Taylor, R.J., T.F. Patterson, and R.J. Harrod. 1994. Systematics of Mexican Spruce - Revisited. Systematic Botany 19: 47-59.
A combination of morphologic, phenolic and terpenoid variables were used in phenetic analyses to determine the patterns of similarity among Picea engelmannii var. mexicana; an undescribed population from Cerro Mohinora, Chihuahua; P. chihuahuana; the recently described P. martinezii (all from Mexico); and P. engelmannii, P. pungens and P. sitchensis from the United States. The Mohinora population loosely grouped with the var. mexicana population, suggesting inclusion in that taxon; both of these populations showed close affinities with U.S. populations of P. engelmannii, supporting retention of varietal status in P. engelmannii. Picea martinezii was similar to P. chihuahuana, but all analyses showed the two taxa to be clearly distinct, justifying the retention of P. martinezii as a separate species. Interestingly, P. engelmannii var. mexicana overlapped in morphologic and phenolic variation with P. sitchensis.