51 August 2002: 453–462 Muldavin Floristic characteristics of Chihuahuan Desert Some floristic characteristics of the northern Chihuahuan Desert: a search for its northern boundary Esteban H. Muldavin Biology Department, University of New Mexico, Albuquerque, New Mexico, 18731, U.S.A. E-mail: muldavin @sevilleta.unm.edu An index of Chihuahuan floristic affinity was constructed to characterize the floristic variation over a 5° lati- tudinal gradient in the northern Chihuahuan Desert, with an emphasis on evaluating northern boundary condi- tions. 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 gradi- ent. The most northern site, the Sevilleta National Wildlife Refuge (34°N latitude), while maintaining a com- plement 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 29°N 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 tran- sitional 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 indi- ces like those presented here can provide an important context for questions in synecology, plant ecology, and conservation biology. KEYWORDS: biome boundaries, Bouteloua eriopoda, Bouteloua gracilis, Chihuahuan Desert, floristic bio- geography, Larrea tridentata. Mexico and west Texas northward to Canada. Yet, B. INTRODUCTION gracilis is also found throughout the Southwest and into The current northern limit of diploid populations of Mexico (Fig. 1b). Granted, as one moves southward, the Larrea tridentata (Sessé & Moc. ex DC.) Coville is just B. gracilis-dominated grasslands become smaller and five km south of Albuquerque, in north-central New less functional as Prairie ecosystems, but when do they Mexico, U.S.A. Given that Larrea is one of the defining also no longer represent that ecosystem in terms of its elements of Chihuahuan Desert scrub vegetation, the floristic elements? Following McLaughlin (1994), the populations near Albuquerque also represent to many the problem then becomes, how do floristic entities segre- northern boundary of the desert itself (Fig. 1a). But does gate themselves in ecological space, and how does this the simple presence of a key indicator species really floristic structure inform our ecological understanding reflect the presence of an entire floristic province or beyond the analysis of dominants or any particular biome? Similarly, Bouteloua eriopoda (Torr.) Torr. is species of interest? also considered to be a Chihuahuan Desert grassland To address these questions, I present a relatively indicator, but it extends even further north into southern simple floristic index based on species distributions and Utah and southern Colorado. Where are the actual north- presence-absence that can be used to characterize grass- ern limits of the Chihuahuan Desert as a floristic entity, lands and scrub communities in terms of their and by extension, an ecosystem? In contrast, Bouteloua Chihuahuan Desert affinity. This index makes use of gracilis (Willd. ex Kunth) Lag. ex Griffiths is a charac- established synecological and phytogeographic classifi- teristic species of the shortgrass and mixedgrass prairies cations that were developed in a multivariate framework of the Great Plains that extend from northeastern New of plot databases and floras, respectively. The vegetation 453 Muldavin Floristic characteristics of Chihuahuan Desert 51 August 2002: 453–462 Fig. 1. Examples of northern Chihuahuan Desert communities at the edges of their range. a, Larrea tridentata/Dasyochloa pulchella scrub near its northern limit on the Sevilleta National Wildlife Refuge in central New Mexico; b, Bouteloua gracilis/Yucca thompsoniana grasslands approaching their southern limit in the Sierra del Carmen Protected Area in northern Mexico. classifications allow the stratification of a problem in an predicated on sound biosystematics for the region, with- ecological context, whereas the floristic classifications out which biogeography and synecological analysis provide a consistent framework categorizing species dis- become problematic at best. tributional characteristics. But the entire enterprise is I use the index, along with distributional information 454 51 August 2002: 453–462 Muldavin Floristic characteristics of Chihuahuan Desert tutes definitive desert conditions. The Jornada and Tularosa basins (33° 20’ N lat) and Otero Mesa (32° 20’N lat) represent intermediate sites that occur in south-cen- tral New Mexico and extend into Texas. The elevations in the Jornada/Tularosa basins and Otero Mesa range from 1,300 m to 1,600 m. These intermediate sites also provide a test of the boundary between the Chihuahuan and Mogollon Floristic Districts as defined by McLaughlin (1992). The furthest northern site is the Sevilleta National Wildlife Refuge (NWR) at 34°20’N lat and 75 km south of Albuquerque. The Sevilleta National Wildlife Refuge offers a special opportunity to analyze northern Chihuahuan Desert boundary conditions because both Larrea and B. eriopoda approach their northern limit there, while extensive B. gracilis stands diminish significantly southward from the refuge. Fig. 2. Study sites were distributed along a five-degree Furthermore, several ecological and floristic maps show latitude axis in northern Chihuahuan Desert. the Sevilleta at or near a northern boundary for Chihuahuan Desert or Madrean entities (Brown & Lowe, 1980; McLaughlin, 1992; Dick-Peddie, 1993; Bailey & on individual species and plant associations, to examine al., 1994; Reichenbacher & al., 1998). floristic structures along a latitudinal gradient from All of the sites have precipitation regimes that are north-central New Mexico to the Sierra del Carmen in dominated by summer rainfall (70–80% of the mean northeastern Mexico. Although a wide variety of grass- annual precipitation). Annual precipitation is similar land and desert vegetation types occurs across this latitu- across the sites, with an average of 245 mm on the dinal gradient (some 500 plant associations among 50 Sevilleta, 265 mm in the Jornada/Tularosa basins, 255 alliances), the emphasis here is on Larrea, B. eriopoda mm on Otero Mesa, and 250 mm in the Sierra del and B. gracilis-dominated vegetation communities as Carmen. Mean annual temperatures gradually increase indicators of floristic and ecological boundaries. In addi- going south: 13.5°C at the Sevilleta; 14.5°C for the tion, these communities are the focus of intensive Jornada/Tularosa basins; 16.3°C on Otero Mesa, and research on desertification processes (Schlesinger & al., 20°C in the Sierra del Carmen Protected Area. Winters 1990), ecological interactions from the plant to biome are relatively mild, even at the Sevilleta (2°C mean win- level (Gosz & Sharpe, 1989; Gosz, 1991; Peters, 2000) ter temperature), but summers can be extremely hot and biodiversity conservation (Muldavin & al., 2000). (31°C is the mean high at the Sevilleta and in the Sierra Hence, there is an opportunity here to bring the knowl- del Carmen). edge base of biosystematics together with biogeography Floristic analysis. — To analyze differences to better inform current ecological and conservation biol- among sites and vegetation communities, a weighted ogy issues in the region. presence-absence floristic index was constructed based on the regional distribution of individual species found among a subset of vegetation plots from the four datasets. First, all selected species in the dataset were MATERIALS AND METHODS evaluated and classified into floristic groups based on S tudy area. — There were several plot datasets their geographic ranges, and then weighting values available that had potential for analyzing floristic pat- applied to each floristic group with respect to their terns. I chose four that were distributed across five Chihuahuan affinity. For every plot in the dataset, the degrees of latitude along the eastern side of the Northern average weight among species present was computed Chihuahuan Desert, and that were from reserves with and used as an index, along with other distributional limited human impacts (Fig. 2). The Sierra del Carmen data, to compare sites and vegetation associations. Protected Area is the furthest southern site at 29°N lati- The four datasets used were originally developed as tude and is located in northern Mexico across the Rio part of vegetation classification and mapping projects Grande from Big Bend National Park, Texas. The Sierra conducted over the past decade by the New Mexico del Carmen