Coexistence of grasses and shrubs in Patagonian steppes: Norm or exception?

Abstract

Questions: (1) Is the co-dominance of grasses and shrubs a general feature of Patagonian steppes? (2) At a local scale, does it change with domestic grazing intensity or decadal changes in precipitation?. Locations: Question 1. Patagonia region in southern South America. Question 2. Shrub–grass steppes (45°24′S, 70°17′W) subjected to different grazing intensity for decades. Methods: We sampled vegetation to estimate perennial grass and shrub cover at two scales: regional and local. At the regional (phytogeographical) scale, we sampled grazed steppes under different climates and soils (n = 324 sites over ca. 487,000 km2). At the local scale, we sampled paddocks subjected to known grazing management, from commercial intensive densities of sheep to total (experimental) exclusion. Also, in the same site, we sampled permanent plots (25 years old) in a grazing exclosure to study the effects of precipitation variation. Results: Mixed communities are the predominant form of community organization in the Patagonia region. At the regional scale, shrub cover was negatively related to grass cover and communities where shrubs had higher cover than grasses represent 54% of the data set. At the local scale for an arid steppe, perennial grasses had higher cover than shrubs, regardless of grazing pressure. The shrub-to-grass ratio was increased only by intensive grazing. Permanent plots in a grazing-excluded paddock indicate that decadal xeric or humid climate periods decrease or increase total cover, respectively. Surprisingly, the relative shrub-to-grass ratio did not change significantly. Conclusions: Although perennial grasses and shrubs cover are negatively correlated, they coexist in the Patagonian steppes under a wide range of climate, soil and grazing conditions. At a local scale, selective herbivory promotes species replacement but did not substantially change the life-form covers. Water shortages, as well as periods of water surplus, similarly affect both life forms without compromising their coexistence.