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

1. The differing abilities displayed by various Rocky Mountain coniferous trees to extend their ranges downslope into altitudes of increasingly hotter and drier climate has been attributed by some investigators to corresponding differences in the ability of their seedlings to tolerate heat, while others explain the same phenomenon on the basis of drought. In an effort to determine which of these is correct, greenhouse studies were made in which seedlings of trees with different altitudinal ranges have been compared in respect to their tolerance of high soil-surface temperatures and their tolerance of atmospheric and soil drought. 2. In general, it was found that the lower the altitudinal distribution of a species the greater the tolerance of its seedlings for high soil-surface temperatures. However, this factor apparently cannot explain the lower altitudinal limits, since (a) the maximum temperature level which each species can tolerate well exceeds the maximum temperatures obtaining in numerous micro-habitats below its altitudinal range; (b) a number of species, including subalpine fir, can be grown far below their natural lower limits without making compensation for the higher soil-surface temperatures there; and (c) the correlation between altitudinal distribution and temperature tolerance is not absolute, ponderosa pine having no greater temperature tolerance than Douglas fir, although it extends to distinctly lower altitudes. 3. When supplied with abundant soil moisture, even the subalpine trees can tolerate an intensity of atmospheric drought far in excess of that to which they are subjected under natural conditions. Furthermore, all species, regardless of altitudinal range, have approximately equal resistance to atmospheric drought. This aspect of drought therefore is apparently of no direct importance in determining the lower limits of the trees. 4. A distinct variation exists in the length of time over which the seedlings of different species can tolerate a lack of growth water; in general, the lower the altitudinal range the longer the period of soil drought which can be endured. Since all researches to date have indicated that the intensity and duration of drought increase downslope, this factor appears to offer at least a partial explanation of the differences in lower range limits. The significance of this difference among the species to endure soil drought is greatly magnified by the fact that the species of low altitude have the most rapid rates of root penetration. Not only are they better able to endure soil drought, but they are more likely to escape its influence. 5. The accuracy with which uniformity of environmental conditions can be maintained in different parts of the same culture varies in versely with the number of experimental plants used, and therefore conclusions based upon a few carefully tended plants are at least equally as valuable as greater numbers of plants but with conditions less uniform. In the temperature tolerance tests in which the fewest numbers of seedlings were used, the same conclusions may be drawn from either half of the lot of seedlings.