In the development of disease, moisture is a contributing factor and, therefore, in cavities or underneath bandages where there is likely to be an acc.u.mulation 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 const.i.tuent 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 quant.i.ty 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.
[Ill.u.s.tration: 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 a.s.similation. 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.
[Ill.u.s.tration: FIG. 91.--A Tree in The Forest. Note the tall stem free from branches and the small, narrow crown.]
(4) Influence of heat: Trees require a certain amount of heat. They receive it partly from the sun and partly from the soil. Evaporation prevents the overheating of the crown. The main stem of the tree is heated by water from the soil; therefore trees in the open begin growth in the spring earlier than trees in the forest because the soil in the open is warmer. Shrubs begin their growth earlier than trees because of the nearness of their crowns to their root systems.
This also explains why a warm rain will start vegetation quickly.
Too much heat will naturally cause excessive drying of the roots or excessive evaporation from the leaves and therefore more water is needed by the tree in summer than in winter.
(5) Influence of season and frost: The life processes of a tree are checked when the temperature sinks below a certain point. The tree is thus, during the winter, in a period of rest and only a few chemical changes take place which lead up to the starting of vegetation. In eastern United States, growth starts in April and ceases during the latter part of August or in early September. The different parts of a tree may freeze solid during the winter without injury, provided the tree is a native one. Exotic trees may suffer greatly from extreme cold. This is one of the main reasons why it is always advisable to plant native trees rather than those that are imported and have not yet been acclimatized. Frosts during mid-winter are not quite as injurious as early and late frosts and, therefore, if one is going to protect plants from the winter"s cold, it is well to apply the covering early enough and to keep it on late enough to overcome this difficulty.
The mechanical injuries from frost are also important. Snow and sleet will weigh down branches but rarely break them, while frost will cause them to become brittle and to break easily. Those who climb and prune trees should be especially cautious on frosty days.
(6) Influence of air: On the under side of leaves and on other surfaces of a tree little pores known as _stomata_ may be found. In the bark of birch and cherry trees these openings are very conspicuous and are there known as _lenticels_. These pores are necessary for the breathing of the tree (respiration), whereby carbonic acid gas is taken in from the air and oxygen given out. The process of a.s.similation depends upon this breathing process and it is therefore evident that when the stomata are clogged as may occur where a tree is subjected to smoke or dust, the life processes of the tree will be interfered with. The same injurious effect results when the stomata of the roots are interfered with. Such interference may occur in cases where a heavy layer of soil is piled around the base of a tree, where the soil about the base of a tree is allowed to become compact, where a tree is planted too deep, or where the roots are submerged under water for any length of time. In any case the air cannot get to the roots and the tree suffers. Nature takes special cognizance of this important requirement in the case of cypress trees, which habitually grow under water. Here the trees are provided with special woody protuberances known as "cypress knees,"
which emerge above water and take the necessary air. See Fig. 18.
Conclusions: From the foregoing it will be seen that trees have certain needs that nature or man must supply. These requirements differ with the different species, and in all work of planting and care as well as in the natural distribution of trees it is both interesting and necessary to observe these individual wants, to select species in accordance with local conditions and to care for trees in conformity with their natural needs.
CHAPTER V
WHAT TREES TO PLANT AND HOW
The following cla.s.sification will show the value of the more important trees for different kinds of planting. The species are arranged in the order of their merit for the particular object under consideration and the comments accompanying each tree are intended to bring out its special qualifications for that purpose.
Conditions for tree growth in one part of the country differ from those of another and these lists, especially applicable to the Eastern States, may not at all fit some other locality.
TREES BEST FOR THE LAWN
DECIDUOUS
1. American elm (_Ulmus americana_)
One of the n.o.blest of trees. Possesses a majestic, wide-spreading, umbrella-shaped crown; is easily transplanted, and is suited to a variety of soils.
2. Pin oak (_Quercus pal.u.s.tris_)
Has a symmetrical crown with low-drooping branches; requires a moist situation.
3. European linden (_Tilia microphylla_)
Possesses a beautiful shade-bearing crown; grows well in ordinary soil.
4. Red maple (_Acer rubrum_)
Shows pleasing colors at all seasons; grows best in a fairly rich, moist soil.
5. Copper beech (_f.a.gus sylvatica_, _alropurpurea_)
Exceedingly beautiful in form, bark, and foliage and possesses great longevity and st.u.r.diness. It is difficult to transplant and therefore only small trees from 6 to 10 feet in height should be used.
6. Coffee tree (_Gymnocladus dioicus_)
A unique and interesting effect is produced by its coa.r.s.e branches and leaves. It is free from insects and disease; requires plenty of light; will grow in poor soils.
7. European white birch (_Belula alba_)
A graceful tree and very effective as a single specimen on the lawn, or in a group among evergreens; should be planted in early spring, and special care taken to protect its tender rootlets.
8. Gingko or Maiden-hair tree (_Gingko biloba_)
Where there is plenty of room for the spread of its odd branches, the gingko makes a picturesque specimen tree. It is hardy and free from insect pests and disease.
9. Horsechestnut (_Aesculus hippocastanum_)
Carries beautiful, showy flowers, and has a compact, symmetrical low-branched crown; is frequently subject to insects and disease.
The red flowering horsechestnut (_A. rubicunda_) is equally attractive.
[Ill.u.s.tration: FIG. 92.--A Lawn Tree. European Weeping Beech.]
10. Sugar maple (_Acer saccharum_)
Has a symmetrical crown and colors beautifully in the fall; requires a rich soil and considerable moisture.
11. Soulange"s magnolia (_Magnolia soulangeana_)
Extremely hard and flowers in early spring before the leaves appear.
12. Flowering dogwood (_Cornus florida_)
Popular for its beautiful white flowers in the early spring and the rich coloring of its leaves in the fall; does not grow to large size. The red-flowering variety of this tree, though sometimes not quite as hardy, is extremely beautiful.
13. j.a.panese maple (_Acer polymorphum_)