Community structure of a southern Chihuahuan Desert grassland under different grazing pressures

The effect of different grazing intensities on the semiarid grasslands of the southern Chihuahuan Desert was studied using a fence-line contrast between a moderately grazed cattle ranch, from which goats and sheep had been excluded for forty-five years, and an ejido, heavily overgrazed for at least the last century. Each plant species density and cover was quantified in three distinctive microhabitats on the ranch and on the adjacent common-use rangelands. The results indicated that three grass species were important in the dynamics of this rangeland. Hilaria belangeri, a stoloniferous, mat-forming grass species, was dominant on the heavily grazed ejido, but is in the process of being replaced by two taller grasses, Bouteloua gracilis and Boutleloua curtipendula, on the more moderately grazed ranch. These data suggest that the dynamics of this system in the semiarid grasslands of the southern Chihuahuan Desert are based primarily on two functional groups of species. Members of the first functional group are stoloniferous and clonal. Their growth form slows the rate of surface water movement, thereby controlling erosion while building up the soil by entrapping debris. In contrast, the second functional group is comprised of non-stoloniferous grasses that are described variously as bunch, tufted or tussock grasses in the literature. These species have the capacity to invade, overtop and replace individuals of the first functional group. They are limited in their lateral growth by their rhizomatous growth habit, which is much less efficient in co-opting space. Finally, there is, at least, a third functional group of much taller grasses present that are limited in these rangelands to refuges by the heavy grazing. These species are usually obligate seed producers. Such species have the potential to replace members of the earlier functional groups by overtopping them. They are limited by their inability to reproduce vegetatively, once established by their seed bank. This research suggests that restoration of the heavily eroded, semiarid grasslands of the southern Chihuahuan Desert must begin with the re-establishment of members of the first functional group. These species have the ability to facilitate the entry of the later functional groups and, in turn, be replaced competitively by them. (C) 2009 SAAB. Published by Elsevier B.V. All rights reserved.