OLD-FIELD SUCCESSIONAL DYNAMICS ON THE INLAND PAMPA, ARGENTINA

The first 10 yr of old-field successional dynamics on the Argentine In land Pampa were studied on a series of adjacent plots established cons ecutively between 1978 and 1989. We examined differences in species ab undance patterns among plots in order to detect the spatial and tempor al variability of succession. Perennial grasses steadily increased in cover and replaced the dominant annual species after 5 yr. Pioneer dic ots persisted in older seral stages with 20 - 23 species/plot. Overall , exotic species (mostly the grasses Lolium multiflorum and Cynodon da ctylon) contributed much to the plant cover in these communities. Nati ve grasses comprised 45 % of total cover at years 7 - 10 of succession , but occurred with less than 7 species/plot. Substantial variation wa s found in the successional pathway, which reflected the particular se quence from annual forbs to short-lived and perennial grasses in the v arious plots. The course of succession was apparently influenced by a 2-yr period of unusually high rainfall. Deyeuxia viridiflavescens, a n ative perennial grass virtually absent before the wet period, spread o ver the study area and dominated seral communities for 3 yr, irrespect ive of plot age. Climatic conditions thus affected the successional tu rnover of life forms by increasing the rate of colonization by perenni al grasses. We further point out the constraints imposed on secondary succession by the life histories of 'available' species.