Our national forests

by Richard H. D. Boerker

Figure 48. The last stage of an insect-attacked tree. The tree is dead and the dry bark is falling off. Lassen National Forest, California. Photo by the author.

Probably the first external evidence of the attack of a bark beetle upon living trees with normal green foliage, is the presence of pitch tubes upon the outer bark. These are small, reddish-brown (later becoming grayish-white) masses of pitch and sawdust, which exude from the small cylindrical entrance made by the adult beetle where it bores through the bark to begin its egg tunnel. Each tube represents the entrance of one or more of these beetles. But we must follow these egg tunnels further, to learn how the actual damage is done to the tree. As soon as the bark beetle has made its entrance through the bark, it starts to work up through the live bark and cambium of the tree, forming a tunnel but little larger than the diameter of the beetle, which is known as the egg gallery, These egg galleries vary in shape from straight to winding, and in length from ten to forty inches. As a rule, male and female beetles work together in one gallery, and the eggs are deposited along the sides of the gallery, often in little pockets. When the tunneling and egg-laying process of the adult beetles is completed, their activity ceases, and they are usually found dead at the upper end of their galleries. The larvæ hatch and begin their work by burrowing across the cambium at right angles to the egg galleries. The complete girdling of the cambium layer is not accomplished until the larvæ have completed their work, and the numerous larval galleries, by joining one another, form a complete gallery around the cambium of the tree, thus cutting off the food supply which is made in the leaves of the tree, from the lower portion of the tree, namely the roots. Since the roots cannot live without nourishment, the tree dies. As soon as the larvæ have completed their development they pupate. Later they develop into adult beetles. These adult beetles issue forth in swarms the following spring, to attack new trees.

The control of insect pests is a difficult matter. On areas where insect depredations are conspicuous and are liable to spread to nearby valuable timber, control measures are undertaken in coöperation with experts from the Bureau of Entomology. In these control projects, crews of men fell the infested trees, strip the bark from them, and burn the bark (usually at a time of the year when the young broods of beetles are still in the bark, namely, fall or winter). Trap trees are sometimes resorted to. In this method, trees are girdled with an ax and thereby weakened to such a degree that beetles are attracted to it. After such a tree has become thoroughly infested in this manner, it is cut down and burned. In the case of a large, conspicuous infestation, an insect reconnoissance is made, in order to obtain an estimate of the percentage of trees that have been killed by insects. When it is possible, the timber is immediately sold. For example, on the Lassen National Forest, the writer several years ago made such an estimate of an infestation caused by the mountain pine beetle, covering over 100,000 acres. The reconnoissance showed that about 35 per cent. of the trees above 12 inches in diameter had been killed. The killed timber was subsequently utilized for telephone and telegraph poles.

There are many administrative measures which are practiced on the National Forests, which aim to prevent insect infestation. The prevention and suppression of forest fires, which form infection courts for insects, is probably the most important one. In all timber sales, old dead snags and slashing, which are breeding places for insects, are disposed of. Through free use and timber sales, insect-killed timber is disposed of and the loss due to insects is reduced to a minimum, besides in many cases destroying the young insect broods.

Tree Diseases. In almost every administrative district there is a Consulting Pathologist, connected with the Bureau of Plant Pathology of the Department of Agriculture, who has charge of all work dealing with the eradication of tree diseases.

A tree disease is really any condition that interferes with the normal functioning of the tree, be this condition caused by fungi, mistletoe, fumes, smoke, frost, sunscald, drought or excess of water in the soil. Parasitic fungi and mistletoes cause most of the tree diseases. Leaf diseases, by killing a greater part of the foliage, destroy the very organs in which food for the growing tissues is prepared. Diseases of the bark intercept the flow of food coming down in the bark from the leaves. Diseases of the sapwood cut off the water supply, which is pumped upward from the roots. Those that attack the roots also affect the water supply of the tree. Diseases of flowers and seeds destroy the faculty of reproduction.

Certain parasites are able to enter the youngest parts of trees, twigs and leaves directly, but the majority of the fungi causing decay of the wood can get into the interior of the living tree only by way of a pin knot or wound. For this reason, every wound caused by lightning, by fire, by man, or by animals, constitutes a menace to infection. Many coniferous trees cover their wounds by an aseptic coat of pitch, which is very effective in preventing the germination and growth of fungus spores. But the less resinous conifers and the hardwood trees do not cover their wounds very effectively; large wounds are not covered at all. Upon exposure by a wound, the sapwood just underneath the bark dies, dries out, and checks. Spores of parasitic fungi enter the cracks, germinate and infect the heartwood. The spores of a heartwood-inhabiting fungus cannot germinate and thrive unless they fall upon the heartwood of the tree. In this way certain diseases of the heartwood, which result in rot or decay, can very frequently be traced directly to fire scars, lightning scars, spike tops, broken limbs or branches, and other mechanical destruction caused by lightning, fire, storms, cloudbursts, or heavy snowfall.

Fire as a cause of wounds is responsible for more cases of heartrot than all other injuries taken together. For this reason the protection of forests from fire is the most important preventive measure that can be taken to eradicate tree diseases. In fact, the best way of controlling diseases is by preventing them, and the Forest officers are endeavoring to eliminate any danger to the health of the forest, to prevent the injury of the trees, and to establish healthy conditions for their growth. This is forest hygiene, and it bears the same relation to the trees and forests as personal hygiene and community sanitation do to persons and communities.

It is impossible to grow a sound and thrifty forest for future generations if there are unhealthful conditions in the forest that are a constant menace to the trees. The first step in this hygienic work is close observation on the part of the Forest officers. The next important step is to prevent the infection and infestation of sound trees by getting rid of all diseased and insect-infested living and dying trees. By means of timber sales and free use, Forest officers very materially help in establishing healthy conditions on the National Forests. There is a clause in most timber sale contracts which requires the cutting by the purchaser of all snags and other unhealthy trees on the area. This measure not only eliminates undesirable trees from a hygienic standpoint, but it also makes it possible to utilize the merchantable timber left in undesirable trees, which would otherwise go to waste. On timber sales, Forest officers who do the marking leave for reproduction only such trees as are perfectly sound and healthy. Mistletoe infested trees, especially, are marked for cutting, for neither in plant nor in animal life can healthy offspring be expected to develop under unhealthful conditions.