In the development of disease, moisture is a contributing factor and, therefore, in cavities or underneath bandages where there is likely to be an accumulation of moisture, decay will do more damage than in places that are dry and exposed to the sun.
(2) Influence of soil: Soil is made up of fine particles of sand and rock and of vegetable matter called humus. A tree will require a certain soil, and unsuitable soils can be very often modified to suit the needs of the tree. A deep, moderately loose, sandy loam, however, which is sufficiently aerated and well supplied with water, will support almost any tree. Too much of any one constituent will make a soil unfit for the production of trees. If too much clay is present the soil becomes “stiff.” If too much vegetable matter is present, the soil becomes “sour.” The physical character of the soil is also important. By physical character is meant the porosity which results from breaking up the soil. This is accomplished by ploughing or cultivation. In nature, worms help to do this for the soil, but on streets an occasional digging up of the soil about the base of the tree is essential.
Humus or the organic matter in the soil is composed of litter, leaves and animal ingredients that have decayed under the influence of bacteria. The more vegetable matter in the humus, the darker the soil; and therefore a good soil such as one finds on the upper surface of a well-tilled farm has quite a dark color. When, however, a soil contains an unusual quantity of humus, it is known as “muck,” and when there is still more humus present we find peat. Neither of these two soils is suitable for proper tree growth.
Fig. 90.—A Tree in the Open. Note the full development of the wide crown with branches starting near the ground. The tree is the European larch.
(3) Influence of light: Light is required by the leaves in the process of assimilation. Cutting off some of the light from a tree affects its form. This is why trees grown in the open have wide-spreading crowns with branches starting near the ground as in [Fig. 90], while the same species growing in the forest produces tall, lanky trees, free from branches to but a few feet from the top as in [Fig. 91]. Some trees can endure more shade than others, but all will grow in full light. This explains why trees like the beech, hemlock, sugar maple, spruce, holly and dogwood can grow in the shade, while the poplar, birch and willow require light. It also explains why, in the forest, the lower branches die and fall off—a process known in Forestry as “natural pruning,” The influence of light on the form of trees should be well understood by all those who plant trees and by those designing landscape effects.
Fig. 91.—A Tree in The Forest. Note the tall stem free from branches and the small, narrow crown.