J. A. Zavala-Hurtado, P. L. Valverde, A. Díaz-Solís, F. Vite, and E. Portilla, “Vegetation-environment relationships based on a life-forms c1assification in a semiarid region of Tropical Mexico”, Rev. Biol. Trop., vol. 44, no. 2A, pp. 581–590, Aug. 1996.