Agriculture has been practised in the Argentine Pampa Region for more than a century. This long history of disturbance has strongly modified the native vegetation of the region. Some original species have disappeared or its area has been remarkably reduced. The objective was to evaluate vegetation changes over time after the abandonment of agriculture practice. Our hypothesis was that over time vegetation in this region will evolve towards a "flechillar" ( Stipa spp. community) similar to the original vegetation. The experiment was conducted at Zavalla (Santa Fe) Argentina (33degrees01?S, 60degrees53?W and 50 m.a.s.l.). Vegetation evolution was studied during 15 years (1982-1997) after abandonment, considering four initial crop situations (i.e. wheat, soybean, pasture and tillage). Periodically, we measured species cover and abundance in 60 subplots. Data was analyzed using multivariate methods. We identified several groups corresponding to two very well defined successional stages. The first group was very small, and showed predominance of annual species; the second group showed a reduction or almost disappearance of annuals and an increase in perennials such as Baccharis salicifolia (Ruiz & Pav.) Pers. and Sorghum halepense (L.) Pers. accompanied by Carduus acanthoides L. Therefore, a secondary succession was developed, with a first stage of annual species dominance followed by long life cycle species. However, even after 15 years there was no reversion of the vegetation to the original community.
