392 P. S. MARTIN 



is found today in narrow, northerly ravines at 8,500 feet. These 

 outposts are in extremely favorable microhabitats, and vertical 

 displacement from a sheltered north slope at 8,500 feet to a level 

 site at 4,500 feet would require climatic change of greater magnitude 

 than the temperature depression encountered between these points 

 (average lapse rate of 0.6° C per 100 meters or a total drop of 7.2° C.) 



Mysteriously, spruce disappeared south of Chihuahua, Mexico, 

 in Postglacial time. Subalpine conifers immediately below treeline at 

 10,000 to 12,000 feet in the transverse volcanic belt of the Mexican 

 Plateau include Pinus hartwegii, Abies, and Cupressus. Superficially, 

 these boreal montane forests appear quite suitable for Picea, and 

 more than one biologist has referred to them casually as "spruce- 

 fir." 



The best record on climatic change in Mexico comes from the 

 sedimentary studies of Sears et al. (1955) and Hutchinson et al. 

 (1956). They demonstrate important climatic fluctuations. How- 

 ever, the correlation of moist climatic intervals in Mexico with 

 Cordilleran glacial advances (i.e., Flint, 1957, p. 233) is considerably 

 less secure than Glacio-pluvial correlations in western North Amer- 

 ica. Biogeographical evidence and climatological theory raise the 

 possibility that Postglacial pluvial periods in the Mexican Plateau 

 are negatively correlated with minor glacial advances at high latitudes 

 (Martin and Harrell, 1957). In the Thermal Maximum there is no 

 sound evidence of drought in the Plateau. 



The presence of Pleistocene spruce in the Valley of Mexico, the 

 biogeography of relict montane plant formations such as Cloud 

 Forest (see below), glacial circ depression on A/Iexican volcanoes 

 (White, 1956), Chirripo in Costa Rica (Weyl, 1955), and other 

 tropical mountains above 13,000 feet makes it convenient to infer 

 climatic cooling at low latitudes during the glacial period. The 

 presence of an extensive North American ice sheet would, however, 

 eliminate the present high-pressure system which brings summer 

 cyclones to Mexico and the Southwest (J. E. McDonald, personal 

 communication) and one wonders if winter Pacific storms would be 

 shifted sufficiently to produce truly pluvial conditions in the Valley 

 of Mexico in the Full-glacial period. Sears et al. (1955, p. 525) inter- 

 preted their Mexican diagrams as climatic oscillations of moist-warm 

 and dry-uncertain, the latter representing the Wisconsin glaciation. 



Within Mexico and Central America some of the strongest indi- 



