ECOSYSTEM CHANGES ASSOCIATED WITH GRAZING INTENSITY ON THE PUNTA-NINFAS RANGELANDS OF PATAGONIA, ARGENTINA

Changes in the vegetation and soil surface were assessed along a grazing intensity gradient on rangelands of the Punta Ninfas area in southern Argentina, Thirty-two transects were sampled in areas with different grazing intensity, Bray-Curtis polar ordination and simple correlation were used to display changes in community composition and measure association between different community attributes. The first axis expressed the changes in species composition along a gradient of grazing intensity, The extremes of the gradient were represented by shrub and grass steppes. Shrub steppes dominated in heavily grazed areas close to permanent water points, while grass steppes dominated in lightly grazed areas in the extremes of the paddocks, A significant negative relation (r = -0.81, p<0.05) between grass and shrub cover suggested that grasses decreased as shrub increased, Flechilla (Stipa tenuis Phil,) and flechilla negra [Piptochaetium napostaense (Speg,) Hackel ap Stuckert.] were the main decreaser grasses while quilembai (Chuquiraga avellanedne Cav.) was the main shrub invading the grass steppes, Uneroded soil surface conditions decreased, and the size and frequency of crusted and desert pavement areas and mounds increased with shrub cover, Three states or stages of range degradation were identified along the gradient of grazing intensity, Grass steppe represented the most desirable state in term of livestock production and soil stability, while shrub steppe represented the most degraded and least productive state.