In the drier forests, Pinus cembroides and Quercus grisea or Q . In areas between 1800-1900 m, Quercus coccolobifolia mixes with Pinus chihuahuana , Q. resinosa , Q. crassifolia and Q. viminea . In areas with deeper soil, Q. sideroxyla can also be found. The dry hillsides with a subtropical climate between 800-1900 m have open forests, in which the dominant species lose their leaves during the dry season. Among them are Acacia pennatula , Bursera spp.
Arborescent cacti are also dominant.
The great gorge of the Mezquital River functions as a biological corridor that provides entrance of tropical elements to the eastern side of the Sierra Madre Occidental, but also functions as the major north-south geographic barrier to various plant species of the sierra. The region's location makes it an important zone of confluence for boreal and equatorial genera, whereas at the species level it is an area where Mexican endemics predominate. Probably because of its physiography and location, it is a zone favourable to speciation - many plant genera are in active evolution. The zone is consequently a refuge for many endemic species of western Mexico and at least sixteen local endemics e.g. Eleocharis svensoniana , Muhlenbergia durangensis , M. michisiensis , Axiniphyllum durangense , Sabazia gonzalezae , Senecio gesneriifolius , S. gonzalezae , Tridax durangensis , Verbesina durangensis and Pavonia durangensis . Endemic species have been found and described in the last ten years but a good part of the region is still unexplored.