The School Book of Forestry

by Charles Lathrop Pack

The leaves contain a material known as chlorophyll, which, in the presence of light and heat, changes mineral substances into plant food. Chlorophyll gives the leaves their green color. The cells of the plant that are rich in chlorophyll have the power to convert carbonic-acid gas into carbon and oxygen. These cells combine the carbon and the soil water into chemical mixtures which are partially digested when they reach the crown of the tree. The water, containing salts, which is gathered by the roots is brought up to the leaves. Here it combines with the carbonic-acid gas taken from the air. Under the action of chlorophyll and sunlight these substances are split up, the carbon, oxygen and hydrogen being combined into plant food. It is either used immediately or stored away for future emergency.

Trees breathe somewhat like human beings. They take in oxygen and give off carbonic-acid gas. The air enters the tree through the leaves and small openings in the bark, which are easily seen in such trees as the cherry and birch. Trees breathe constantly, but they digest and assimilate food only during the day and in the presence of light. In the process of digestion and assimilation they give off oxygen in abundance, but they retain most of the carbonic acid gas, which is a plant food, and whatever part of it is not used immediately is stored up by the tree and used for its growth and development. Trees also give off their excess moisture through the leaves and bark. Otherwise they would become waterlogged during periods when the water is rising rapidly from the roots.

After the first year, trees grow by increasing the thickness of the older buds. Increase in height and density of crown cover is due to the development of the younger twigs. New growth on the tree is spread evenly between the wood and bark over the entire body of the plant. This process of wood production resembles a factory enterprise in which three layers of material are engaged. In the first two of these delicate tissues the wood is actually made. The inner side of the middle layer produces new wood while the outer side grows bark. The third layer is responsible for the production of the tough, outer bark. Year after year new layers of wood are formed around the first layers. This first layer finally develops into heartwood, which, so far as growth is concerned, is dead material. Its cells are blocked up and prevent the flow of sap. It aids in supporting the tree. The living sapwood surrounds the heartwood. Each year one ring of this sapwood develops. This process of growth may continue until the annual layers amount to 50 or 100, or more, according to the life of the tree.

One can tell the age of a tree by counting the number of annual rings. Sometimes, because of the interruption of normal growth, two false rings may be produced instead of a single true ring. However, such blemishes are easy for the trained eye to recognize. Heartwood does not occur in all varieties of trees. In some cases, where both heartwood and sapwood appear, it is difficult to distinguish between them as their colors are so nearly alike. Because it takes up so much moisture and plant food, sapwood rots much more quickly than heartwood. The sapwood really acts as a pipe line to carry water from the roots to the top of the tree. In some of our largest trees the moisture is raised as high as 300 feet or more through the sapwood.

Strange though it may seem, trees fight with each other for a place in the sunlight. Sprightly trees that shoot skyward at a swift pace are the ones that develop into the monarchs of the forest. They excel their mates in growth because at all times they are exposed to plenty of light. The less fortunate trees, that are more stocky and sturdy, and less speedy in their climb toward the sky, are killed out in large numbers each year. The weaker, spindly trees of the forest, which are slow growers, often are smothered out by the more vigorous trees.

Some trees are able to grow in the shade. They develop near or under the large trees of the forest. When the giants of the woodland die, these smaller trees, which previously were shaded, develop rapidly as a result of their freedom from suppression. In many cases they grow almost as large and high as the huge trees that they replace. In our eastern forests the hemlock often follows the white pine in this way. Spruce trees may live for many years in dense shade. Then finally, when they have access to plenty of light they may develop into sturdy trees. A tree that is a pigmy in one locality may rank as a giant in another region due to different conditions of growth and climate. For example, the canoe birch at its northern limit is a runt. It never grows higher than a few feet above the ground. Under the most favorable conditions in Florida, where this species thrives, such trees often tower to a height of 125 feet.

In sheltered regions the seeds of trees may fall, sprout and take root close to their parent trees. As a rule, the wind plays a prominent part in distributing seed in every section of the country. Pine and fir seeds are equipped with wings like those of a bird or an airplane. They enable the seeds to fly long distances on the wind before they drop to the ground and are covered with leaves. Maple seeds fly by means of double-winged sails which carry them far afield before they settle. Ash seeds have peculiar appendages which act like a skate-sail in transporting them to distant sections. Cottonwood seeds have downy wings which aid their flight, while basswood seeds are distributed over the country by means of parachute-like wings. The pods of the locust tree fall on the frozen ground or snow crust and are blown long distances from their source. On the other hand, oak, hickory, and chestnut trees produce heavy seeds which generally remain where they fall.

Squirrels are the most industrious foresters in the animal world. Each year they bury great quantities of tree seeds in hoards or caches hidden away in hollow logs or in the moss and leaves of the forest floor. Birds also scatter tree seed here, there, and everywhere over the forests and the surrounding country. Running streams and rivers carry seeds uninjured for many miles and finally deposit them in places where they sprout and grow into trees. Many seeds are carried by the ocean currents to distant foreign shores.

The decay of leaves and woodland vegetation forms rich and fertile soils in the forests, in which conditions are favorable for the development of new tree growth. When living tree seeds are exposed to proper amounts of moisture, warmth and air in a fertile soil, they will sprout and grow. A root develops which pushes its way down into the soil, while the leaf-bud of the plant, which springs from the other end of the seed, works its way upward toward the light and air. This leafy part of the seed finally forms the stem of the tree. But trees may produce plenty of seed and yet fail to maintain their proper proportion in the forest. This results because much of the seed is unsound. Even where a satisfactory supply of sound fertile seed is produced, it does not follow that the trees of that variety will be maintained in the forest, as the seed supply may be scattered in unfavorable positions for germination. Millions of little seedlings, however, start to grow in the forest each year, but only a small number survive and become large trees. This is because so many of the seedlings are destroyed by forest fires, cattle and sheep grazing, unfavorable soil and weather conditions, and many other causes.

Beech and chestnut trees and others of the broad-leaved type reproduce by means of sprouts as well as by seed. Generally, the young stumps of broad-leaved trees produce more sprouts than the stumps of older trees which have stood for some time. Among the cone-bearing trees reproduction by sprouts is rare. The redwood of California is one of the few exceptions. The pitch pine of the Eastern States produces many sprouts, few of which live and develop into marketable timber.