Effects of grazing on patch structure in a semi‐arid two‐phase vegetation mosaic

Abstract

Abstract. Question: What are the grazing effects in the spatial organization and the internal structure of high and low cover patches from a two‐phase vegetation mosaic? Location: Patagonian steppe, Argentina. Methods: We mapped vegetation under three different grazing conditions: ungrazed, lightly grazed and heavily grazed. We analysed the spatial patterns of the dominant life forms. Also, in each patch type, we determined density, species composition, richness, diversity, size structure and dead biomass of grasses under different grazing conditions. Results: The vegetation was spatially organized in a two‐phase mosaic. High cover patches resulted from the association of grasses and shrubs and low cover patches were represented by scattered tussock grasses on bare ground. This spatial organization was not affected by grazing, but heavy grazing changed the grass species involved in high cover patches and reduced the density and cover of grasses in both patch types. Species richness and diversity in high cover patches decreased under grazing conditions, whereas in low cover patches it remained unchanged. Also, the decrease of palatable grasses was steeper in high cover patches than in low cover patches under grazing conditions. Conclusions: We suggest that although grazing promotes or inhibits particular species, it does not modify the mosaic structure of Patagonian steppe. The fact that the mosaic remained unchanged after 100 years of grazing suggests that grazing does not compromize population processes involved in maintaining patch structure, including seed dispersal, establishment or biotic interactions among life forms.

Abundance and spatial patterning of coexisting perennial grasses in grazed shrublands of the Patagonian Monte

“…We found evidence for the effect of grazing on the abundance of perennial grass species with different functional traits but the spatial patterning of these species with respect to the shrubby matrix was not differentially affected by grazing. Further, the cover of the shrubby matrix was not affected by grazing although increased density and reduced diameter of shrub patches were observed in sites disturbed by grazing as reported elsewhere (Adler et al, 2001;Bisigato and Bertiller, 1997;Bertiller et al, 2002a;Cipriotti and Aguiar, 2005).…”

Vertical structure and spatial associations of dominant tree species in an old-growth temperate forest

“…For the bivariate O-ring statistic, when compared with different height classes, we hypothesized that higher height classes suppress the recruitment and growth of lower ones, but lower height classes do not affect higher ones (Nakashizuka, 2001;Cipriotti and Aguiar, 2004). Therefore, we analyzed our data based on an antecedent condition null model that randomizes the locations of the lower height classes and keeps the locations of the higher ones fixed (Cipriotti and Aguiar, 2004). We focused on the spatial associations of the sapling height class (S) and the lower height class (L) to the upper height class (U).…”

Herbivory resistance traits in populations of Poa ligularis subjected to historically different sheep grazing pressure in Patagonia

“…On the contrary, the genotypic change by selection operating through vitality is not constrained by the life span of the adult individuals. In the case of Patagonia, there is solid evidence that grazing promoted a reduction in the numbers of individual of P. ligularis (Cipriotti and Aguiar 2005a;Perelman et al 1997). Mortality induced by grazing may not be at random.…”

“…In January 2002 genets of P. ligularis were collected from exclosured, grazed and overgrazed paddocks (Cipriotti and Aguiar 2005a). Genets from the exclosure population were collected from three paddocks ungrazed for the last 45, 30, and 20 years.…”