Huizache, an area located in the southern extreme of the Chihuahuan Desert Region, in San Luis Potosí, Mexico
With 75 species recorded, this area has the highest concentration of cactus species in the American Continent, in comparison with other comparably-sized regions. The outstanding diversity of cactus species in the Huizache area may be explained by the relatively favorable climate of this desert area, its environmental heterogeneity, and its location in a transitional position between three regions with distinct cactus floras: Chihuahuan Desert Region, Queretaroan-Hidalgoan Arid Zone, and Tula-Jaumave Valleys. Species richness is not distributed evenly in the area; the zones of highest species concentration are primarily located in the lowland valleys, in the northwestern portion of the grid square, where typical Chihuahuan Desert conditions prevail. Of the 75 cactus species found in the area, 63% are endemic to the Chihuahuan Desert. Among these, ten species are outstanding for being endemic or nearly endemic to the Huizache area. The Huizache grid square is part of a recently declared natural reserve, The Real de Guadalcázar Natural Reserve.
only 1.1% (6900 km2) of the ecoregion is under formal conservation (Dinerstein et al. 1999).
Mapimí Biosphere Reserve and Cuatro Ciénegas, do not correspond with the areas of high cactus species richness and endemism (Hernández and Bárcenas 1995, 1996; Gómez-Hinostrosa and Hernández 2000; this paper).
area is dominated by extensive lowland plains, which are interrupted by numerous mountain ranges of variable altitude. The most conspicuous of these is the Sierra La Trinidad mountain range, whose highest peak reaches 2260 m in elevation, giving the area considerable climatic and vegetational diversity
[the plains get 300mm rainfall/yr] [up to 800mm on the mountain tops] [temp in plains highest (avg 20-22C) but decreases with altitude]
Figure 1. Location of the Huizache grid square. [pdf pg 3]
[cactus populations are hard to survey. Spotty distribution. Many species are difficult to identify.]
[grid off area for examination]
1211 specimens were incorporated into the National Herbarium of Mexico (MEXU)
incorporated into the Database of Cactus Collections from North and Central America (Hernández et al. 1993).
The Huizache cactus flora was compared phytogeographically with several other well-known areas within the CDR: Mier y Noriega (Gómez-Hinostrosa and Hernández 2000), Doctor Arroyo (Hernández-Valencia 1981), Sierra de la Paila (Villarreal 1994), Cuatro Ciénegas (Pinkava 1984), and Mapimí (Ruiz de Esparza 1988). Comparisons were also made with three areas of the Queretaroan-Hidalgoan Arid Zone (QHAZ): Xichú, San Luis de la Paz, and San Miguel de Allende (Bárcenas 1999). In order to do this, the pairwise similarities of all possible combinations of cactus areas were calculated using Jaccard?s qualitative index (Rohlf 1998).
With a total of 75 native species recorded (Table 1), we confirmed that this relatively small region has a higher number of cactus species than any other area of comparable size on the American Continent
Table 1. Species of Cactaceae occurring in the Huizache grid square, San Luis Potos´i, Mexico. Taxonomic concepts based on Hunt (1999) with a few exceptions [pdf pg 6]
Mammillaria crinita subsp. leucantha (= M. moellervaldeziana) and Pelecyphora aselliformis [found just outside the Huizache boarder]
Huizache grid square has more cactus species than reputedly rich areas of comparable size that have been studied intensively, such as Mier y Noriega, Nuevo León (56 spp., Gómez-Hinostrosa 1998; Gómez-Hinostrosa and Hernández 2000), Cuatro Ciénegas, Coahuila (48 spp., Pinkava 1984), La Paila, Coahuila (44 spp., Villarreal 1994), Xichú, Guanajuato (56 spp., Bárcenas 1999), and Mapimí, Durango (30 spp., Cornet 1985; Ruiz de Esparza 1988). Also, the Huizache grid square has a similar number of species as do the states of Arizona (78 spp., Lehr 1978), Texas (62 spp., Correll and Johnston 1970), Guanajuato (92 spp., Bárcenas 1999), Baja California (65 spp.), and Baja California Sur (64 spp., Rebman et al. 1999). Likewise, it has a similar number of species as all of the Central American countries combined (74 spp., Bravo and Arias 1999), and a substantially higher number than Cuba, and most South American countries (Hunt 1999).
Figure 3. Comparison of species richness of Cactaceae in several Mexican regions. Data from various sources (see text). [pdf pg 8]
Tehuacán?Cuicatlán Valley, in the Mexican states of Puebla and Oaxaca, where species richness is also very high (76 native species, Arias et al. 1997). However, this area is more than three times larger (10 000 km2) than that of the Huizache. In contrast to the relatively inconspicuous species of Cactaceae found at the Huizache area, there is a predominance in the Tehuacán?CuicatlánValley of large, columnar or profusely-branched, arborescent species (e.g., Cephalocereus columna?trajani, Mitrocereus fulviceps, Myrtillocactus spp., Neobuxbaumia spp., Pachycereus spp., Stenocereus spp., Polaskia spp., Escontria chiotilla). On the other hand, in the Huizache the only conspicuous species are the arborescent Myrtillocactus geometrizans and Stenocereus griseus, and two barrel cacti (Echinocactus platyacanthus and Ferocactus pilosus). [colder in North inhibits large catus]
[large column cactus limited to] areas influenced by tropical climatic regimes (e.g., Tehuacán Valley, Balsas Basin, Tehuantepec Isthmus, and the Sonoran Desert).
[high diversity b/c] the relatively favorable climate of the area, its environmental heterogeneity, and the fact that its location is in a transitional position between the domains of at least three diverging cactus floras.
Although some North American cactus species can tolerate very low temperatures and extremely dry environments (Gibson and Nobel 1986; Nobel 1994), most Mexican species appear to lack the ability to live in such extreme climatic conditions.
occur exclusively in relatively shaded areas, where higher precipitation results in a denser vegetation cover (e.g., Coryphantha wohlschlageri, Ferocactus echidne, Mammillaria schiedeana, Selenicereus spinulosus, Turbinicarpus knuthianus).
Although the CDR and the QHAZ are geographically separated by a complex of mountain ranges, which results in a more mesic climate, the floristic similarities between these areas are evident (Rzedowski 1973; Rzedowski and Calderón 1988, 1995), suggesting a long history of floristic interchange and perhaps a common origin [but are still very distinct, lots of endemism]
Figure 4. Dendrogram showing floristic affinities between regions located in the Chihuahuan Desert (Huizache, Mier y Noriega, Dr Arroyo, La Paila, Cuatro Ci´enegas, and Mapim´i), and the Queretaroan-Hidalgoan Arid Zone (Xich´u, San Luis de la Paz, and San Miguel de Allende). Cophenetic value, r = 0.95309. [pdf pg 10]
Located east of the Huizache, the Tula-Jaumave Valleys are two regions very well known to cactus collectors for a profusion of unique, endemic species
Usually, the collecting sites located in or near the lowland valleys, where the creosote bush scrub develops, contained the highest number of species; however, the richest collecting sites were those located in the interface between this plant association and the rosetophyllous thorn scrub. In contrast, the sites located at the top of the mountain ranges were the poorest in terms of cactus diversity.
(63%) of the cactus species occurring in the Huizache grid square are endemic to the CDR and its adjacent areas
Only Echinocactus platyacanthus, Echinocereus cinerascens, E. pectinatus, E. pentalophus, Ferocactus echidne, F. histrix, F. latispinus, Mammillaria heyderi, M. magnimamma, M. prolifera, M. sphaerica, M. uncinata, Myrtillocactus geometrizans, Selenicereus spinulosus, Stenocactus sp., Stenocereus griseus, and several species of Opuntia extend their ranges well beyond the confines of this region.
Likewise, eleven out of the 18 cactus genera found in the Huizache are essentially endemic to the CDR (Ariocarpus, Astrophytum, Echinocactus, Epithelantha, Leuchtenbergia, Lophophora, Neolloydia, Sclerocactus, Stenocactus, Thelocactus, and Turbinicarpus; see Hernández and Godínez 1994).
Moreover, although the Huizache area is small, it is remarkable because it holds ten cactus species endemic or nearly endemic to its limits (Ariocarpus bravoanus, Coryphantha pulleineana, C. odorata, C. villarensis, C. wohlschlageri, Echinocereus waldeisii, Mammillaria aureilanata, M. microthele, M. surculosa and Turbinicarpus knuthianus)
The high frequency of narrowly endemic species supports the hypothesis proposed by Hernández and Bárcenas (1995, 1996) that the Huizache, specifically the lowland valleys of the northwestern portion of the grid square, acted as a refuge area during the Pleistocene climatic changes.