Some floristic characteristics of the northern Chihuahuan Desert: a search for its northern boundary

An index of Chihuahuan floristic affinity was constructed to characterize the floristic variation over a 5degrees latitudinal gradient in the northern Chihuahuan Desert, with an emphasis on evaluating northern boundary conditions. The index was based on evaluating the ranges of 494 species from 590 vegetation plots from Bouteloua eriopoda and B. gracilis grasslands and Larrea tridentata scrub, distributed among four sites along the gradient. The most northern site, the Sevilleta National Wildlife Refuge (34degreesN latitude), while maintaining a complement of primarily southern distributed species, had the lowest index values and the fewest Chihuahuan endemics (3%) and Southwestern desert species (19%) in general. At the intermediate sites (Jornada/Tularosa basins and Otero Mesa), one and two degrees further south, index values increased conspicuously along with number of Chihuahuan endemics (7%) and the Southwestern desert species (37%). At 29degreesN latitude, the Sierra del Carmen Protected Area in northern Mexico was the furthest southern site. It represented typical hot and dry Chihuahuan Desert conditions where Chihuahuan endemics comprised 29% of species complement and Southwestern desert species made up 55%. With respect to floristic boundaries, the Sevilleta was clearly transitional to the southern Shortgrass Prairie and Intermountain (Great Basin-Colorado Plateau) provinces. While it supported several grassland and shrubland associations that maintained their overall Chihuahuan character, there were also several associations with primarily northern affinities, and also a set unique to the Sevilleta, reflecting its ecotonal nature. The results show that sound biosystematics, biogeographical analyses and indices like those presented here can provide an important context for questions in synecology, plant ecology, and conservation biology.