Location and General Description
Situated in central Mexico, this region is surrounded by the Sierra Madre Occidental Mountains to the west, by the Sierra Madre Oriental Mountains to the east, and by the rugged Trans-volcanic belt to the south. The region constitutes the southernmost portion of the Mexican central meseta, which is an extensive portion of flatlands running south from the Mexico-US border in the state of Chihuahua.
There are some elevational relief’s in this region, but they do not get above 2000 m. The great majority of the substrate on which the scrub formations grow is of sedimentary origin, and is composed of pure sedimentary rocks and some metamorphic elements, which are the result of volcanic activity in the tertiary period of this region. However, some areas that are nearer the Sierra Madre Oriental and Occidental Mountains also have soils of volcanic origin. The climate is dry and hot, with precipitation levels below 500 mm/year (Marroquín et al. 1981).
The plant associations are as variable as the specific conditions of the soil, temperature and exposure in the slopes and valleys. In the state of Hidalgo, areas of moderate slope and rocky terrain are occupied by Agave lechuguilla, Hechtia podantha, Mammillaria magnimama, M. uncifera, Ferocactus lastispinus. In similar areas but with deeper soils, Mimosa biuncifera and Acacia farnesiana are abundant. Between 2265-2295 m., the dominant species become Opuntia robusta, O. streptacantha, Cilindropuntia imbricata, Zaluzania augusta, Mimosa biuncifera, and Opuntia spinulifera. In these areas the herbaceous stratum is composed of Gramineae (Rzedowski 1978). Growing on the slopes of mountains with volcanic soils, Fouquieria-Opuntia is the most common plant association, which occur along with Eysenhardtia polystachya. Another type of scrub habitat (dominated by Machaonia coulteri) is characterized by plants with highly reduced leaves, growing on limestone soils, and containing species of the genera Agave, Dasylirion, Opuntia, Condalia and Fraxinus. The Opuntia scrub grows on slopes of volcanic mountains where precipiation is between 300-600 mm/year. These areas are composed of O. streptacantha, O. leucotricha, Myrtillocactus geometrizans and Yucca decipiens. As the region approaches the Valley of Mexico the dominant species become O. streptacantha, Zaluzania augusta, Yucca filifera, Schinus molle and Mimosa biuncifera.
The central Mexican matorral is one of the most extensive floristic and physiographic provinces of Mexico, partly because past geological events promoted the diversification of the biota. This region was historically isolated from other arid regions in Mexico and has established its own endemics (Morafka et al. 1992, cited in Challenger 1998). Orogenic activity fragmented the central meseta and the surrounding areas (that now constitute three of the most important mountain ranges of Mexico) and the subsequent isolation of the region favored the differentiation and evolution of endemic species in each of the fragmented portions. Therefore, the matorral of central Mexico is characterized by its high concentration of endemic species. In the state of Hidalgo, the vegetation is composed by three different kinds of xeric scrub (Parga-Mateos et al. 1996), which is a characteristic that contributes to increased diversity and richness of arid plant species. Some of these elements are considered as relicts.
The region has a high number of endemic plants species (Parga-Mateos et al. 1996). The diversity of cacti is important in Hidalgo, because there are nearly 17 species in a very small area. It is also considered among the high priority areas for conservation of succulents (Oldfield 1997). Although this region is closely related to the Chihuahuan desert, it is physically separated from it by a series of pine-oak forest patches. This ecological "isolation" has favored the local speciation of some plants, and there are three endemic genera: Dyscrithothamnus (Asteraceae), Neoeplingia (Lamiaceae), and a genus of the family Verbenaceae (González-Medrano & Chiang-Cabrera 1988). Other endemic plants include Scutellaria molangitensis, Salvia hidalguense, Randia hidalguense, Bursera medranoana and Dalea zimapanica.
This region is considered among the richest zones in México in terms of its herpetofauna (Flores-Villela 1993; Challenger 1998); it is also a region of high reptilian endemism. In terms of the avifauna, arid regions such as this one are considered as some of the richest in endemic species (Escalante-Pliego et al. 1993). The Central Meseta Province in the state of Querétaro has nearly 100 species of birds, of which 6.5% are endemic (Navarro et al. 1993). CONABIO has identified a number of terrestrial priority sites in this ecoregion, including: Sierra de Álvarez and Sierras Santa Bárbara-Santa Rosa (Arriaga et al. 2000). A number of important areas for bird conservation have been identified in this ecoregion, including: Sierra de Santa Rosa, Reserva de la Biósfera Sierra Gorda, Sótano del Barro, El Zamorano, Sierra del Abra –Tanchipa, and Sierra Fría (Benitez et al. 1999).
Vast areas of this region have been modified and the habitat is partially destroyed in many areas. According to Challenger (1998) most of the region is perturbated and does not contain the original biota’s that are characteristic of this southernmost portions of the Chihuahuan desert (of which this region is derived). However, some protected areas have been created to preserve what remains of the original xeric scrub vegetation, and are contributing in the efforts to re-populate these areas.
Types and Severity of Threats
The main threats to this region are field clearing for agricultural purposes and for cattle, sheep and goat farming as well as grazing; urban growth is the second strongest threat. As a result of urban expansion into arid lands, erosion of the soil and water pollution are severely damaging the native habitat. In addition, many species of cacti have reached endangered status because they are intensely extracted from the region and illegally sold in public markets on both national and international levels. Agricultural activities displace many burrowing vertebrates (that build underground refuges), which make them more susceptible to predation. Cattle farming may also displace other vertebrates by directly competing with them. For example, sheep and goats compete with native vertebrates for water by eating water-storing cacti that the native fauna also need for their water supply (Oldfield 1997).
Justification of Ecoregion Delineation
This matorral ecoregion is limited to lower elevations and valleys of the sub-Sierra Madre Occidental and sub-Trans-volcanic belt of Mexico, and forms a coarse gradient with the Meseta Central matorral to the north. This unique matorral ecoregion is host to a number of endemic species (see description above for details). Delineation’s for this ecoregion follow INEGI (1996), from which we lumped the following vegetation classifications: "sarcocaulous matorral", "crasicaulous matorral", and portions of "agricultural landuse". Reference was also made to Rzedowski (1978), and reviews and revisions to the linework were done by expert opinion at several ecoregional priority setting workshops (CONABIO 1996 and 1997).
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Prepared by: Alejandra Valero, Jan Schipper, and Tom Allnutt
Reviewed by: In process