Biological diversity in the Madrean pine-oak woodlands
The Madrean Pine-Oak woodlands hotspot includes Mexico’s main mountain chains, namely the Sierra Madre Occidental, the Sierra Madre Oriental, the Trans-Mexican Volcanic Belt, the Sierra Madre del Sur, and the Sierra Norte de Oaxaca, as well as isolated mountaintop islands in Baja California (particularly around the Sierra de la Laguna). Although the vast majority of the hotspot’s 461,265 km2 lie within Mexico, several scatterend patches occur in the southwestern United States (represented by numerous Madrean Sky Islands, a series of about 40 mountain-tops in southern Arizona, Texas and New Mexico).
The complex geological history of the Madrean Pine-Oak Woodlands is evidenced by its rugged mountainous terrain, high relief and deep canyons. The Trans-Mexican Volcanic Belt, which runs from west to east across central Mexico, and serves as a bridge connecting the Sierra Madre Occidental and the Sierra Madre Oriental, is the highest mountain chain in the hotspot, including the peaks of Pico de Orizaba (5747 meters) and Popocatépetl (5452 meters). The climate of the hotspot is primarily temperate, with annual precipitation varying between 500 and 2500 millimeters (mm), largely depending on slope and aspect.Pine and oak forests are the characteristic vegetation type in the hotspot, ranging from monospecific stands of either pines (Pinus) or firs (Abies) to almost pure stands of oak (Quercus). In between these two extremes, different regions have varying combinations of species, with some more dominant than others. The pine-oak woodlands have an insular-type distribution by virtue of being surrounded by more extensive floristic provinces, generally tropical or arid. This feature is particularly noticeable in the northern Mexican Highlands and the Madrean Sky Island Archipelago.
Unique and Threatened Biodiversity
The Madrean Pine-Oak Woodlands are home to about 5300 species of flowering plants, a quarter of the Mexican flora. Although it is difficult to gauge the level of endemism because of the incompleteness of inventories, high-end estimates suggest that as many as 75 percent of these species may be found nowhere else on Earth. Mexico is an important center of diversity for both pines and oaks, with 44 of the 110 recognized pine species and over 135 species of oak, more than 30 percent of the world’s species in this genus. Of these oak species, more than 85 are endemic to Mexico. Two endemic species of oak are found only in the Sierra Madre Occidental, Quercus carmenensis and Q. deliquescens. Forests in the Baja California Peninsula have an important diversity of pine trees, including Pinus lambertiana, which can grow as high as 70 meters and produces pine cones 70 centimeters in length.
Roughly 525 bird species are found in the hotspot, of which more than 20 are endemic to the hotspot. Most of these endemics occur in the middle Sierra Madre Occidental, the Trans-Mexican Volcanic Belt and the southern Sierra Madre Oriental. Four Endemic Bird Areas, as defined by BirdLife International, overlap with the hotspot, and together support more than 15 restricted-range species, many of them threatened, including the short-crested coquette (Lophornis brachylopha, CR), blue-capped hummingbird (Eupherusa cyanophrys, EN), white-tailed hummingbird (E. poliocerca, VU), white-throated jay (Cyanolyca mirabilis, VU), bearded wood-partridge (Dendrotyx barbatus, VU), and dwarf jay (Cyanolyca nana, VU). Tragically, the imperial woodpecker (Campephilus imperialis, CR), currently listed as Critically Endangered and once the largest woodpecker in the world, is now almost certainly extinct, and the slender-billed grackle (Quiscalus palustris), once endemic to the marshlands around Mexico City, was driven to extinction early last century. Of the three bird genera endemic to the hotspot, two are monotypic: Euptilotis, represented by the eared quetzal (E. neoxenus), and Xenospiza, represented by the Sierra Madre sparrow (X. baileyi, EN). The third endemic genus, the remarkable Rhynchopsitta parrots, has two species, the thick-billed parrot (R. pachyrhncha, EN) and the maroon-fronted parrot (R. terrisi, VU).
Nearly 330 mammals are thought to occur in the hotspot, although only six are endemic. Even with so few endemic species, there are two endemic genera, both monotypic: Zygogeomys, represented by the Michoacán pocket gopher (Z. trichopus, EN), and Romerolagus, represented by the zacatuche, or volcano rabbit (R. diazi, EN). The volcano rabbit, which is endemic to the Trans-Mexican Volcanic Belt surrounding Mexico City, is one of the world’s smallest and most unusual rabbits. It has small, round ears and, unlike other rabbits, makes high-pitched penetrating sounds. The rabbit lives on the slopes of the Iztaccíhuatl and Popocatépetl Volcanoes and in 16 small fragments of habitat on the slopes of the Pelado Volcano and the Tláloc Volcano in the mountain complex south of Mexico City. Frequent burning of the zacatón bunchgrasses, which the rabbit uses to construct and protect its burrows, as well as ever-increasing encroachment of settlements from the expansion of Mexico City, pose a threat to the survival of this charismatic species.
More than 380 species of reptiles occur in the Madrean Pine-Oak Woodlands, nearly 40 of which are endemic. There is also one endemic reptile genus, Rhadinophanes, which is represented by a single species, the graceful mountain snake (R. monticola). Among lizards, there are 50 species of the genus Sceloporus, six of which are endemic, and 14 species of Anolis, two of which are endemic. The Colubridae family of snakes is also particularly well represented, with nearly 150 species, more than 15 of which are endemic.
Amphibian diversity in the hotspot is remarkable, with nearly 200 species, about 50 of which are endemic. Among frogs, there are 38 species of Hyla, with nine endemics, and 34 species of Eleutherodactylus, with seven endemics. Among salamanders, which are typically restricted to northern temperate zones, the hotspot holds 63 species in the family Plethodontidae, with 27 endemics, and 13 species from the family Ambystomatidae, with five endemics.
Surprisingly, given the “sky island” characteristic of much of the hotspot, there are more than 80 fish species present here, nearly 20 of which are endemic. This is in part due to the fact that Mexico’s overall fish fauna is extremely diverse, even at high elevations and in semi-arid regions. Many of these species come from the minnow family Cyprinidae. In central Mexico, the fauna is composed mostly of lake-adapted fishes in high-elevation lakes that were created by tectonic plate activity and volcanism. These species include several live-bearing fishes in the family Goodeidae and a number of silversides in the endemic Mexican genus Chirostoma.
The best-known invertebrates in the Madrean Pine-Oak Woodlands are the hotspot’s approximately 160-200 butterfly species, of which about 45 are endemic. Perhaps the most spectacular feature pf the hotspot's invertebrate fauna is the annual overwintering of millions of monarch butterflies (Danaus plexippus) in the pine forests of Michoacán. Each fall, about 100 to 150 million monarchs migrate south from eastern North America and form giant clusters on the boughs and trunks of trees in the oyamel fir (Abies religiosa) dominated ecosystem. Only about 30 of these overwintering sites exist, covering 10-25 hectares (ha) each. This wildlife spectacle, which attracts about 200,000 people per year, is a major economic boon to the communal landholders that manage the main butterfly sanctuaries in the Monarch Butterfly Biosphere Reserve. However, in January 2002, disaster struck, as more than 80 percent of the butterflies in some of the colonies died, and as many as 250 million dead monarchs littered the ground. Although the immediate cause of this devastation was a severe winter storm, deforestation has increased the susceptibility of monarchs to such storms by removing the forests that provide them with protection from cold and wet conditions.
Biogeography and evolutionary formation
The formation of this ecoregion is quite complex as expressed by its considerable geographical extent and involved topography consisting of numerous discrete mountain ranges. In portions of the ecoregion, the flora have a more tropical than temperate affinity, as measured by the family taxonomic level; for example the southern reaches of the Baja Peninsula illustrate such a tropical plant affinity. In elements farther north such as the southwestern USA, there is a more marked shift to temperate family affinity, such as the Huachuca Mountains of Arizona, where there is also a particularly high rate of endemism.
For the ecoregion as a whole there is a notably high endemism, particularly for some genera such as oaks. The region has been relatively stable in major tree dominance based upon pollen core records dating back at least 9000 years before present; over that range of time, the present palette of oaks and pines that are dominant have been relatively stable.
The principal threats to the forests of the Madrean Pine-Oak Woodlands are logging and the use of this region for drug trafficking and unlawful immigration. The exploitation of pines and, to a lesser extent, oaks for timber has increased, in some areas becoming indiscriminate. Additionally, many non-timber forest products are also being used unsustainably. For example the vascular epiphyte (Tillandsia usneoides) is extracted for Christmas ornamental purposes in Mexico, and a large variety of mushrooms in pine-oak forests, including species in the genera Amanita, Leccinum, Russula, and Boletus, are extracted for culinary use. Fire is partly a natural process in this ecosystem, but intentional burning to foster regeneration of fresh sprouts for expanded livestock grazing or to clear land for agricultural purposes is altering habitats throughout the region.
In southern Arizona, 90 years of fire suppression by federal and state agencies and modification of fuels by grazing and other causes, has led to conifer forests at higher elevations changing from open-grown ponderosa pine (Pinus ponderosa) to dense stands of mixed conifers.The fragmented nature of the hotspot makes it difficult to arrive at a reliable estimate of the amount of original vegetation remaining intact. Pine-oak forests once covered about 21 percent of Mexico, but these remaining forests now cover no more than about eight percent. Such estimates, however, fail to account for stands that have been impacted by fire and overgrazing, and so it seems reasonable to expect that no more than about 20 percent of the hotspot's original vegetation can be considered pristine.
Conservation Action and Protected Areas
Although some 27,000 km2 (around six percent of the hotspot’s land area) is under some form of protection, only 8900 km2 (two percent of the land area) are in protected areas in IUCN categories I to IV. The percentage of protected area is much higher in the USA than in Mexico. Most of the endemic terrestrial vertebrate species in Mexico that are still unprotected are located on the southern slopes of the Sierra Madre del Sur and, to a lesser extent, on the western slopes of the Sierra Madre Occidental and the eastern slopes of the Sierra Madre Oriental, making this hotspot an important conservation priority.
One of the most important protected areas in the hotspot is the 563-km2 Monarch Butterfly Biosphere Reserve in Michoacán, which was decreed in 1986. Other Mexican protected areas include the 1396-km2 Sierra de Manantlán Biosphere Reserve in Jalisco, which protects very diverse pine-oak forests, with some 33 species of Quercus. The largest protected area in the hotspot, and one of the largest in Mexico, is the 1774-km2 Cumbres de Monterrey National Park in the Sierra Madre Oriental. In the Sierra Madre Occidental, the 48-km2 Cumbres de Majalca National Park, the 58-km2 Cascada de Basaseachic National Park and the 93-km2 La Michilía Biosphere Reserve conserve notable stands of pine-oak formations. In Baja California, the 1124-km2 Sierra de la Laguna Biosphere Reserve was established in 1994 to protect an island of intact pine-oak vegetation.
On the American side of the hotspot, Big Bend National Park in Texas covers 3245 km2. Most of the pine-oak woodlands in the Madrean Sky Islands of the United States are protected, although the majority are in U.S. Forest Service land. Nevertheless, several smaller, highly protected reserves also occur, some operated by private owners, such as The Nature Conservancy, and others designated as U.S. National Monuments or U.S. Wilderness Areas. Several conservation NGOs are working in the Madrean Pine-Oak Woodlands, including the Fondo Mexicano para la Conservación de la Naturaleza, which is working to enhance social participation to prevent fires in key areas. PRONATURA/PRONATURA NORESTE, which was established in 1997, works in the Sierra Madre Occidental and Sierra Madre Oriental to conserve and promote the sustainable use of natural resources. The Sierra Madre Alliance, a network of Mexican and international partners working in the Sierra Madre Occidental in Chihuahua, aims to preserve biodiversity and restore the functioning of forested ecosystems through local participation.
In the United States, the Sky Island Alliance, formed in 1992, has worked with agencies in Mexico to create a Sky Islands Wildlands Network Conservation Plan to consider the needs of all stakeholders and formulate a framework for conservation in this region. An important private initiative has been the work of CEMEX to protect Madres del Carmen, in the northern part of Coahuila. Since 1999, the cement company has purchased, and effectively protected, 700 km2 of land, including several new pieces that bring El Carmen to the U.S. border and make it contiguous with Big Bend National Park in Texas. As a result, the two protected areas now form a 20,000 km2 conservation unit that includes some of the most important remaining tracts of the Chihuahuan Desert wilderness and significant portions of the Madrean Pine-Oak Woodlands Hotspot.
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