LONG-TERM SPECIES CHANGE IN A URUGUAYAN GRASSLAND

This paper reports on species changes which occured in plots in a Urug uayan grassland after 55 yr of continuous grazing. The study was based on the pioneering work of B. Rosengurtt who sampled 10 plots in 1935; in 1990 we resampled five of them. Comparative studies were based on rank-frequency, diversity, evenness and multivariate analyses. Diversi ty was measured as species richness S, Shannon's index H and its expon ential form N = e(H), evenness E = N/S and Gini's coefficient of hiera rchy G. Species-richness of the plots increased significantly. Species which entered the sites are mostly weedy species, while those that ha ve decreased are mostly palatable forage species. Evenness, however, r emained constant. Floristic change was chiefly associated with time, a nd secondarily with plot soil-type. During the 55 yr that elapsed betw een both observation periods, a dramatic floristic change has occurred in this grassland site.