Abstract
We studied the spatial pattern of plant richness and diversity along topographic gradients in two pairs of plots with 3 and 4 versus 9 and 10 years of abandonment from cultivation, in the Inland Pampa of Argentina. Vegetation in each plot was sampled for species cover every 10 m along a transect running parallel to the maximum local slope. Observations began after 2 years of unusually severe floods and continued for over 6 years. Community richness and diversity both increased with topographic height only the first year after flooding, in young and old successional plots. Richness and diversity decreased over time in upper and intermediate topographic positions, converging toward lesser values 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 successional development. Thus, maximum diversity occurred at an early stage of succession in upper, infrequently flooded sites. Spatial and temporal patterns of plant diversity in this successional system did not generally conform with predictions from intermediate disturbance models. We propose that control of species diversity in old-field vegetation changed with time since flooding from physical stress to strong biotic interactions. Keywords: pampean grasslands, secondary succession, disturbance and stress gradients, species richness, vegetation structure.
Published Version
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