disturbance mode ls. We propose that control of species diversity in old-field vegetati on changed with time since flooding from physical stress to strong bio tic interactions.