TEMPERATURE.
The fact that some trees, as paper birch and white spruce, grow only in cold regions, and some, as rubber trees and cypress, only in the tropics, is commonplace; but a fact not so well known is that it is not the average temperature, but the extremes which largely determine the habitat of trees of different kinds. Trees which would not live at all where there is frost, might flourish well in a region where the average temperature was considerably lower. On the other hand, provided the growing season is long enough for the species, there is no place on earth too cold for trees to live. Fig. 60.
Fig. 60. Northern Forest,—Young Spruce Growing Under Yellow Birch. Santa Clara, New York. U. S. Forest Service.
In general, cold affects the forest just as poor soil and drought do, simplifying its composition and stunting its growth. In Canada there are only a few kinds of trees, of which the hardwoods are stunted; south of the Great Lakes, there is a great variety of large trees; farther south in the southern Appalachian region, there is a still greater variety, and the trees are just as large; and still farther south in tropical Florida, there is the greatest variety of all. The slopes of a high mountain furnish an illustration of the effect of temperature. In ascending it, one may pass from a tropical forest at the base, thru a belt of evergreen, broad-leaved trees, then thru a belt of deciduous broad-leaved trees, then thru a belt of conifers and up to the timber line where tree life ceases. Figs. 61, and 62.
Fig. 61. Mixed Hardwoods on Lower Levels. Spruce and Balsam Dominate on Higher Elevations. Mt. McIntyre, Adirondack Mountains, New York. U. S. Forest Service.
Fig. 62. Scrub Growth on Mountain Top. Mt. Webster, New Hampshire. U. S. Forest Service.