In southern Arizona and northern Sonora, the neotropical vine snake (Oxybelis aeneus) lives in temperate oak woodland, canyon riparian woodland, desert-grassland, and pine-oak woodland communities at 1160 to 1650 m elevation. Perennial plants in an oak woodland habitat in the Atascosa Mountains in the center of the Arizona distribution of the species, have strong biogeographical affinities with the grasslands and woodlands of the southwestern United States and the northern Sierra Madre Occidental in Mexico. The plant communities are wholly different in vegetation structure and only 2.2% of the Atascosa site perennials reach tropical lowlands near Alamos in southern Sonora. The restricted area occupied by 0. aeneus in southern Arizona is characterized by relatively mild winter temperature and relatively high annual precipitation that falls mainly as summer monsoon rain. The northerly limit of Oxybelis in southern Arizona is apparently limited by winter freezing above 1650 m and by summer aridity below 1160 m. The northward range limit is associated with final failure of tropically-evolved morphology. physiological tolerances, and behavior under continental physiographic-climatic constraints.