The Oak (Ward)/Chapter XI
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The Cultivation of the Oak, and the Diseases and Injuries to Which it is Subject
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THE CULTIVATION OF THE OAK, AND THE DISEASES AND INJURIES TO WHICH IT IS SUBJECT.
The oak has been cultivated in all kinds of ways, but by far the best timber is produced in what is called "high forest"—that is, the young trees all start at the same age and planted much closer together than they will be later on, their number being lessened period after period by successive removals until there is left a forest of large trees at equal distances. As it takes from 140 to 200 years to bring such a crop of timber to maturity, we may easily understand that such are rarely met with except as State forests, and the governments of various countries keep them going at various ages: one set of plantations will be ten, another twenty, a third thirty years old, for instance, when a given set is ready to be finally cut over for heavy timber.
There are many difficulties, however, in cultivating pure oak woods, and the custom of mixing other trees is a common one, for the young oaks need much light; and yet, if each plant has the space given it necessary to allow of this light, it grows into a short and spreading tree instead of rising up into a tall, straight one. The forester usually gets over these difficulties by planting beech, or silver fir, or some other species among the oaks, but in such a way that the oaks are never completely shaded by the other trees—that is to say, he keeps the trees at different ages, the beach, hornbeam, silver fir, spruce, etc., only being allowed to just close in the forest, leaving the leaf-crowns of the oaks to be fully exposed to the light above. The oak grows faster than the beech or spruce, for instance, while young, and so keeps its head easily above the others for a time. Very often the oak is cultivated pure at first, and then, when the oaks are becoming too crowded and he has to thin them, the forester puts in the silver fir or beech, which prevents the light coming in to the lower parts of the young oak-trees, and consequently prevents the development of lower branches, which would give the spreading, squat habit he wishes to prevent. For without light the leaves of the lower twigs of course can not make the materials to strengthen and thicken the latter into branches, and so they die off, and the trunk remains a straight, clean cylinder.
Although oaks are often raised from seed, a number of veteran trees being allowed to stand for many years in order to scatter the acorns, yet in by far the greater number of cases the plants are put in artificially, the long tap-roots being first cut in order to make them throw out lateral rootlets. It is also a common practice to cut back oaks, and allow them to sprout into what is known as coppice—that is to say, numerous buds which would not have developed at all are impelled to grow up into twigs and branches (stool-shoots) from the lower parts of the cut tree. It was very usual at one time to grow oak in this way for the sake of the bark, which was employed in tanning, the trees being cut back again and again, and renewing the coppice growth after each cutting.
There are various other modes of growing oak in forests, but, whatever the system employed, the following facts have to be borne in mind and provided for: The oak is a tree that requires a soil of great depth, and sufficiently open to allow of the free penetration of air and water to the subsoil; consequently many soils, otherwise rich enough, are unsuited for the culture of this tree. Again, young seedlings and plants are apt to suffer from frost unless they are protected by suitable mixtures of other plants; but such mixtures must be chosen properly, for this tree demands light and space to a degree greater than most other European trees except the larch, birch, and one or two others, and rapidly suffers if shaded or unduly crowded. Further, as compared with other European trees, the oak is a tree of the plains, and requires a relatively high temperature. These requirements also accord with its adaptation to deep, rich, well-drained soil, and, taking it all round, we have to regard the oak as a tree which makes considerable demands on the locality (soil and climate) where it grows. In return for this, however, it yields the best of all temperate timbers.
As we have seen, the forester has to exercise considerable forethought—the outcome of long experience—in growing oak so as to obtain long, clean stems. The natural habit of the tree is to form a short, thick bole and a widely spreading crown, the main branches of which come off not far from the ground. To compel the stem to elongate into a long pole he has to plant other trees with it (as we have seen, beech, spruce, etc.), which, while they keep the light off the lower parts of the oaks, do not overtop them. This makes the trees long and spindly at first, as they run up their leaf-crowns higher and higher, and it is part of the forester's art to select the exact time when he may cut away some of the nurse trees and let in just enough, and not too much, light and air, so that the crowns of the oaks shall fill out more and thicken the stems. For it must never be forgotten that the timber is laid on from substance prepared in the leaves.
The natural shape, so to put it, of an oak-tree is that of a wide-spreading, short-stemmed mushroom, and such a shape is realized in the open; the forester compels it to lengthen its stem as much as possible before he lets it extend its crown. Hence he aims at length first, and then lets the tree put on timber in the mass. He does this, of course, by taking advantage of the tree's peculiarities, and one of these is that it grows very rapidly when young. It will be obvious that the skilled forester also has to aim at getting as much timber as possible on the ground in a given time, and in the case of a tree like the oak his calculations have to be well made beforehand, for the tree may have to stand for from 120 to 200 years before it is cut. Left alone it may live for 1,000 years, but the proportion of good timber in trees after a certain age rapidly diminishes—a fact that has also to be reckoned with.
It is quite different, however, when trees are required for seed purposes. The oak hardly bears fruit at all before it is fifty to sixty years old, and seventy to eighty years is a better age for the purpose; but, as with other trees, to produce really good seed the oaks must be isolated, or nearly so, so that they get the maximum of light and air. Consequently a modification of procedure has to be made when seed-trees are required.
When the fruiting period has once been reached the tree goes on producing acorns every year; but it is noticed that heavy crops of good seeds only recur every five (or perhaps three) years or so, the yield in the intervals being inconsiderable. This is in accordance with Hartig's discovery that in the beech, for instance, the tree goes on storing up nitrogenous materials and salts of phosphorus and potassium during the first seventy or eighty years of its life, and then suddenly yields these stores to the seeds; the drain is so exhausting that it requires three to five years to re-store sufficient of these substances for another "seed-year." The season or weather is also concerned in the matter.
Of course there are very many other details to be considered in the technical cultivation of the oak, but enough has been said to give the reader a general account of the procedure, and I now pass to the subject of the dangers and diseases which threaten the tree at various periods in its development, and the timber afterwards.
The diseases and injuries to which the oak is subject are very numerous and various, although, compared with some other indigenous trees, it suffers remarkably little from the different dangers which await it at all stages in the course of its long life from the seedling to the aged tree. Some of these are referable to the exigencies of the non-living environments—the climate, soil, etc.; others are due to the attacks of living organisms, both vegetable and animal—from the weeds which smother the young seedlings by keeping the light from them, to man himself, who injures the trees in various ways. The earliest struggles of the young seedling are with the weeds, slugs, and insects of various kinds that invade the territory on which the acorn has germinated; and of course the baby plant has also to contend against any inclemencies of climate or unsuitableness of soil that it may meet with. Owing to such vicissitudes very many of the seedlings never obtain the dimensions of a plant at all, and in some seasons the mortality is enormous. Other destructive agents during these early phases of the life of the oak are cattle and deer, which not only tread down the shoots but also nibble them off, and mice, squirrels, etc., do their share of injury, as also do wood-pigeons and other birds. In the north of Europe the young plants suffer terribly from the ravages of a fungus named Rosellinia, the mycelium of which sends its branches into the roots and kills them, consequently entailing the death of the plant. The larvae of various insects also damage the roots and bring about injuries which may prove fatal. Cynips corticalis produces galls on the lower parts of the stems.
When the plant has passed into the condition of a sapling its dangers are for the most part of quite other nature, the injurious fungi especially being different. The chief diseases of the roots now arise from their spreading into unsuitable soil, the drainage of which may be incomplete, and thus bring about a sodden, acid, ill-aerated condition. The want of oxygen and the low temperature combine to kill the root-hairs and young rootlets, and the leaves above part with their water faster than it can be supplied from below, and they turn yellow and die off, the branches dry up, and the tree dies.
Other dangers arise from the persistent overshadowing of other trees, which slowly kill the young oaks by depriving their leaves of light; the offending trees playing the same inimical part, in fact, that grass and weeds, etc., play towards the small seedlings. Or the roots may be too thickly set in the soil if the trees are too crowded, and each suffers from over-competition with others.
Much mischief is effected by the attacks of insects of various kinds. The caterpillars of certain moths (especially Cnethocampa and Tortrix), for instance, eat off the loaves in June, and then form large masses of mingled débris, skins, etc., as they pass into the pupa stage in July. The denudation of the leaves brought
Fig. 40.—Tortrix viridana, the green oak-moth, the larvæ of which eat off the young leaves. (Altum.)
about by such caterpillars is apt to be very exhaustive to the trees, for although they put forth new foliage in July and August, it must not be forgotten that these new leaves are constructed from materials which should have gone to the general stores in the tree, and from which new wood, for instance, would have been developed.
Of other animals which injure oaks I may mention the various cattle, which bite off or rub the bark and buds; hares, squirrels, mice, etc., which nibble roots and buds and destroy the acorns, etc.; and a few birds; and certain beetles, which bore into the wood.
Among the pests belonging to the vegetable kingdom the following may be selected from a large number: The honeysuckle occasionally twists tightly round the young stem, and in course of time so compresses the cortex that the formative materials from the leaf-crown have to pass in a spiral course between the coils of the strangling plant, and the tightly-squeezed parts may be starved as the tree thickens, and even the death of the cambium may follow, especially if one or two of the honeysuckle coils come to lie nearly horizontally round the stem.
As a rare event the mistletoe is found on the oak. A much commoner parasite of the same family is Loranthus europœus, which does considerable damage to oaks in some parts of Europe. The sticky seeds are carried into the trees by thrushes. Here they germinate, and send their roots, or haustorial strands, into the cortex of a branch as far as the cambium, where they spread and feed on the contents of the young wood- and cambium-cells, causing malformations of the injured branch at the spot attacked, owing to the hypertrophy of the tissues, to which abnormal quantities of food materials now flow (Fig. 41); and frequently bringing about the death of the upper parts of the branches owing to the paucity of water at those parts, the parasite taking much of that which reaches the injured place, and the impoverished wood allowing less to pass than it would normally have done.
Among the fungi there are several enemies to the oak-tree. The leaves are attacked by Phyllactinia, one
Fig. 41.—Loranthus europœus. A. Lower part of stem attached to branch of oak, both denuded of cortex. B. Longitudinal section through one of the haustorial strands, showing its progress year by year, as the branch thickens. C. Transverse section, through a branch which has long been badly infested with the Loranthus; a a, dead remains of old haustorial strands; b b, young Loranthus plants developed as buds from the older ones. The asterisks mark still younger specimens. (Hartig.)
of the mildews, which forms white networks, like spiders' webs, on their surfaces. Numerous small ascomycetous fungi are found on the dying and dead leaves, but these do not directly injure the living tree.
Other fungi are found in the cortex, and one of the most interesting of these is a red Nectria, the spores of which germinate on the bark, but can not infect the tree unless there is a wound in the neighborhood. However, owing to the numerous small cracks and ruptures due to the injuries caused by insects, hail, frost, etc., the mycelium easily gains access to the cortex and cambium, and feeds on the contents of the cambium-cells, which it destroys. The consequence of the irritations set up is the formation of canker-like knots on the branches, and the injury may be great enough to destroy smaller ones, and occasionally even a large one.
Unquestionably the most important of the diseases to which the older oak-trees are subject are those which result in the destruction of the timber.
There are about six or eight of the fungi known popularly as toadstools — technically as Hymenomycetes — which are able to injure and even destroy the timber of standing oaks, and while each of these pests does the damage in its own peculiar way, they show considerable similarity in general behavior. In the first place, these fungi are unable to penetrate the bark of sound trees, and their hyphæ always gain access to the timber by means of actual wounds and exposed surfaces of wood, such as the cracks caused by frost or by the bending down of heavy branches under the weight of a load of snow, or the ruptured ends of broken branches blown off by strong gales or struck by falling trees, or places where animals have removed the bark, where cart-wheels have abraded the larger roots, and so on. Once inside, the hyphæ of these fungi pierce the vessels, cells, etc., of the wood by excreting soluble ferments which dissolve the substance of their walls, and feed on the products of solution. Hence they damage the timber in two ways—they riddle it through and through by myriads of minute apertures, and thus ruin its structure, and they
Fig. 42.—Piece of oak destroyed by Thelephora Perdix, showing the characteristic markings due to the action of the fungus. (R. Hartig.)
reduce its substance by dissolving it and converting it to their own uses. The wood, therefore, loses in strength and in weight, and becomes "rotten." There are differences in detail as to the mode of destroying the elements of the wood, but the final result is much the same in all cases: some of the fungi destroy the vessels, fibers, etc., by dissolving their walls from inside, while others destroy the part common to contiguous cells, etc., and thus first isolate the elements and then complete the destruction. A series of very interesting researches by
Fig. 43.—Oak timber destroyed by the fungus Hydnum diversidens: a shows the medullary rays on the tangential section; b, a mass of felted mycelium. (R. Hartig.)
Hartig has demonstrated that the presence of these timber-destroying fungi can be detected from the markings and discolorations they produce in the wood; those due to Hydnum diversidens, Thelephora Perdix, Polyporus sulphureus, P. igniarius, P. dryadeus, and Stereum hirsutum being all different, and in some cases so characteristic that the merest glance suffices to diagnose the disease (cf. Figs. 42 to 45).
There is yet another disease of oak timber to be noticed, and one which causes great havoc in buildings
Fig. 44.—Oak damaged by Polyporus igniarius, a very common timber fungus. (R. Hartig.)
where the ventilation is bad and the air damp. This is the too well known dry-rot, due to the destructive action of the fungus Merulius lacrymans, a hymenomycete allied to the preceding, but differing from them in not attacking the standing timber. The spores of this fungus are able to infect oak planks, beams, etc.; and the mycelium rapidly spreads on and in the wood, destroying the cell-walls, and causing the wood to shrink and crack and warp, and finally to fall to pieces. Thorough ventilation is fatal to the fungus and stops the rot.
Fig. 45.—Oak wood destroyed by Polyporus dryadeus, showing the very characteristic markings, like insect tunnels in a deep red brown matrix. (R. Hartig.)
Fig. 46.—Piece of oak-bark with fructification of Polyporus sulphureus.
Fig. 47.—Piece of oak attacked by Polyporus sulphureus, the yellowish white mycelium of which is seen at c and d. (R. Hartig.)
Fig. 48.—Transverse section of oak-timber destroyed by Polyporus sulphureus. (R. Hartig.)
Fig. 49.—Highly magnified longitudinal radial section of a piece of oak destroyed by a timber fungus, showing the ravages of the hyphæ in the various tissues. (R. Hartig.)
cells multiply and form the gall. This is a mere outline sketch of the matter, however, for the differences in behavior are enormous. Each insect causes the formation
Fig. 50.—Portion of the spore-bearing hymenium of Merulius lacrymans, the fungus of "dry rot."
of a specific kind of gall, differing in shape, size, color, and other characters from those caused by other gall-insects. There are many kinds, and only a few can be mentioned here. Each species of oak may have its own galls also, those on the American oaks differing from those on the European species, but some are common to more than one species. The insects which produce
Fig. 51.—An oak-leaf with several kinds of Cynips galls on it: a, gall produced by Cynips scutellaris; b, C. divisa; c, Neuroterus Reaumurii; e, Biorhiza renum; f, Neuroterus ostreus. (Frank.)
the commonest English oak-galls are nearly all members of the Cynipideæ, a group of hymenoptera which lay their eggs in the young tissues of various plants, especially oaks and roses.
Some of the resulting galls are discoid, such as the "oak-spangles" of our woods; others, again, are spherical, such as the common leaf-galls so well known in England, and the so-called oak-apple; then there are the "artichoke galls," produced by the partial metamorphosis of the buds of the oak in which the Cynips has laid its egg, and many others.