Perennial grass species richness and diversity on Argentine rangelands recovering from disturbance

Abstract

The influence of types of disturbance (hereon referred to as treatments) on natural vegetation was determined on richness and diversity of perennial grasses in semiarid rangelands of Argentina (40º 39'S, 62º 54'W). Studies were performed in adjacent areas which received different treatments: (1) untreated control; (2) burning; (3) herbicide application (shrub control); (4) Old Field 1 or (5) Old Field 2 (areas previously subjected to mechanical soil disturbance for 25 years; Old Field 1 had been exposed to a more severe grazing than Old Field 2 previous to cropping); and (6) overgrazing. During 1984-1992, annual aboveground biomass was determined per perennial grass species within each treatment after clipping permanent plots (n=30). Grass species richness and diversity were higher (p<0.05) on the Control than on sites receiving a more severe or continuous grazing (Old Field 1 and Overgrazing). With a few exceptions, the Control, Shrub Control and Old Field 2 treatments had a similar (p>0.05) perennial grass species richness and diversity. Changes in grass species diversity among years within each treatment were due to changes in both richness and relative biomasses of the studied species. The declining trends in grass species richness or diversity during 1984-1992 had equal slope in all treatments, indicating that the reduction rate of these variables over time was the same independent of disturbance type. Principal Coordinate Analysis showed that the similarity of species composition was greater among years within any treatment than among treatments in any given year. However, more than 69% of all perennial grass species were desirable perennial grasses in all treatments and years.