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 inhabited by basin big sagebrush (Artemisia tridentata ssp. tridentata) and mountain 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 sites and hybrid zone. Canonical correspondence analysis and ordination showed that the two big sagebrush subspecies and their hybrids are each associated with different groups of species, and occupy edaphically distinct habitats. Thus, the hybrid zone occurs at an ecotone. Moreover, the hybrid zone is not in a more disturbed habitat than the parental zones. Indeed, annuals comprise a significantly smaller fraction of the total vegetation in the hybrid zone than in either parental zone. Similarly, introduced species represent a significantly greater percentage of the plant cover in the parental areas than in the hybrid zone. We did not observe a population density trough as predicted by the dynamic equilibrium model. Our findings are consistent with the predictions of the bounded hybrid superiority model, which postulates that hybrids occupy unique habitats.