The Madrean pine–oak woodlands are subtropical woodlands found in the mountains of Mexico and the southwestern United States. They are a biogeographic region of the tropical and subtropical coniferous forests and temperate broadleaf and mixed forests biomes, located in North America. Conservation International estimates the woodlands' original area at 461,265 km². The woodlands are surrounded at lower elevations by other ecoregions, mostly tropical and subtropical deserts and xeric shrublands, forests, and grasslands. Woodland areas were isolated from one another and from the pine–oak woodlands of the Sierra Madre Occidental to the south by the warming and drying of the climate since the 1st century CE. Distribution Mexico The Madrean pine–oak woodlands are found at higher elevations in Mexico's major mountain ranges, the Sierra Madre Occidental, the Sierra Madre Oriental, the Trans-Mexican volcanic belt, the Sierra Madre del Sur, the Sierra Madre de Oaxaca, the Peninsular Ranges of the Baja California Peninsula. Madrean sky islands There are also approximately 27 enclaves in southern Arizona and New Mexico and in western Texas, where they are known as the "Madrean sky islands". The major Madrean "sky island" ranges in Arizona are the Dragoon Mountains, Chiricahua Mountains, Pinaleño Mountains, Santa Catalina Mountains, Rincon Mountains, Santa Rita Mountains, and Tumacacori Highlands. In New Mexico, the Sacramento Mountains and Guadalupe Mountains, which extend into Texas, as well as the Davis Mountains and Chisos Mountains, are also forested Madrean sky islands. Flora The pine–oak woodlands are composed of stands of oak (Quercus), pine (Pinus), douglas-fir (Pseudotsuga) and fir (Abies). The trees generally occur in mixed stands, though monospecific stands are sometimes found. The pine–oak woodlands are home to one-quarter of Mexico's plant species, and Mexico is home to 44 of the 110 species of pine and over 135 species of oak, over 28% of the world's oak species. Plant species descended from Madro-Tertiary flora, Madrean ancestor species, are an important element of the California chaparral and woodlands ecoregion. Madrean pine–oak woodlands ecoregions The World Wildlife Fund recognizes several distinct Madrean pine–oak woodlands ecoregions, based on geographic distribution and species mix. They include: Sierra Juárez and San Pedro Mártir pine–oak forests of the Sierra de Juárez and Sierra de San Pedro Mártir ranges of the northern Baja California peninsula. Sierra de la Laguna pine–oak forests of the Sierra de la Laguna in the southern Baja California peninsula. Sierra Madre Occidental pine–oak forests extend along the Sierra Madre Occidental