This Province occupies all of the Sierra Madre Oriental and, therefore, the southwestern part of the state.
The vegetation in general is Pine-oak Forest, in which the most common trees are Abies religiosa, Pinus flexilis, P. patula, P. montezumae, P. teocote, Populus tremuloides, Juniperus flaccida, Quercus arizonica, Q. clivicola and Q. polymorpha.
In his study of plants of the Gómez Farías area, Martin (1958) recorded several different types of vegetation, which in part can be placed in the Potosian Biotic Province, especially those types that occur to the northwest of the Cloud Forest. In addition to the Cloud Forest, Martin recognized Humid Pine-oak Forest, Dry Oak-pine Forest, Chaparral, Thorn Forest and Scrub, and Thorn Desert.
The only mammal endemic to the Potosian Province in Tamaulipas is Cryptotis pergracilis pueblensis. Other mammals that occur mainly in this Province are: Sorex saussurei; Notiosorex crawfordi; Glaucomys volans herreranus; Cratogeomys castanops planifrons; Perognathus nelsoni; Liomys irroratus alleni; Reithrodontomys fulvescens griseoflavus; Microtus mexicanus subsimus; Ursus americanus eremicus; Conepatus leuconotus texensis; and Odocoileus hemionus.
The fauna of this Province is a mixture of elements with tropical affinities on the east side of the Sierra Madre and with those of the Mexican Plateau on the west side.
Chihuahuan Biotic Province
This Province occurs in Tamaulipas only in a small portion of the Central Plateau physiographic region and occupies the southwesternmost part of the state.
The vegetation is of two types: Desert or Mesquite-grassland. The last is like that described for the Tamaulipan Biotic Province. In the Desert type the dominant plants are the cactus, Opuntia leptocaulis, and yuccas, Yucca filifera and Y. potosina. Subdominants are mariola, guayule, Agave lechugilla, A. stricta or Larrea divaricata. Along stream banks mesquite, Prosopis juliflora, can be found.
No endemic mammals of the Chihuahuan Province are known in Tamaulipas. Mammals that occur principally in this Province are: Dipodomys merriami atronasus; D. ordii durranti; Peromyscus melanophrys consobrinus; P. difficilis petricola; Onychomys torridus subrufus; and Neotoma albigula subsolana.