Sierra Madre pine-oak forests
The Sierra Madre Pine-Oak Forests are a biodiverse montane ecosystem located in southern Mexico, characterized by a mix of pine and oak trees along with a variety of unique species. This biome is noted for its high levels of endemism, with many species found nowhere else in the world, and serves as a convergence point for flora and fauna from North and Central America. The forests are divided into three distinct ecoregions: Sierra Madre Occidental, Sierra de la Laguna, and Sierra Madre Oriental, each supporting a rich variety of conifer and oak species.
Temperatures in this region typically range from 60 to 68 degrees Fahrenheit, with significant annual rainfall contributing to a humid climate. The diverse vegetation includes cloud forests and a variety of epiphytes, while the fauna boasts over 300 bird species and significant populations of mammals like jaguars and gray wolves. However, these forests face significant threats from human activity, including logging and agricultural conversion, which impact their ecological integrity. Concerns have also been raised regarding the potential effects of climate change, particularly for species that are highly sensitive to temperature variations and restricted to higher elevations. These factors contribute to ongoing conservation challenges in maintaining the biodiversity and health of the Sierra Madre Pine-Oak Forests.
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Subject Terms
Sierra Madre Pine-Oak Forests
Category: Forest Biomes.
Geographic Location: Mexico.
Summary: These forests represent a rich mixture of tropical and temperate species, some of them endemic.
The Sierra Madre Pine-Oak Forests biome is a montane ecosystem with high levels of biodiversity and endemism (species found nowhere else), and a convergence of species from North and Central America. It is possible to see jaguar stalking deer among lush oak forests, shrouded in orchids and other epiphytes (air plants). To the casual observer, these pine and oak forests of southern Mexico may not seem as lush as the tropical rainforests, and in plants that is certainly the case, but the Sierra Madres are a crossroad for many vertebrate fauna. Due to the strong elevational gradient, the region supports disproportionately high animal diversity.
This biome is made up of three geographically distinct ecoregions: Sierra Madre Occidental pine-oak forests, Sierra de la Laguna pine-oak forests, and Sierra Madre Oriental pine-oak forests. A total 27 species of conifer trees and 21 oak species have been recorded in these mountains, making them a center of diversity for these tree groups. However, oak and pine are highly desirable timber species, making this region one of the montane biomes most heavily affected by human activity. Impacting the forest areas are logging, conversion to agriculture and pasture, and broadening human settlement.
The climate is temperate and humid; temperatures average 60–68 degrees F (16–20 degrees C), and annual mean precipitation is 27–157 inches (700–4,000 millimeters).
Vegetation
Oak forests gradually merge into pine forest moving upward along an elevational gradient. Oak forests dominate at 4,921–8,202 feet (1,500–2,500 meters), transitioning to cloud forests at 7,546 feet (2,300 meters), and pine-oak forests and some dispersed pine clusters live at still higher elevations. The most representative families are Pinus, Quercus,Asteraceae, Fabaceae, Poaceae, and Euphorbiaceae. In total, 12 main types of oak trees are found within these conifer forests.
The more humid forest areas contain cloud forests and numerous epiphytes, among them Odontoglossum spp., Tillandsia prodigiosa, and Peperomia galioides; scrub (Eupatorium spp. and Ternstroemia pringlei); and herbs (Smilax moranensis, Spigelia longiflora and Salvia spp.). These forests house a mix of vegetation with neotropical and boreal elements, including Oreomunnea mexicana and Weinma.
The pine-oak forests in the northern reaches of the biome contain many endangered species, including trees such as Hickels fir (Abies hickelii) and Mexican Cyprus (Cupressus bethamii var. lindleyi) as well as many species of endemic ferns and water lilies.
Fauna
Among the many natural phenomena in these forests is the migration of the monarch butterfly (Danaus plexippus). Monarchs migrate from Mexico to the United States and Canada, and over three generations they make the round-trip journey back to the Mexican highlands to breed en masse.
Birds are among the most diverse vertebrate taxa here, including more than 300 species from the north and south, including the Mexican jay (Aphelocoma ultramarina), Mexican chickadee (Parus sclateri), maroon-fronted parrot (Rhynchopsitta terrisi), thick-billed parrot (R. pachyrhyncha), Strickland’s woodpecker (Picoides stricklandi), and Montezuma quail (Cyrtonyx montezumae). The imperial woodpecker (Campephilus imperialis), the world’s largest woodpecker, was once widespread in this biome, but today is thought to be extinct due to loss of old-growth habitats.
Among the mammals found here are large predators such as the jaguar (Panthera onca) and gray wolf (Canis lupis)—this is perhaps the only part of the world where these predators overlap—as well as white-tailed deer (Odocoileus virginianus), Buller’s chipmunk (Tamias bulleri), endemic Zacatecan deer mouse (Peromyscus difficilis), and rock squirrel (Spernophilis variegatus).
The number of reptile and amphibian species in this biome is quite high, including at least three species of rattlesnakes: rock rattlesnake (Crotlaus lepidus), twin-spotted rattlesnake (C. pricei), and ridge nose rattlesnake (C. willardi).
Environmental Threats
There are numerous threats to the native forest, including overgrazing, clearing of forests for timber and fuelwood, and human development. Because these forests are restricted to mountainous areas, there is growing concern that global warming will force the more temperature-sensitive species to move toward the summits until there is nowhere left to go. This is a special concern for the many species restricted to high-elevation areas, where slight changes in climate will have irreversible effects on restricted populations.
Bibliography
Avila-Gonzalez, Heriberto, Jesus Guadalupe Gonzalez-Gallegos, Guadalupe Munguia-Lino, and Arturo Castro-Castro. "The Genus Sisyrinchium (Iridaceae) in Sierra Madre Occidental, Mexico: A New Species, Richness and Distribution." Systematic Botany, vol. 47, no. 2, June 2022, pp. 319-34. DOI: 10.1600/036364422X16512564801641. Accessed 1 Sept. 2022.
Barbour, M. G. and W. D. Billings, eds. Forests of the Rocky Mountains. North American Terrestrial Vegetation. Cambridge, UK: Cambridge University Press, 1988.
Brown, David E., ed. Biotic Communities: Southwestern United States and Northwestern Mexico. Salt Lake City: University of Utah Press, 1994.
Collar, N. J., L. P. Gonzaga, N. Krabbe, A. Madroño-Nieto, L. G. Naranjo, T. A. Parker et al. Threatened Birds of the Americas. The ICBP/IUCN Red Data Book, Part 2, 3rd Ed. Cambridge, UK: International Council for Bird Preservation, 1992.
Stattersfield, A .J., M. J. Crosby, A. J. Long, and D. C. Wege. Endemic Bird Areas of the World, Priorities for Biodiversity Conservation. Cambridge, UK: BirdLife International, 1998.