Soil Temperature Versus Drought as a Factor Determining Lower Altitudinal Limits of Trees in the Rocky Mountains

Abstract

The 2 leading theories to account for lower altitudinal limits emphasize drought and high temp., respectively. In an effort to determine which of these is the more correct, greenhouse studies were made in which seedlings of trees with different altitudinal ranges were compared as to their tolerance of high soil-surface temp., of atmospheric drought, and of soil drought. Lethal surface temps. for high-altitude spp. began between 40-45[degree]C, while pinon was not killed by 70[degree]. Although temp. tolerance is related to altitudinal distribution, reasons are presented for the belief that this is not a critical limiting factor. All of the spp. tested had approx. equal resistance to atmospheric drought, and since the tolerance of all was relatively great, the importance of this factor also is minimized. Subalpine spp. proved incapable of enduring > 1 day with soil moisture at or below the wilting coeff., pinon survived this condition for approx. 12 days, and other spp. exhibited intermediate degrees of tolerance. The significance of this greater ability of low-altitude spp. to tolerate soil drought is greatly enhanced by the rapid rate of penetration of their tap roots, and it is concluded that differences in species'' adaptations to the soil drought factor is the most important factor limiting downward distribution.