Characterization of a narrow hybrid zone between two subspecies of big sagebrush (Artemisia tridentata, Asteraceae): VII. Community and demographic analyses

We examined the floristic and vegetative composition of the communities inh abited by basin big sagebrush (Artemisia tridentata ssp. tridentata) and mo untain big sagebrush(A. t. ssp. vaseyana) and their hybrids in Clear Creek Canyon near Richfield, Utah. We also examined the demographic structure of the sagebrush populations in their native habitats. The species composition , proportions of annual plants, perennial forbs and grasses, shrubs, rock, litter, bare ground and total vegetative cover differ among the parental si tes and hybrid zone. Canonical correspondence analysis and ordination showe d that the two big sagebrush subspecies and their hybrids are each associat ed with different groups of species, and occupy edaphically distinct habita ts. Thus, the hybrid zone occurs at an ecotone. Moreover, the hybrid zone i s not in a more disturbed habitat than the parental zones. Indeed, annuals comprise a significantly smaller fraction of the total vegetation in the hy brid zone than in either parental zone. Similarly, introduced species repre sent a significantly greater percentage of the plant cover in the parental areas than in the hybrid zone. We did not observe a population density trou gh as predicted by the dynamic equilibrium model. Our findings are consiste nt with the predictions of the bounded hybrid superiority model, which post ulates that hybrids occupy unique habitats.