Fig. 64. Tolerant Maple. The trees are too slender to stand alone. U. S. Forest Service.
Fig. 65. Intolerant Aspen, a "nurse" of Tolerant Spruce. U. S. Forest Service.
This matter of tolerance has also much to do with the branching of trees. The leaves on the lower branches of an intolerant tree will not thrive, with the result that those branches die and later drop off. This is called "cleaning," or natural pruning. Intolerant trees, like aspen and tulip, Fig. 66, clean themselves well and hence grow with long, straight boles, while tolerant trees, like spruce and fir, retain their branches longer.
Fig. 66. Intolerant Tulip. Notice the long, straight boles. U. S. Forest Service.
The distribution of a species may also be determined by geographical barriers, like mountain ranges and oceans. This is why the western forests differ radically from the eastern forests and why the forest of Australasia is sharply distinct from any other forest in the world.
Any one or several of these factors, soil, moisture, heat, and light, may be the determining factor in the make-up of a forest, or it may be that a particular tree may survive, because of a faster rate of growth, thus enabling it to overtop its fellows and cut off their light. The struggle for survival is constant, and that tree survives which can take the best advantage of the existent conditions.
Besides these topographical and climatic factors which help determine the distribution of trees, a very important factor is the historical one. For example, the only reason by which the location of the few isolated groves of big trees in California can be accounted for is the rise and fall of glacial sheets, which left them, as it were, islands stranded in a sea of ice. As the glaciers retreated, the region gradually became re-forested, those trees coming up first which were best able to take advantage of the conditions, whether due to the character of their seeds, their tolerance, their endurance of moisture or whatever. This process is still going on and hardwoods are probably gaining ground.
Besides these external factors which determine the composition and organic life of the forest, the trees themselves furnish an important factor in their methods of reproduction. These, in general, are two, (1) by sprouts, and (2) by seeds.