This Trans-Mexican Volcanic belt pine-oak forest ecoregion connects the Sierra Madre Occidental and Sierra Madre Oriental opening its species diversity to both eastern and western regions while serving as a center of diversity itself.
At the middle of this ecoregion the range is about 60 miles wide and 400 miles long and contains 13 of the highest peaks in Mexico. Some of these peaks rise into the Arctic Zone adding yet another habitat and increasing the possibilities of species diversity in this ecoregion. This ecoregion however also holds some of the most populated states in Mexico, including Mexico City.
Location and General Description
The Trans-volcanic belt is a large mountain range running from west to east in the central portion of Mexico.
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.
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. 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; it is recognized as an important center of taxonomic diversity. Nixon considers them as a center of diversity for the genus Quercus, and for the family Asteraceae, with 370 endemic species. Styles 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, 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.
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.
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 (4,000 kilometers (km)). These forests house some of the highest diversity of reptiles and amphibians in Mexico such as the Mexican beaded lizard (Heloderma horridum). 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.
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 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). 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.
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. 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, 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. 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.
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 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 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. Linework was reviewed by experts during ecoregional priority setting workshops in Mexico.
Additional Information on this Ecoregion
- For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
- To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion.
- World Wildlife Fund Homepage
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