Location and General Description
The Trans-volcanic belt is a large mountain range running from west to east in the central portion of Mexico. It is composed of igneous rocks (piroclastic and andesitic), and is home to the tallest peaks in Mexico: the Pico de Orizaba (5,747 m) and the Popocatépetl (5,452 m). Many valleys and basins divide the region, which is considered as a group of active volcanic mountains of recent origin. Intense volcanic and orogenic activity has allowed the formation of many fluvial deposits; the soils in this region are important in their ability to retain water. The forests play an important role as a "rain trap"; they contribute to the re-fill of underground aquifers that supply water to the nearby towns (López-García et al. 1996). The climate is temperate and the levels of humidity vary according to the altitude, with snow being common in the tallest peaks of the mountain range (>3000 m).
Pine forests grow at elevations of 2,275-2,600 m; pine-oak at 2,470-2,600 m; and pine-cedar at more than 2,700 m. Oak forests in the western portion of the mountains are composed of Quercus resinosa and Juniperus flaccida; in Jalisco (near the coast) the dominant species are Q. magnifolia, Q. conspersa, and Q. peduncularis. Quercus rugosa, and Q. laurina grow at more than 2,400 m. above sea level. The transition areas between pine-oak forests and tropical deciduous forests south of the Trans-Volcanic belt are composed of Quercus glaucoides (Rzedowski 1988). The eastern portion of the mountains, near the Valley of Mexico, is composed of Q. mexicana, Q. crassifolia, Q. laeta, and Q. deserticola. In the transition between the Trans-Volcanic belt and the more xeric vegetation of Veracruz, near the Papaloapan river, Q. glaucoides, Q. glaucophylla, Q. liebmannii, and Q. obscura are the dominant species. The pine forests are composed mainly of Pinus montezumae, although in more humid areas, P. pseudostrobus is the dominant species. In drier areas with more shallow soils, P. rudis and P. teocote are the dominant elements. At low elevations (below 2,000 m) P. oocarpa and P. michoacana are more common and are sometimes interspersed with P. herrerae, P. pringlei and P. leiophylla, along with some individuals of Quercus. Above 3,000 m throughout the Trans-Volcanic belt, the forests are a strict combination of Pinus hartwegii and Abies religiosa. The herbaceous stratum is well developed and epiphytes are abundant.
The Trans-Volcanic belt is the tallest mountain range in Mexico (Rzedowski 1988); it is recognized as an important center of taxonomic diversity. Nixon (1993) considers them as a center of diversity for the genus Quercus, and for the family Asteraceae, with 370 endemic species (Turner and Nesom 1993). Styles (1993) proposed that since these mountains connect the biotas of the Sierra Madre Occidental and Sierra Madre Oriental, they are crucial in the evolutionary history of the genus Pinus. The high levels of volcanic activity in the area gave rise to many microhabitats that allowed radiation and speciation of taxa (Challenger 1998), contributing to the high diversity which is characteristic of this region. Pine-oak forests are also important because most of the endemic vertebrates of Mesoamerica, restricted-range vertebrates, and plants of the family Lamiaceae, occur exclusively in pine-oak forests (Flores-Villela & Gerez 1994; Ramammoorthy & Elliott 1993).
The Sierra de Manantlan is part of the western zone of this ecoregion. This sierra is considered a center of diversification for families such as the Malvaceae, and possibly Compositae and Malpighiaceae. Endemic plants from the Manantlan Sierra include Zea diploperennis, Agrostis novogaliciana, Populus guzmanantlensis, Croton wilburii, Cnidoscolus autlanensis and Vernonia pugana. The discovery of Zea diploperennis, a disease-resistant relative of maize, in 1977 triggered the study of the vegetation in the Sierra (Cuevas Guzman et al. 1997).
The forests in the state of Michoacan are a unique point in the migratory route of the monarch butterfly (Danaus plexippus). These butterflies come to hibernate in this ecoregion, and undertake the largest migratory route of any insect (4000 km; Challenger 1998). These forests house some of the highest diversity of reptiles and amphibians in Mexico such as the Mexican beaded lizard (Heloderma horridum) (Flores-Villela 1993). The inter-montane lakes are a habitat for many endemic species of frogs and axolotls and constitute the center of diversity for the axolotls of Mexico (Robles-Gil 1993).
The pine-oak forests also contain a large number of endemic birds, more than most of the other ecosystems in Mexico (except for xeric scrubs). Stattersfield (1998) lists Sierra Madre sparrow (Xenospiza baileyi) and the green-stripe brush finch (Atlapetes virenticeps) as restricted to this ecoregion and Sierra Madre Occidental. The grey-barred wren (Campylorhynchus megalopterus), banded quail (Philortx fasciatus), common bobwhite (Colinus virginianus), great horned owl (Bubo virginianus), white-tipped dove (Leptotila verreaxi), long-tailed wood-partridge (Dendrortyx macroura), and montezuma quail (Cyrtonyx montezumae) are also present in this diverse ecoregion. This ecoregion also contains many endemic mammals, including the volcano rabbit (Romerolagus diazi), and the Mexican volcano mouse (Neotomodon alstoni) (Robles-Gil 1993). Other mammals include the puma (Puma concolor), bobcat (Lynx rufus), grey fox (Urocyon cinereoargenteus), Mexican cottontail (Sylvilagus cunicularis), and 50% of all Mexican. The region is important in terms of diversity and endemism levels, because they probably served as refuge for holartic species that were forced to migrate to temperate areas when the Central Mexican Plateau became drier and hotter, during the Pleistocene (Fa & Morales 1993).
Pine-oak forests of the Trans-Volcanic belt have been extensively exploited. The pine-trees of this region are the most appreciated for their wood quality, together with the pine trees of Sierra Madre Oriental and the Sierra Madre Occidental (Challenger 1998). The forests have been logged to clear land for agricultural purposes, but also to allow the building of highways and other roads. Species threatened due to selective exploitation include Cedrela odorata, Swietenia humilis, Fraxinus uhdei, Juglans major, Tilia mexicana, Abies religiosa, Guaiacum coulteri, Talauma sp. and Magnolia iltisiana.
Almost half of the total Mexican population lives in the states that are part of the Trans-Volcanic belt (Toledo et al. 1989), thus pine-oak forests in this region have suffered the effects of large urban congregations. The pine-oak forests of Mexico City, for example, have declined due to the effects of extremely high levels of pollution.
Throughout the whole area covered by the Trans-Volcanic belt, a number of places have been designated to protect the pine-oak forests; almost thirty areas exist, covering approximately 3.5% of the original extent of the forests (Challenger 1998). However, full representation of the characteristic plant and animal associations of the region can only be achieved through protection of a higher number of distinctive and large areas. One of the largest protected areas is the Sierra de Manantlan Biosphere Reserve (IUCN category V) with an area of 1,396 km2. This Reserve is the site for long-term ecological studies and conservation projects of Zea diploperennis and similar species as well as research on habitat, mammal and bird conservation (Reserva de la Biosfera Sierra de Manantlan 1990).
Types and Severity of Threats
The traditional practice of burning down most of the lower stratum of the forest to provide food for cattle is the most significant threat to pine-oak forests. Rzedowski (1988) considers this the main reason for the destruction of pine-oak forests in Mexico, and up to 80% of the area covered by them is burned for these purposes. As this precious habitat is destroyed, the risk or extinction for many increases. The abrupt topography of the region favors accelerated rates of erosion, a process that is all the more aggravated by removal of the plant coverage. Intense erosion of the mountain slopes, in turn, contributes to poor water retention, and therefore the loss of available water for the system. Human settlements and logging activities in the area threaten the complex composition of microhabitats that are important for endemic species.
Justification of Ecoregion Delineation
These montane pine and oak forests of the Trans-Mexican Volcanic Belt occur along ridge tops, valleys, and slopes in a patchwork distribution and are host to a number of endemic species. Linework for this ecoregion follows the INEGI (1996) current landcover maps, encompassing all "pine-oak forests", "oak with pine forests", and "pine forest" classifications within the Sierra de la Laguna region, as well as portions of "low open forest", "mesophyll montane forest", "low deciduous forest", "matorral", and agricultural activities which fall within these parameters. Classification and justification follow Rzedowski (19789). Linework was reviewed by experts during ecoregional priority setting workshops (CONABIO 1996 and 1997) in Mexico.
Challenger, A. 1998. Utilización y conservación de los ecosistemas terrestres de México. Pasado, presente, y futuro. Conabio, IBUNAM y Agrupación Sierra Madre, México.
CONABIO Workshop, 17-16 September, 1996. Informe de Resultados del Taller de Ecoregionalización para la Conservación de México.
CONABIO Workshop, Mexico, D.F., November 1997. Ecological and Biogeographical Regionalization of Mexico.
Cuevas-Guzman, R., Benz, B., and E. J. Jardel-Pelaez. 1997. Sierra de Manantlan Region and Biosphere Reserve. S. D. Davis, V. H. Heywood, O. Herrera-MacBryde, J. Villa-Lobos, and A. C. Hamilton, editors. Centres of Plant Diversity: A Guide and Strategy for their Conservation, Vol. 3 The America. IUCN, WWF, Oxford, U.K.
Fa, J.E. & Morales, L.M. 1993. Patrones de diversidad de mamíferos de México. In T.P. Ramamoorthy, R. Bye, A. Lot, and J. Fa, editors, Diversidad Biológica de México. Orígenes y Distribución. Instituto de Biología, UNAM. pp. 315-354.
Flores-Villela, O. 1993. Herpetofauna de México: distribución y endemismo. In T.P. Ramamoorthy, R. Bye, A. Lot, and J. Fa, editors, Diversidad Biológica de México. Orígenes y Distribución. Instituto de Biología, UNAM. pp. 251-279.
Flores-Villela, O., and P. Gerez. 1994. Biodiversidad y conservación en México: Vertebrados, vegetación y uso de suelo. Conabio and UNAM, México.
Goldman, Edward A. & Moore, Robert T. 1946. The biotic provinces of Mexico. Journal of Mammalogy. 26: 4: 347-361.
INEGI Map (1996) Comision Nacional Para el Conocimiento y Uso de la Biodiversidad (CONABIO) habitat and land use classification database derived from ground truthed remote sensing data Insitituto Nacional de Estastica, Geografia, e Informática (INEGI). Map at a scale of 1:1,000,000.
López-García, J., Manzo-Delgado, L.L. & Nava-Soria, P. 1996. Capacidad de carga , ecoturismo y desarrollo sustentable en la Reserva Especial de la Biósfera "Mariposa Monarca", Santuario Cerro Pelón (Ejido Capulín, Estado de México). Memorias del 1o. Simposio sobre protección en Areas Naturales Protegidas, 18-20 diciembre 1996. Valle de Bravo. PROFEPA, México.
Nixon, K.C. 1993. El género Quercus en México. In: Ramamoorthy, T.P., Bye, R., Lot, A. & Fa, J. (Eds). Diversidad Biológica de México. Orígenes y Distribución. Instituto de Biología, UNAM. pp. 435-448.
Perry, Jesse P. Jr., 1991. The Pines of Mexeco and Central America, Timber Press, Portland, Oregon.
Ramamoorthy, T.P. & Elliott, M. 1993. Lamiaceae de México: diversidad, distribución, endemismo y evolución. In: Ramamoorthy, T.P., Bye, R., Lot, A. & Fa, J. (Eds). Diversidad Biológica de México. Orígenes y Distribución. Instituto de Biología, UNAM. pp. 501-526
Reserva de la Biosfera Sierra de Manantlan. 1990. Retrieved (2001) from http://www.unep-wcmc.org/protected_areas/data/sample/0543q.htm.
Robles-Gil, P., Ceballos, G. & Eccardi, F. 1993. Mexican Diversity of Fauna. Cemex and Sierra Madre, Mexico.
Rzedowski, J. 1978. Vegetación de Mexico. Editorial Limusa. Mexico, D.F., Mexico.
Rzedowski, J. pers.comm. at CONABIO Workshop, 17-16 September, 1996. Informe de Resultados del Taller de Ecoregionalización para la Conservación de México.
Strattersfield, A.J., Crosby, M.J., Long, A.J., and Wege, D.C. 1998. Endemic bird areas of the World, priorities for biodiversity conservation. BirdLife International, Cambridge, UK.
Styles, B.T. El género Pinus: su panorama en México. In: Ramamoorthy, T.P., Bye, R., Lot, A. & Fa, J. (Eds). Diversidad Biológica de México. Orígenes y Distribución. Instituto de Biología, UNAM. pp. 385-408.
Toledo, V.M., Carabias, J., Toledo, C. & González-Pacheco, C. 1989. La producción rural en México: alternativas ecológicas. Colección Medio Ambiente, no. 6. Fundación Universo Veintiuno, México.
Turner, B. L. & Nesom, G. L. 1993. Biogeografía, diversidad y situación de peligro o amenaza de Asteraceae de México. In: Ramamoorthy, T.P., Bye, R., Lot, A. & Fa, J. (Eds). Diversidad Biológica de México. Orígenes y Distribución. Instituto de Biología, UNAM. pp. 545-562.
Prepared by: Alejandra Valero, Jan Schipper, Tom Allnutt, and Christine Burdette
Reviewed by: In process