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Transactions of the Highland and Agricultural Society of Scotland
On Thinning Plantations as applicable in Practical Forestry

By Christopher Young Michie, Forester, Cullen House, Cullen.
[Premium—Five Sovereigns.]

Practical forestry, I consider, may be defined as signifying the growing of the greatest quantity of the most valuable wood or timber, upon the smallest piece of ground, in the shortest period of time.

To grow a large quantity of wood is a very desirable thing, but the operations of forestry may and often are so conducted as to increase the quantity at the expense of the quality. We have all seen large bulky trees, so coarse and knotty and open in the grain, as to be unfit for almost anything. Scots fir, for example, grown on rich loam, or on certain descriptions of moss soil, is of such a rapid growth as to render the wood useless for anything except fuel. Quantity of wood is therefore no index to good forestry, but when combined with quality, the case is essentially altered. The largest and best are qualities sought for in a tree, and I hope to show how they are to be produced.

The size and quality of trees are in themselves very good, but wood as well as gold may be bought too dear, or cost too much, and if gold may be bought too dear, wood may also be grown at too great expense; and when this is the case, practical forestry is not successfully carried out. One thing above most others very materially influences the value of wood, that is the cost or value of the ground it occupies during its period of growth. If the ground, for example, at 10s. per acre can be made to grow as much timber of equal quality as another acre •can at 15s., it must appear evident that the former is the most profitable, and only such should be planted. As certain descriptions of ground, however, grow certain species of trees better than others, an important consideration here arises as to what species of trees to plant upon the different kinds of soil. The importance of this matter is paramount, because, in the first place, when once the trees are planted they in a sense grow of themselves, unaided by man. Hence it becomes every planter's duty to see well to it that only suitable trees are planted.

The length of time which a single tree or crop of trees takes to attain its highest possible value is an important matter, because if one forester can grow a crop of trees as valuable in fifty years as another can in sixty years, then ten years would be thereby gained, which would represent one-sixth of the producing cost, &c. When all these and other relative matters are kept distinctly in view, the operations of practical forestry become better understood, and the modes of carrying them out greatly simplified.

In addition to practical forestry, however, we have sheltering forestry, which consists principally of belts, stripes, groups, and single trees, the object of which is to produce warmth and shelter to animals in the fields, and to dry and ameliorate the climate. This branch of forestry is indeed more an auxiliary to farming, and a means of making farms pay, than that of producing profitable returns from planting. Indeed, trees grown either as small groups or narrow belts will not fulfil the conditions laid down for practical forestry.

We have also ornamental forestry, differing from both the other two in almost every respect. Ornamental forestry comprises hedge-row trees, certain lines and groups distributed over the landscape, and single trees so grown, either alone or combined, as to produce certain well-known but indescribable effects.

Experimental forestry is also another branch which embraces the pinetum, shrubbery, and certain departments of the nursery. It is designed to grow trees of new importation, to see what they will attain to, find out how certain species of trees enjoy or dislike each other's presence, how certain trees thrive in different kinds of soil, &c. These are all interesting, instructive, and important branches of forestry, and should each be studied and practised separately, and in accordance with their importance and worth.

In this paper, however, the writer intends to confine himself exclusively to that branch of practical forestry termed thinning. If thinning were rightly understood and properly attended to, pruning would be almost unnecessary, for it is either from superabundance of room on the one hand, or too little on the other, or from having stood too closely together at one stage of their growth and receiving too much thinning at another, that produces most of the necessities for pruning. I shall endeavour briefly to show how far and to what extent thinning is necessary to produce the desired results of practical forestry. I have stated (be it observed) that quantity is required—I mean quantity of timber, not number of trees, for while it is true that two sixpences are of value equal to one shilling it does not follow that two small trees are of equal value to one large one. Two trees containing 20 feet each may be of as much value as one containing 40 feet, but two trees containing only 10 feet each are not so valuable as one containing 20 feet.

The following is the system of thinning practised by the writer, and which, after a trial of many years upon an extensive scale, he finds attended with such favourable results as to inspire him with confidence to recommend it to others:—-

No. 1 is an enclosure of 500 acres, and between six and twelve years planted—the youngest part six and the oldest twelve. It is situated between 500 and 800 feet above the level of the sea, and covers both sides and top of the hill, which extends north and south. The exposure is severe on all sides, but most so on the north and east, which slopes towards the sea, which is distant about four miles. On that side all evergreens, including the Scots pine, are seriously injured by the sea air in winter.

The plants are composed of Scots pine, larch, and a slight mixture of Norway spruce and Silver fir.

The quality of the soil and subsoil varies considerably, but consists chiefly of a mixture of sand, clay, and gravel as the surface soil, and gravel, less or more inclined to moor pan, as a subsoil. The natural herbage consists of heath, some parts whins, with a profusion of those grasses indigenous to such soils.

The ground was all well drained previous to planting, and all well fenced, partly with turf dykes, partly with wire and posts, and partly with wooden paling and posts.

In consequence of the long interval between commencing to plant and finishing it, difference of quality of soil and exposure, there is a corresponding difference in growth all over the plantation. In some parts the trees are over 15 feet high, while in others they are scarcely as many inches.

With such an extent of plantation before him the writer had to consider well what course to pursue with the thinning. He saw that to allow such an extensive plantation as that to grow to any considerable height before thinning would be attended with disastrous consequences; and in order to avoid this, he commenced work on all parts where the trees had attained 6 to 8 feet high, and cleared openings or shooting roads, as they are termed. The roads are laid off 100 yards apart each way, thus dividing the whole plantation into squares containing 10,000 square yards each. The roading was done by contract, the open space specified to be 16 feet wide, and all trees, heath, and whins to be cleared off and thrown clear of the sides at least 3 feet. The trees, heath, and whins all to be kept separate, and laid in small heaps amongst the growing trees in such a manner as not to injure any of them.

The trees and whins to be cut level with the natural surface of the ground, and all heath and other herbage to be mown with the whin scythe and raked off. The work of roading was let in two separate contracts. By the one the work was done at three farthings per lineal yard, and by the other at one penny per yard. At the above rates the contractors earned for themselves, and those working under them, fair wages, say from 18s. to 20s. per week. The heath, whins, and thinnings were sought after by the tenant farmers and peasantry in the district, and carted off by them without making any charge. Having first determined upon the base line as the starting-point, the whole subsequent operations were carried on by means of the cross staff and poles used in land measuring. In the midst of a dense forest or large plantation like this, it is impossible to carry the lines straight and parallel without some squaring instrument, of which the cross-staff is the most convenient for practical use amongst common labourers. The reasons why it is desirable to have the roading done before thinning is commenced, is in order to guide the latter operations. Any one practically acquainted with thinning young plantations knows the unpleasant effects of a bewildering and interminable thicket, and the confinement and want of air and sunshine. I conclude that ten men do as much work in one month in a well-roaded and properly laid off plantation as they would do in six weeks in one without roads or openings. Another reason for making the roads the first work in connection with thinning is to afford facilities to the keepers for killing rabbits and game. I recommend making all the roads straight and parallel with each other, as far at least as the nature of the ground will admit. The advantages of the parallel straight roads are also experienced during the cover shooting, by affording the beaters the opportunity of coming into and keeping line at the end of every 100 yards, and the roads themselves determine the distances at which the guns are to be placed. Straight and parallel lines of roads are recommended, but cannot always be carried out in practice. Roads leading up hill must of course be curved or serpentine, and as no road can be led through a morass or swamp, or over a precipice, all such places must be avoided, and this cannot be done without deranging the plan of squares and straight lines. My plan is, when a deviation of road is unavoidable, to confine it to the one particular road, and not derange the general lines of all the roads.

The work of thinning is proceeded with according to the size of the trees; and here special attention should be paid to their height. As a rule I thin when the remaining crop stands 6 to 8 feet high, at which period they should be about 8 feet apart, or say 700 trees per acre. I know this practice is called in question by some, and condemned by others, and few approve or practise it. On these and other accounts I am all the more desirous of stating reasons, and giving my experience of the matter. First, an active man with a proper hand-bill (not hedge-bill), which is the only proper implement I know of for such work, will thin an acre in two days, and this represents six shillings per acre added to the original expenditure of forming the plantation, for until such time as actual income is derived from the plantation, the whole expenditure must be charged against it. The thinnings at this stage I put no money value upon. They are mostly collected and carted away by the tenantry and others; but the cost and labour of collecting and clearing are so great, that no money is charged for them except a small sum for such as are on the margin of the plantation, and convenient to clear. The second reason for thinning at this early period is, in order to give the remaining crop full freedom and liberty to develop their lower side branches. It is those branches situated upon the lower part of the stem of the tree that supplies food and nourishment to the roots, and unless they are preserved vital at this critical period of the tree's existence, it very soon ceases to develop itself and make wood. In fact, it ceases to grow to anything like satisfaction at that very period when it should be making wood faster than at any other period of its history. If only it could be kept in mind that the loss of every lower branch of a young tree is just a corresponding loss of roots, and as the one suffers so does the other, it would be a good and profitable thing for woodland proprietors.

To the preservation of the lower branches of the celebrated larch forests of the Duke of Athole, more than anything else, may be attributed their successful growth. The larch there were planted 6 feet apart, and that distance, admitting that all the trees grew, allowed all the lower branches to grow 3 feet in length all round ; but as many of them would no doubt decay, and from accidents and other causes perish, many of the trees would thereby produce their lower branches twice that length, hence the unparalleled results of the growth of the larch in these forests.

Having witnessed so much injury inflicted upon young plantations, and some entirely ruined, by the lower branches being interfered with at a stage of growth too early, I would recommend in the strongest possible terms, the special attention of all who have the management of plantations to this particular aspect of the subject. It is often asked what rule can be given, and how it may be known when either individual trees or plantations have the exact and proper quantity of branches upon them. The rule for this is, as far as any rule can be given, to maintain a due proportion of girth to the height of the tree, and these proportions are girth in inches to feet in height. For example, a tree 12 feet high, should girth a little above the swell of the root, 12 inches; a tree 24 feet high, 24 inches, and so on up to 30 or 35 feet in height, to a corresponding number of inches in girth. When trees attain the above height, thinning should be entirely discontinued, and frequently it should not be prolonged after the trees are from 20 to 25 feet in height, but allow the plantation to grow undisturbed (except by cutting down dead or decaying trees) till it is ripe for cutting down and clearing the ground.

That there is a danger of old trees having too many, as well as too few branches, I fully admit; but there is no danger of young trees having too many, and if the rule given should be observed, there will be no superfluity of branches at any time,, for if the proportional girth is too great, it can soon (if their are sufficient trees upon the ground) be reduced.

Having found out by careful inspection, that a well proportioned, and properly balanced tree measures as many feet in. height as it girths in inches, I was further anxious to know what proportional weight the branches of such trees bore to the stem on which they grew, and in order to ascertain this, I cut down, in the process of thinning, some specimen trees of the following species, and after carefully weighing them, I found the following results:—

The height and girth in the above table are not exactly proportional, as it is very difficult in making a selection to find them so, but the approximation is sufficiently near for the purpose of showing the requirements of practical forestry according to my definition. The form or outline of the tree up to that period when thinning should be discontinued should be conical or tapering, both in the stem and general form of the tree. After thinning is discontinued the shape of the tree alters, both in the stem and branches ; the latter wither and fall off, till only the top is covered, and the former gradually changes from a cone to a cylinder. The cause of the latter change is in the increase of woody deposit near to the live branches, and decrease of it when the branches have withered and fallen off. During this stage of growth, on dissecting a tree, it is found that the zones of wood near the vital branches are much thicker than at a distance from them, and the further distant the thinner they are. Every possible effort should be made in the thinning of plantations to preserve the proper quantity of live branches upon each individual tree, for if once the vitality is destroyed the best future skill will be defective in restoring it. Another reason for early thinning is in order not at any time to check the growth of the trees, for it must always be borne in mind that the immediate effect of thinning is to check the growth of the standing trees, and this is done in at least two different ways.

It is very consoling to be told that if thinning is done gently, no evils will result from it. The most gentle mode of thinning that can be practised is to cut down one tree that stands too close to another. If many such trees are cut, the thinning may be termed severe; and if only a few are cut, it may be termed gentle. "With words and terms, however, we may do as we please, but with the effects produced upon the standing tree, by removing one that stood close by it, and for whose sake it was cut down, we have some inquiries to make. When two trees have grown up side by side, for many, or it may be only a few years, they have formed such an affinity for each other that separation becomes a painful ordeal, so far as trees can be supposed to sympathise with, or feel, which for my part I do not believe in to the same extent as some do. Whether trees are affected through feelings or not is of little importance, since it is certain they are influenced in two other ways at least—namely, mechanically and chemically. That thinning injuriously affects trees both ways there is little or no doubt; and we shall first see bow they are injured mechanically. When two trees grow near each other the branches on the confronting sides are less developed than on the opposite sides, and the roots underneath are developed in a corresponding manner; and if one of the trees is removed by thinning, the whole of the weak side of the remaining tree is exposed suddenly, and the wind acting upon it strains the tender weak roots to such an extent as often to uproot the tree altogether. But apart from actually uprooting and blowing over the tree, the roots are strained and fractured so severely that they lose their vitality. Any injury inflicted upon resinous trees, whether upon branch, stem, or root, is succeeded by an accumulation of rosin, and this, when it occurs to the roots, is fatal to them, as it obstructs the sap vessels and stops the circulation.

Prom observing the sickly aspect of a very extensive plantation after being thinned, and being called upon to assign a cause for it, which I was at the time unable to do, I turned my attention exclusively to the subject, and, after much labour, concluded I had solved the problem. The plantation consisted principally of Scots pine, with a mixture of larch and spruce and some hardwoods.

All species of trees did not present the same sickly appearance, nor did the same species on all soils.

The Scots pine was of all others the most sickly, especially upon the hard gravelly soils, and the larch least affected of any. Of all forest trees the Scots pine is the most impatient of any artificial interference, and suffers more from thinning than any other species. In this case, after thinning the trees lost their natural dark-green colour, and assumed a faint light-green. The leaves became shorter, and presented a clustered appearance. Some foresters, on giving their opinion upon it, said it required more thinning; that the crop was too great for the poor gravelly soil to bear. Some thought it blighted by some atmospherical influence, and others said it had attained maturity, and ought to be cut down.

After weighing all arguments, and duly examining the whole case, I came to the conclusion that the thinning, and thinning alone, had done the mischief. From a very wonderful provision in nature, the branches of a tree are so spread out as to shade the surface of the ground underneath which the delicate tender roots are spread, and thus keep them uniformly cool, and never at any time scorched or unduly heated by the direct rays of the sun. A plantation, therefore, like that under consideration, which had grown up till about forty years old without ever receiving a regular thinning, may well be understood to have so far adapted itself to its circumstances as to be seriously and injuriously affected by any change such as thinning. The trees —including stem, branch, and root—were what may be termed acclimatised, or rather habituated, and therefore thinning produced a change upon them equivalent to removal to a different soil and climate.

There is much said about acclimatising of plants, which applies only to that part above ground, but there is little or nothing said about the roots of the plants, although the latter are equally as important as the former.

Now, if a pine or fir plantation such as this, grown upon a dry gravelly soil, with the roots extended and ramified all over, and within an inch of the surface of the ground, is it either unreasonable, strange, or unlikely, that, when a sudden opening is made by cutting one or more trees, and letting in a stream of sunny rays to heat or scorch the delicate, tender, sensitive roots, so long nursed, protected, and shaded under a canopy of branches— is it strange, I would again ask, that a change should take place with the roots of the trees, or that the heat of the sun should crystalize the fluids in the roots, and stop the flow of sap which was wont to nourish the tree? To this chemical change in the roots I attribute the sickly appearance referred to.

Another Scots fir plantation was thinned at about thirty-five years' growth, and had not been thinned during the preceding fifteen years. After thinning it became sickly and death-like, and but for the important place it occupied in the landscape would have been all cut down. It however was allowed to stand, and after the fourth or fifth year began to assume its natural colour, and is now in a fair state of health.

After a few years the trees usually recover, as the result of having made new roots suited to their new condition of life; but while some recover, others go back and perish altogether.

From the foregoing results, it must appear obvious that thinning is a very delicate and precarious operation, and is attended with much danger and risk to a crop of trees. If thinning could be entirely dispensed with, so much the better, and in the case of natural forests, where no artificial thinning has ever taken place, there are to be found many hundreds of acres of wood which no artificial forest or plantation can compare with in point of value. Any one who has examined the forests on Deeside, on Balmoral, Invercauld, and Mar Estates, or Rothiemurchus, Glenmore, and Abernethy, on Speyside, and many others both at home and abroad, will support that testimony. One piece of a few acres on Rothiemurchus estate is worthy of special notice. When I examined it a few years ago, the trees stood on an average 9 feet apart—some of them as wide as 15 feet, and others as close as 2 feet. The market value of it per acre at the time I saw it, allowing the trees to be all sound—which I am certain they were not—was at least £300 per acre. The ground itself is the poorest possible—a light, sandy gravel, with a crisp dry herbage of heath and moss, certainly not worth over 1s. 6d. per acre per annum for grazing purposes.

The question here arises, How are plantations to be managed that have been so thickly planted as to require thinning to prevent the trees from growing up disproportionately small? The answer is, thin early enough, and complete the operation before the side branches touch each other, and before any of them decay. This is advisable, not only for the preservation of the branches themselves, but in order that no unfavourable change be produced upon the roots of the trees, by admitting a degree of heat and air amongst them to which they have not been accustomed, and which they cannot endure.

In all forest operations by far too little attention is paid to the roots of the trees. They are often planted in soil so saturated with water that no air can penetrate it, and reach the rootlets—hence the lingering and sickly state the trees remain in for years after being planted; and it is only after they grow to such a height that the action of the wind upon the trees shakes and loosens the soil that they begin to grow freely.

Most plantations under thirty years old may be benefited by thinning if the trees have sufficient branches; but where they have stood so closely together as to destroy their vitality to two-thirds their entire height all hopes of restoring them is at an end, and if any thinning is done in such cases it does not improve but injure the crop, by retarding its growth.

All thinning, I would again repeat, should be commenced before the side branches touch each other, and continued till the trees stand about 8 feet apart, after which they may very safely be handed over to Nature to perfect and complete their growth, the forester meanwhile only to cut and remove dead or sickly trees till the crop is ripe, when it should be cut, cleared, and the ground replanted.

All that has previously been said about thinning applies only to pine and fir plantations; and now a few words about hardwoods.

I have an extensive hardwood plantation under my care which had never been thinned up to twenty years old. It consists of oak, ash, elm, beech, sycamore, lime, &c, At the age of twenty it received a moderate thinning, and from that time to the present (twenty-five years) it has received every possible attention. Some of the trees are over 1 foot diameter, but the greater part are not quite 6 inches, and some not even 4. The soil is good, chiefly of a loamy nature, and a good depth.

The remarkable disparity of growth amongst the trees impresses one with a desire to know the reason why some are so large and others so small, being all of the same age and grown on the some soil. The explanation is simply this,—Those that were confined, and thereby deprived of their lower branches, are the small ones; while those that had most room are the largest.

It is a common impression that hard wooded trees, though denuded of their branches when young, will recover them after being thinned. That such is not the case I have ample proof of; for here there are hundreds of trees as bare as poles, with only a tuft of branches on the top, which have had ample room for many years to develop their side branches, had it been in the order of Nature for them to do so. Some trees—as the oak —do make an effort to reproduce their lateral branches, but when the effort at all succeeds it is at the sacrifice of the top growth, so that what is gained on the one hand is lost on the other. It is very difficult to know how to treat a plantation successfully that has once been neglected in thinning. Cutting down and allowing a new crop to grow from the stools is sometimes recommended; but this plan is attended with at least one very serious objection—namely, the circumstance that the scion springing from an old stool produces a tree in character, form, and habit exactly like the parent from which it springs. A dwarfed and stunted tree produces one dwarfed and stunted, and so on. Since, then, so very little can practically be done to recover neglected plantations, the greater is the necessity for preventing them from going wrong.

To all rules there are exceptions, and to this the larch appears to be so; for, of all others, the larch gains most by thinning and suffers least from it. It is very impatient of confinement, and enjoys freedom and liberty, although they come even late. On bare pole-like trees I have seen lateral branches formed and developed beyond anything I have witnessed in any other forest tree. Unless the trees are sound and healthy, however, no lateral growth will take place by thinning. There is something, also, very remarkable about its roots; I know some old stools still vital from which the trees were cut more than twenty years ago. How they continue vital and yet produce no shoots is a profound mystery, and all the more so that no other stools remain in the same state.

The system of thinning young plantations for profit is very objectionable. Not that there is any wrong in disposing of the thinnings to the best advantage; but the profits spoken of as derived from thinnings have done much to mislead proprietors, and induce them to injure, if not ruin, their woodlands. I saw a plantation lately which had been thinned for pit-props, and it was sad to see most of the fine grown and best proportioned trees cut down, and the coarse and weakly ones left as the crop, many of the latter so weakly that they could not sustain their own weight. A report set forth this plantation as an example of profit, and showed that it yielded, as thinnings in a given time, from L.8 to L.10 per acre. The same report should have stated how much such thinnings had reduced the value of the plantation.

On some estates a large revenue is derived from what are termed thinnings, although the plantations are so thin already that they are suffering from it. I know young Scots pine plantations being thinned containing only 300 trees per acre, and some also containing only half that number. Thinning is a very general term, and is understood and practised very differently. Cutting down the tender sapling as a weed of a few years' growth is termed thinning, and the operation of felling all but the last tree of the matured old forest is known by the same term. The two greatest errors amongst foresters are—being too late in commencing to thin, and continuing the operation too long. It does much good if done early, and equally much harm if done late.

"The most valuable crop of oak timber I ever saw," says Mr J. S., an extensive timber-merchant, "was upon the Duke of Devonshire's property in that county, when the trees stood from 6 to 8 feet apart." The best I ever saw was a few acres of Scots pine on Rothiemurchus estate, Strathspey, when the trees stood from 2 feet to 15 feet apart.

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