Sierra Juarez and San Pedro Martir pine-oak forests
The Sierra Juarez and San Pedro Martir pine-oak forests is a relatively small ecoregion, found in several disjunctive units within the northern part of the Baja Peninsula. Covering two mountain ranges, this area is rich in biodiversity and provides habitat for the threatened Bald eagle and California condor. Greater than average rainfall compared to Baja as a whole induces a Mediterranean climate here. This is one of the few places within Mexico where such a Mediterranean climate occurs; moreover, a number of endemic flora species area found within this dry forest ecoregion.
This ecoregion is codified as NA0526 by the World Wildlife Fund, and is within the Temperate Coniferous Forests biome. Displaying low faunal species richness, a total of 289 vertebrate taxa are found in the Sierra Juarez and San Pedro Martir pine-oak forests. The inaccessibility of the mountains has kept much of this regions habitat intact; however, the increasing price of timber is a serious threat to these pine forests.
Location and general depiction
The region is located in two important mountain ranges in the state of Baja California, Mexico: the Sierra de Juarez and the Sierra de San Pedro Martir. Both mountain ranges belong to the physiographical province of Baja California, and constitute the northernmost elevated peaks of the state. The mountainous range that descends into a large portion of Baja California becomes more abrupt at Juarez and San Pedro Martir; the eastern slope is steeper than the western. Altitudes range between 1100-2800 meters (m). The granitic mountains of Juarez and San Pedro Martir have young rocky soils and are poorly developed, shallow, and low in organic matter.
The climate is temperate subhumid with winter rains; its precipitation levels (400-700 millimeters per annum) are the second highest in the entire peninsula. The climatic conditions have led some researchers to consider the region among the few within Mexico with a Mediterranean climate. There are many rivers in this region which drain the mountain slopes (e.g. Las Palmas, Guadalupe, San Antonio, San Telmo and El Rosario).
The dominant tree species are Pinus quadrifolia, P. jeffreyi, P. contorta, P. lambertiana, Abies concolor, and Libocedrus decurren. The herbaceous stratum is formed by Bromus sp. and Artemisia tridentata. Epiphytes and fungi are abundant throughout the forests.
The region consists of islands of coniferous forests in an enormous desert of arid terrains; it thus contains a unique biota composed of many endemic and endangered species. Conifer forests in the Sierra de Juárez are among the most important of Mexico because of the diversity of pine trees they contain. At least ten species of Pinus inhabit the steep slopes of the mountains of Baja California. The conifer forests are unique to Mexico because they are the only multi-species Mediterranean-climate forests in the country. Not only are pine trees diverse in this region, but the forests of north Baja California constitute the only habitat in Mexico for the species Pinus contorta murrayana, which at the Sierra de San Pedro Martir reaches its most meridional limit of distribution. San Pedro Martir is also home to some of the largest pine trees in Mexico: the 70 meter tall Pinus lambertiana, with giant cones of 70 centimeters in length. Pinus lagunae, which is also a member of these communities, is restricted to the peninsula of Baja California. Pine-oak forests in general contain high numbers of restricted distribution vertebrate species. This ecotone between temperate forests and the desert represents an ideal habitat for many avian species; some birds consume the pinyons from pine trees and place them for the winter in underground stores, where misplaced or forgotten pine seeds find an ideal habitat to grow.
Characteristic mammals of the ecoregion include: Ornate shrew (Sorex ornatus), Puma (Puma concolor), Fringed Myotis bat (Myotis thysanodes), California chipmunk (Tamias obscurus), Bobcat (Lynx rufus), Coyote (Canis latrans), San Joaquin kit fox (Vulpes macrotis) and Bighorn sheep (Ovis canadensis).
Numerous birds are present in the ecoregion, including the rare Bald eagle (Haliaeetus leucocephalus), California condor (Gymnogyps californianus), Pinyon jay (Gymnohinus cyanocephalus), and White-breasted nuthatch (Sitta carolinensis).
A number of different reptilian taxa are found in these oak-pine forests; representative reptiles here are: the Banded rock lizard (Petrosaurus mearnsi); Common checkered whiptail (Cnemidophorus tesselatus), who is found in sparsely vegetated areas; Coast horned lizard (Phrynosoma coronatum), often found in locales of sandy soil, where individuals may burrow to escape surface heat; Night desert lizard (Xantusia vigilis), who is often found among bases of yucca, agaves and cacti; and the Baja California spiny lizard (Sceloporus zosteromus).
The Pacific chorus frog (Pseudacris regilla) is an anuran found within the Sierra Juarez and San Pedro Martir pine-oak forests as one of its western North America ecoregions of occurrence. The only other amphibian in the ecoregion is the Western toad (Anaxyrus boreas).
A great portion of Sierra de Juarez and San Pedro Martir coniferous forests are still intact, mostly due to the inaccessibility to the mountains. However, diverse threats such as cattle farming and intense fires have seriously deteriorated some patches of land. Fires in 1989 and 1996 consumed 70 square kilometers (km2) and 60 km2 of forest, respectively. The common mistletoe (Phoradendron flavescens) has widely invaded many of the trees in the forest. Many of the trees are very old (80 to 250 years), which contributes to their susceptibility to infection by various parasites. Some species of insects also build galleries or excavate holes in the tree cortex, which weakens the trees but also promotes invasion by other bark-eating insects. Cattle-grazing contributes to low rates of regeneration, and this aggravates the already serious threat that fires and parasites place on coniferous forests. The California condor (Gymngyps californianus) once soared above the mountains of north Baja California, but disappeared around 1940 due to disturbance of the habitat. Many species that inhabit these forests are endemic to the ecoregion.
Type and severity of ecological threats
The location of the temperate forests in the tall peaks of the mountains makes isolation from other similar ecosystems inevitable. The only patches of temperate forests close to this region are the pine-oak forests of Sierra de la Laguna, in the southernmost part of Baja California, and the pine-oak forests of the Sierra Madre Occidental Mountains. However, vast extensions of the Sonora and California desert stretch between these forests; thereby creating the only refuge for temperate species in a desert of arid lands. This increases the value of conserving of pine-oak forests and coniferous forests in west Mexico, because the forests represent a natural reservoir of unique animal and plant species. Approximately 37% of the pine-oak forests in Mexico have succumbed to agricultural pressure and logging.
The following ecoregions have some perimeter contact with the Sierra Juarez and San Pedro Martir pine-oak forests:
- Sonoran Desert, to the east
- Baja California Desert, to the south and surrounding southern disjunctive units of the pine-oak forests
- California coastal sage and chaparral, to the west and surrounding northern disjunctive units
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