Floristic changes related to grazing intensity in a Patagonian shrub steppe

Abstract

Sheep grazing in and and semiarid Patagonia have the natural vegetation as their exclusive source of food This paper studies the flonstic changes that occur as a response to grazing in the Stipa speciosa, Stipa humilis, Adesmta campestris, Berberis heterophylla , and Poa lanugmosa community in the SW of Chubut (Argentina) by companson of community characters between areas with different degrees of histoncal grazing Stands located on typical zonal soils with sandy cover were sampled, including pairs of stands from neighbouring fields with different grazing history and two areas where large herbivores had been excluded for 27 and 9 yr, respectively Species cover values were recorded m 37 samples of ca 500 m 2 each A PCA species ordination with the aid of site biplot graphs and species cluster analysis allowed for the identification of five groups of species with differential response to grazing Relative cover of shrub species was positively correlated with grazing intensity, but total plant cover and flonstic nchness showed a less obvious decreasing tendency Diversity was not correlated with this gradient Some of the decreasing species belong to the group shared with the more mesic grassy steppe, and some of the increasing ones belong to the one shared with the more xenc low shrub steppe These results are in accordance with models which propose that continuous grazing promotes a xerophytization of and and semiarid plant communities