CHAPTER VI.

NATURAL ENEMIES OF THE FOREST.

The natural enemies of the forest--as distinct from its human enemies--fall into three groups: (1) Meteorological, (2) Vegetable, (3) Animal.

METEOROLOGICAL FORCES.

_Wind._ "Windfalls" are not an uncommon sight in any forest. Frequently only small areas are blown down, one large tree upsetting a few others, or again a vast region is destroyed by great storms, Fig. 74. An area of many square miles in Florida covered with long-leaf pine was thus destroyed several years ago. The "slash" thus formed, when well dried, is particularly liable to catch fire and burn furiously. Windfalls are especially common among shallow-rooted trees, as hemlock, basswood and spruce, on sandy soil and on shallow soil underlaid with solid stone, especially where open spaces give the wind free sweep. It follows that an unbroken forest is a great protection to itself. The only precautions against wind therefore, that can be taken by the forester, are to keep the forest unbroken by selecting only the larger trees for felling or to cut down a given tract by beginning at the side opposite the direction of prevailing storms and working toward them.

In sandy regions, the wind does immense harm by blowing the sand to and fro in constantly shifting dunes, Figs. 75 and 76. These dunes occupy long stretches of the Atlantic coast and the shore of Lake Michigan. Such dunes have been estimated to cover 20,000 square miles of Europe. Along the Bay of Biscay in France, the sand dunes formerly drifted in ridges along the shore, damming up the streams and converting what was once a forest into a pestilential marsh. This region has been reclaimed at great expense by building fences along the shore to break the wind and thus keep the moving sand within limits. In this way a million acres of productive forest have been obtained.

On the other hand winds are beneficial to the forest in scattering seeds, weeding out weak trees, and developing strength in tree trunks.

_Drouth_ both injures the foliage of trees and causes defects in the grain of wood, the latter appearing as "false rings." These arise from the effort of the tree to resume growth when the water supply is restored. See p. 19.

_Water._ Certain trees have become accustomed to living in much water, as cedar and cypress have in swamps, and certain trees have become accustomed to periodical floods, but other trees are killed by much water. So when lumbermen make a pond which overflows forest land, the trees soon die, Fig. 77.

_Lightning_ frequently blasts single trees, and in dry seasons may set fire to forests. This is a much more important factor in the west than in the east,--in the Rockies, for instance, where there are electrical storms without rain.

_Fires_ will be considered later under man's relation to the forest.

_Snow and ice_ often bring serious harm to saplings by permanently bending them over, Fig. 78, or by breaking off tops and branches.

_Frost_ kills young plants; and sudden changes in temperature seriously affect grown timber, producing "frost checks" and "wind shakes." When there is a sudden fall in temperature, the outside layers of the tree, which are full of sap, contract more rapidly than the inner portions, with the result that the tree splits with a sudden pistol-like report, the check running radially up and down the tree. This is called a "frost check" or "star shake," Fig. 41._a_, p. 47, and such wounds rarely heal, Fig. 79.

On the other hand when the temperature rapidly rises, the outside layers of the tree expand so much more rapidly than the inside, that they separate with a dull muffled chug, the check extending in a circular direction following the annual rings. Such checks are often called "wind shakes" and "cup shakes," Fig. 41._c_, p. 47. These injuries are found in regions where sudden changes of temperature occur, rather than in the tropics or in very cold climates.

VEGETABLE ENEMIES.

Under this head may be classed, in addition to fungi, a number of unrelated plants, including such as: moosewood and dogwood, Fig. 80, which crowd out young trees; vines, like bitter-sweet, which wind about trees and often choke them by pressure, cutting thru the bark and cambium; saprophytes, which smother the foliage of trees, of which Spanish moss, Fig. 47, p. 201, is an example; and finally such parasites as the mistletoes, which weaken and deform the trees.

The most important of the vegetable enemies of trees are fungi. It should be remembered, however, that, without the decay produced by them, the fallen trees would soon cover the ground, and prevent any new growth, thus destroying the natural forest.

Every tree, as has been noted (p. 17), is composed of two parts, one part, including leaves, young branches, roots and sap-wood, living, and the other part, namely, the heart-wood, practically dead.

Fungi that attack the live parts of a tree are called parasites, while those that live on dead trunks and branches are designated as saprophytes. The line, however, between these two classes of fungi is not well defined, since some parasites live on both living and dead wood. The parasites are of first importance, for, since they kill many trees, they control to a large extent the supply of living timber.

Nearly all parasitic fungi have two portions, an external fruiting portion which bears the spores--which correspond to the seeds of flowering plants--and an internal portion consisting of a tangle of threads or filaments, which ramify the tissues of the tree and whose function is to absorb nutriment for the fungus. Fungi are classified botanically according to the spore-bearing bodies, their form, color, etc.

The parasitic fungi which are especially destructive to wood are those that have naked spores growing on exposed fruiting surfaces (the _Hymenomycetes_). In toadstools (the _agarics_) these exposed surfaces are thin, flat plates called gills. In the polypores, which include the shelf fungi, the spore surfaces are tubes whose openings constitute the pores. In the dry-rot, or tear fungus (_Merulius lacrymans_), the spore surfaces are shallow cavities.

Some varieties, called _true_ parasites, develop in uninjured trees, while others, called _wound_ parasites, can penetrate the tissues of trees, only where a cut or injury makes a suitable lodgment for the spores. Some fungi attack only a single species of trees, others whole genera; some attack only conifers, others deciduous trees, while a few attack trees of nearly all kinds alike.

Fungal spores when brought in contact with a wound on a tree or other suitable place, and provided with suitable conditions of growth, germinate, penetrate the tissues and grow very rapidly. These spores send out long threads or filaments which run thru the cells lengthwise and also pierce them in all directions, soon forming a network in the wood called the mycelium.

Rotting, in a large number of cases, is due to the ravages of fungi. This sometimes shows in the color, as the "red rot" of pine or the "bluing" of ash. Sometimes as in "pecky" or "peggy" cypress, the decayed tracts are tubular. More commonly the decayed parts are of irregular shape.

The decay of wood is due to the ravages of low forms of plant life, both bacteria and fungi.

A few of the more destructive forms may be noted.

_Trametes pini_ (Brot.) Fr. Foremost among the timber destroying fungi is the large brown "punk" or "conch" found in its typical development on the long-leaf and short-leaf pines, _Pinus palustris_ and _Pinus echinata_, Fig. 81. The fruiting bodies form large masses which grow out from a knot, oftentimes as large as a child's head. They are cinnamon brown on the lower surface, and much fissured and broken, on the black charcoal-like upper surface. This fungus probably causes four-fifths of the destruction brought about by the timber destroying fungi. It occurs on most of the conifers in the United States which have any value as lumber trees, and brings about a characteristic white spotting of the wood, Fig. 82, which varies with the kind of tree attacked. (Von Schrenk, _Agric. Yr. Bk._, 1900, p. 206.)

Of the shelf fungi, which project like brackets from the stems of trees, and have their pores on their under surfaces, one of the commonest in many localities is the yellow cheese-like _Polyporus sulphureus_, Fig. 83. This is found on oak, poplar, willow, larch, and other standing timber.

Its spawnlike threads spread from any exposed portion of cambium into the pith-rays and between the annual rings, forming thick layers of yellowish-white felt, and penetrating the vessels of the wood, which thereupon becomes a deep brown color and decays.

Of the umbrella-shaped gill-bearing fungi, a yellow toadstool, called the honey mushroom (_Agaricus melleus_), is a good example, Fig. 84.

This fungus, of common occurrence in the United States as well as in Europe, is exceedingly destructive to coniferous trees, the white pine in particular suffering greatly from its attacks. It also fastens upon various deciduous species as a parasite, attacking living trees of all ages, but living as well upon dead roots and stumps and on wood that has been cut and worked up, occurring frequently on bridges, railroad ties, and the like, and causing prompt decay wherever it has effected an entrance. The most conspicuous part of the fungus is found frequently in the summer and fall on the diseased parts of the tree or timber infested by it. It is one of the common toadstools, this particular species being recognized by its yellowish color, gills extending downward upon the stem, which is encircled a little lower down by a ring, and by its habit of growing in tufts or little clumps of several or many individuals together. It is also particularly distinguished by the formation of slender, dark-colored strings, consisting of compact mycelium, from which the fruiting parts just described arise. These hard root-like strings (called rhizomorphs) extend along just beneath the surface of the ground, often a distance of several feet, and penetrate the roots of sound trees. By carefully removing the bark from a root thus invaded the fungus is seen in the form of a dense, nearly white, mass of mycelium, which, as the parts around decay, gradually produces again the rhizomorphs already described. These rhizomorphs are a characteristic part of the fungus. Occurring both in the decayed wood from which they spread to the adjacent parts, and extending in the soil from root to root, they constitute a most effective agency in the extension of the disease. * * *

External symptoms, to be observed especially in young specimens recently attacked, consist in a change of the leaves to a pale sickly color and often the production of short stunted shoots. A still more marked symptom is the formation of great quantities of resin, which flow downward thru the injured parts and out into the ground. (_Forestry Bulletin_ No. 22, p. 51.)

Of the irregular shaped fungi, one of the most destructive is a true parasite, _i.e._, one that finds lodgment without help, called _Polyporus annosus_ and also _Trametes radiciperda_, Fig. 85. It is peculiar in developing its fructifications on the exterior of roots, beneath the soil. Its pores appear on the upper side of the fructifications. It attacks only conifers.

Its spores, which can be readily conveyed in the fur of mice or other burrowing animals, germinate in the moisture around the roots: the fine threads of "spawn" penetrate the cortex, and spread thru and destroy the cambium, extending in thin, flat, fan-like, white, silky bands, and, here and there, bursting thru the cortex in white, oval cushions, on which the subterranean fructifications are produced. Each of these is a yellowish-white, felt-like mass, with its outer surface covered with crowded minute tubes or "pores" in which the spores are produced. The wood attacked by this fungus first becomes rosy or purple, then turns yellowish, and then exhibits minute black dots, which surround themselves with extending soft white patches. (Boulger, p. 73.)

Of the fungi which attack converted timber, the most important is "dry rot" or "tear fungus" (_Merulius lachrymans_), Fig. 86. It flourishes on damp wood in still air, especially around stables and ill ventilated cellars. It gets its name lachrymans (weeping) from its habit of dripping moisture.

The fungus destroys the substance of the timber, lessening its weight and causing it to warp and crack; until at length it crumbles up when dry into a fine brown powder, or, readily absorbing any moisture in its neighborhood, becomes a soft, cheese-like mass. * * * Imperfectly seasoned timber is most susceptible to dry rot: the fungus can be spread either by its spawn or by spores, and these latter can be carried even by the clothes or saws of workmen, and are, of course, only too likely to reach sound wood if diseased timber is left about near it; but on the other hand dry timber kept dry is proof against dry rot, and exposure to really dry air is fatal to the fungus. (Boulger, p. 75.)

About all that can be done to protect the forest against fungi is to keep it clean, that is, to clear out fallen timber and slash, and in some cases to dig trenches around affected trees to prevent spreading or to cut them out and destroy them. Such methods have heretofore been too expensive to employ in any ordinary American forest, but the time is at hand when such action will prove profitable in many localities.

For the preservation of cut timber from decay, several methods are used. Fungi need heat, air, moisture and food. If any one of these is lacking the fungus cannot grow. Air and heat are hard to exclude from wood, but moisture and food can be kept from fungi. The removal of moisture is called seasoning, and the poisoning of the food of fungi is a process of impregnating wood with certain chemicals. Both these processes are described in _Handwork in Wood_, Chapter III.

ANIMAL ENEMIES.

The larger animals working damage to our forests are chiefly rodents and grazing animals. Beavers gnaw the bark, while mice and squirrels rob the forest of seed and consequently of new trees. The acorns of white oak are particularly liable to be devoured because of their sweetness, while those of red and black oak, which afford timber of comparatively little value, are allowed to sprout, and thus come to possess the land. Hogs annually consume enormous quantities of "mast," _i.e._, acorns or other nuts, by pasturing in oak and other forests. They, together with goats and sheep, Figs. 87 and 88, deer and cattle, work harm by trampling and browsing. Browsing destroys the tender shoots, especially of deciduous trees, but trampling entirely kills out the seedlings. The cutting up of the soil by the sharp cleft hoofs injures the forest floor, by pulverizing it and allowing it to be readily washed away by storms until deforestation may result, as was the case in France after the Revolution. It has cost the French people from thirty to forty million dollars to repair the damage begun by the sheep. In this country, this matter has become a very serious one on the Pacific Coast, where there are enormous flocks of sheep, and therefore the government is trying to regulate the grazing on public lands there, especially on steep slopes, where erosion takes place rapidly.[1]

[Footnote 1: The evils of grazing are increased by the fact that fires are sometimes started intentionally in order to increase the area of grazing land.]

The most destructive animal enemies of the forest are the insects. The average annual loss of trees in the United States from this cause alone has been estimated to be one hundred million dollars.

Insects have two objects in their attack on trees, one is to obtain food, as when they are in the larval stage, and the other is to provide for offspring, as do certain beetles.

The number of insect enemies of the forest is enormous. At the St. Louis Exposition, there were on exhibit nearly three hundred such insects. These belong to some twenty orders, of which the beetles (_Coleoptera_), which have horny wings and biting mouth parts, and the moths and butterflies (_Lepidoptera_), with membraneous wings and sucking mouth parts, are the most destructive. Insects attack every part of the tree, the seed, the shoot, the flower, the root, the leaf, the bark and the wood, both standing and cut.

Of the fruit and seed pests, the most destructive are weevils, worms and gall insects.

Of the twig and shoot pests, beetles, weevils and caterpillars are the worst.

Among insects that attack roots, the periodical cicada (17 year old locust) may be noted.

The leaf pests are far more serious. They include the true and false caterpillars, moths, gall insects and plant lice.

Of the bark pests, the bark beetles are the most destructive. These are also called Engraver Beetles from the smoothly cut figures which are their burrows under the bark, Figs. 89, 90, 91.

Many pairs of beetles make a simultaneous attack on the lower half of the main trunk of medium-sized to large trees. They bore thru the outer bark to the inner living portion, and thru the inner layers of the latter; they excavate long, irregular, longitudinal galleries, and along the sides of these at irregular intervals, numerous eggs are closely placed. The eggs soon hatch and the larvae at once commence to feed on the inner bark, and as they increase in size, extend and enlarge their food burrows in a general transverse but irregular course, away from the mother galleries (see illustration). When these young and larval forms are full grown, each excavates a cavity or cell at the end of its burrow and next to the outer corky bark. (Hopkins, _Agric. Yr. Bk._, 1902.)

Some of the species attack living trees, causing their rapid death, and are among the most destructive enemies of American forests.

All of the above indirectly affect both the quantity and quality of the wood supply. They can be studied more in detail in the publications of the U.S. Bureau of Entomology.

Of the insects directly attacking wood, the most important are the ambrosia or timber beetles, the borers, the ants, and the carpenter bees. The most remarkable feature of the beetle is the manner of its boring into the harder parts of the wood. Its jaws are particularly constructed for this work, being heavy and strong. The boring is done something after the manner of countersinking, and the jaws are believed to be self-sharpening, by reason of the peculiar right to left and left to right motion.

_Ambrosia_ or _timber beetles_, Fig. 92. This class of insects attacks living, dead, and felled trees, sawlogs, green lumber, and stave-bolts, often causing serious injury and loss from the pin-hole and stained-wood defects caused by their brood galleries. The galleries are excavated by the parent beetles in the sound sap-wood sometimes extending into the heart-wood, and the young stages feed on a fungus growth which grows on the walls of galleries. (Hopkins, Entom. Bulletin No. 48, p. 10.) The growth of this ambrosia-like fungus is induced or controlled by the parent beetles and the young are dependent on it for food. (Hopkins, _Agric. Yr. Bk._, 1904.)

There are two general types or classes of these galleries, one in which the broods develop together in the main burrows, the other, in which the individuals develop in short separate side chambers extending at right angles from the primary gallery, Fig. 93. The galleries of the latter type are usually accompanied by a distinct staining of the wood, while those of the former are not. (Hopkins, _Agric. Yr. Bk._, 1904, p. 383.)

_Bark_ and _wood borers_, Fig. 94. This class of enemies differs from the preceding in the fact that the parent beetles do not burrow into the wood or bark, but deposit their eggs on the surface. The elongate, whitish, round-headed (_Cerambycid_), flat-headed (_Buprestid_), or short, stout (_Curculionid_) grubs hatching from these eggs cause injury by burrowing beneath the bark, or deep into the sap-wood and heart-wood of living, injured and dead trees, sawlogs, etc. Some of the species infest living trees, Fig. 95, causing serious injury or death. Others attack only dead or dying bark and wood, but this injury often results in great loss from the so-called wormhole defects. (A. D. Hopkins, _Entom. Bull._, No. 48, p. 10.)

The pine sawyers are among the most troublesome pests in the mill yard, and their large, white larvae often do much damage to logs by eating great holes thru their solid interior. While burrowing in the wood the larvae make a peculiar grating sound that may be heard on quiet nights at a considerable distance. This is a familiar sound in the lumber camps of the North, and has probably given rise to the name of the pine sawyers by which these insects are known. (_Forestry Bulletin_, No. 22, p. 58.)

_Powder-post beetles_, Fig. 96. This is a class of insects representing two or three families of beetles, the larvae of which infest and convert into fine powder many different kinds of dry and seasoned wood products, such as hickory and ash handles, wagon spokes, lumber, etc., when wholly or in part from the sap-wood of trees. Oak and hemlock tan-bark is sometimes injured to a great extent, and the structural timbers of old houses, barns, etc., are often seriously injured, while hop poles and like products are attacked by one set of these insects, the adults of which burrow into the wood for the purpose of depositing their eggs. (Hopkins, _Forestry Bulletin_ No. 48, p. 11.)

_Timber worms_, Fig. 97. This class of true wood-boring "worms," or grubs, are the larvae of beetles. They enter the wood from eggs deposited in wounds in living trees, from which they burrow deep into the heart-wood. Generation after generation may develop in the wood of a tree without affecting its life but the wood is rendered worthless for most purposes by the so-called wormhole and pinhole defects resulting from their burrows. The same species also breed in the wood of dying and dead standing trees, and in the stumps and logs of felled ones, often for many years after the trees are felled. One species sometimes attacks freshly sawed oak lumber, new stave bolts, etc. They are among the most destructive enemies of hardwood forest trees, especially in reducing the value of the wood of the best part of the trunks. (Hopkins, _Forestry Bulletin_ No. 48, p. 10.)

The _carpenter worms_, Fig. 98. These are large pinkish caterpillars which are the larvae of stout-bodied moths. They enter the bark and wood of living oak, locust, poplar and other trees, from eggs deposited by the moths in the crevices of uninjured bark, or in the edges of wounds. They burrow deep into the solid wood, where they live for two or three years before transforming to the adult. The wood is seriously injured by the very large wormhole defects, and while the life of the tree is but slightly, if at all, affected by the earlier attacks, the continued operations of this class of borers year after year, finally results in the decay of the heart-wood, or a hollow trunk and a dead top. (Hopkins, _Forestry Bulletin_, No. 48, p. 11.)

_Columbian Timber-beetle_ One of the commonest wormhole defects in white oak, rock oak, beech, and tulip ("whitewood" or "yellow poplar") is one known to the lumber trade as grease spots, patch-worm, or black holes, Fig. 99, steam boats, Fig. 100, etc., caused by the Columbian timber beetle (_Corthylus columbianus Hopk_.) The characteristic feature of this wormhole defect, which will enable it to be readily recognized in oak and beech, is transverse series of two or more black holes about the size of the lead in an ordinary lead pencil, with a streak of stained wood extending with the grain two or three or more inches each side, as in Fig. 99. In quarter-sawed oak or split or sawed staves, a short longitudinal section of one of these black holes is seen attended by the stained streak on one side of a thick or curly growth or grain, Fig. 100. It is this form which is called "steamboats." In whitewood (yellow poplar) the black holes are attended by very long black, greenish, or bluish streaks, sometimes five or six feet long. When this is common in the lumber it is called "calico poplar." Fig. 101 represents the characteristic appearance of this defect greatly reduced. (Hopkins, _Agric. Yr. Bk._, 1903, p. 327.)

_Carpenter bees._ The work of this class of woodboring bees is shown in Fig. 102. The injury consists of large augerlike tunnels in exposed, solid dry wood of buildings and other structures. It is most common in soft woods, such as pine, poplar, redwood and the like. (Hopkins, _Agric. Yr. Bk._, 1904, p. 390.)

_Horn tails._ This is a class of borers which are the larvae of the so-called wood wasps. They may enter the exposed dead wood of wounds of living trees, but more commonly attack the wood of dead standing conifers and hard woods, in the sap-wood of which they excavate irregular burrows, which are packed with their borings. When the adults emerge they leave the surface perforated with numerous round holes. Water and fungi entering these holes cause a very rapid decay of the wood. (Hopkins, _Entom. Bull._ No. 48, p. 11.)

The tunnels of these various wood pests are most frequently to be seen in chestnut, ash, hickory, oak, tulip, and cypress.

One would think that with such an array of enemies, the forest would hardly survive, but on the other hand there are many enemies of these pests. The most destructive are the predaceous and parasitic insects. Many insects are simply predaceous, pouncing upon and destroying such other insects as they can overcome. Still others are parasites, some external, but most of them living within the bodies of their victims where they pass their entire larval life. The eggs are laid on or in the body of the victim, so that as soon as one hatches, it has suitable food. The ichneumon fly, Fig. 103, is such a parasite; it destroys millions of insect pests. It has a long and peculiar ovipositor with which it drills a hole into the tree and deposits the egg in a burrow of the Pigeon Horntail, a wood wasp that burrows into deciduous trees. The larva soon finds its victim, the grub of the Pigeon Horntail, and lives on it to its destruction.

It would seem that it is a hopeless task to control the insect enemies of forest trees and forest products or to prevent losses from their ravages, but the writer is informed by Dr. A. D. Hopkins, the expert in the Bureau of Entomology in charge of forest insect investigations, that the results of their investigations show conclusively that there are many practical and inexpensive methods of control now available thru the suggestions and recommendations in recent Department publications on forest insects, as well as thru direct correspondence with the Department. These methods are based on the principle of prevention and not on that of extermination. It has been shown that thru proper adjustment of the details in management of forests and of the business of manufacturing, storing, transporting, and utilizing the products a large percentage of the losses can be prevented at small additional expense, and that even when considerable cost is involved the amount saved will often represent a handsome profit.

THE NATURAL ENEMIES OF THE FOREST.

REFERENCES:[A]

(1) Meterological.

Pinchot, _Primer_ I, pp. 75-76. Roth, _First Book_, _pp._ 198-202. Bruncken, pp. 27-29.

Water. Roth, _First Book_, p. 27.

Snow, ice and frost. Pinchot, _Primer_, I, p. 76. Bruce, _For. and Irr._, 8: 159, Ap. '02.

(2) Vegetable.

Roth, _First Book_, p. 4. Boulger, pp. 70-75. Spaulding, _For. Bull._, No. 22. Ward, Chaps. V, VI, VII. Sickles, pp. 41-45. von Schrenck, _For. Bull._, No. 41, Pl. III. Sherfesee. _For. Circ._ No. 139. von Schrenck, _Bur. Plant Ind. Bull._ No. 36. von Schrenck, _Bur. Plant Ind. Bull._ No. 32. von Schrenck, _Agric. Yr. Bk._, 1900, p. 199.

(3) Animal.

Grazing. Pinchot, _Primer I_, pp. 69-73, II, p. 73. Pinchot, _Agric. Yr. Bk._, 1898, p. 187 Coville, _For. Bull._ No. 15, pp. 28-31. Roth, _First Bk._, p. 130, 178.

Insects. Comstock, passim. Hopkins, _Agric. Yr. Bk._, 1902, pp. 265-282. Roth, _First Book_, pp. 115-130. Howard, _Entom. Bull._, No. 11, n. s. Hopkins, Spaulding, _Entom. Bull._, No. 28. Hopkins, _Entom. Bull._, No. 48. Hopkins, _Agric. Yr. Bk._, 1903, pp. 313-329. Hopkins, _Agric. Yr. Bk._, 1904, pp. 382-389, Figs. 43-56. Pinchot, _Primer_, I, p. 73. Felt, N. Y. _State Museum Bull._, 103, Ent. 25. Hopkins, _Entom. Bull._ No. 32. Hopkins, _Entom. Bull._ No. 56. Hopkins, _Entom. Bull._ No. 58. Spaulding and Chittenden, _For. Bull._ No. 22, pp. 55-61.

[Footnote A: For general bibliography, see p. 4.]