The extent to which a closed stand may be opened depends on several conditions. The kind or kinds of tree that compose the stand, the nature of the soil, the character of the undergrowth, the purpose for which the timber is grown, all play a part in determining the degree of thinning. This is one of the many matters in forestry that cannot be reduced to a rule, but must be based on a study of each woodlot. There are, however, several con siderations which indicate the extent to which a woodlot may be thinned. The classes into which trees in a closed stand gradually become separated, in the course of their struggle for existence, are of assistance in selecting trees for removal. Four classes are usually distinguished : (1) dominant, (2) intermediate, (3) suppressed, and (-1) dead. Dominant trees are those that have their crowns in the light ; they have kept ahead of the others in height-growth. Intermediate trees are those that still have their crowns in the light, but are somewhat backward, and are destined to become suppressed in the near future. Suppressed trees are those that stand slightly below the intermediate class and will probably die within a few years. Now, moderate thinning would involve the removal of such of the intermediate trees as are interfering with the best development of the dominant ones. Care should be taken not to open up the stand to such an extent that undesirable undergrowth will result. In the case of shallow-rooted species, like the spruce, the stand should not be opened up too much or it will become liable to windfall. The cover must be broken into enough, however, to stimulate the growth of the remaining trees, or very little good will have been accomplished by the operation. In no case should the cover be broken to such an extent that it will not close in two or three years.
Whether suppressed and dead trees should be removed depends principally on whether they con tain enough wood to make their removal, along with the remainder, worth while. Some stimulation will result from the removal of certain of the suppressed trees, but most of them are so far behind the dominant trees that are to compose the final stand that their presence or absence has little effect, one way or the other, on the development of the dominant ones. Yet it often pays to remove some of the suppressed, and sometimes even a part of the dead trees, while the more important thin ning is in progress, although, except in extraor dinary cases, it would not pay to go into a stand for suppressed and dead trees alone. On the general principle of cleaning a stand of all useless material that might add to the dissemination of disease or increase the danger from fire, it is sometimes ex pedient to remove dead and suppressed trees, when it can be done without extra cost, while thinning is being done. On the other hand, it is sometimes desirable to retain the suppressed trees, or a part of them, in order to keep the ground as well shaded as possible.
Certain species in a mixed stand are more desir able than others. If it comes to a choice between two trees of different species, other things being equal, the more desirable kind will be left. For example, a white ash and a yellow birch tree are standing side by side, and the conditions demand that one should be removed ; the birch would he removed and the ash should be allowed to grow, for white ash logs sell for over twice as much as yellow birch.
A defect in an individual of a desirable kind may render it less valuable than a tree of inferior kind. For example, a decayed spot in the ash mentioned above may have made its removal pref erable to that of the yellow birch.
The shape of the crown and its position relative to surrounding crowns are of special importance. The processes of respiration and assimilation are effected in the foliage which composes the crown of the tree. The crown of a tree is its lungs and stomach, so that the development and health of the crown are closely related to the growth and health of the tree ; and when a decision is to be made, the position, shape and health of the crown should be given great weight.
In addition to the above considerations, which should be studied in determining the extent to which a thinning should be carried, another method, though a rough one, may be found useful. The amount of wood standing on the area to be thinned is estimated, and a percentage of the vol ume of the stand is removed. For example, a given stand would run twenty-six cords to the acre ; about four cords an acre, or 15 per cent of the volume of the stand, would be removed in a moderate thinning.
One of the advantages of thinning that has not been mentioned, and which should not be over looked, is that it may be combined with other operations in practice, although in theory quite distinct. As an example of this, an improvement thinning may sometimes be combined with har vesting a part of the final crop.
How to treat a mismanaged woodlot.
There is no better way to outline the treatment of mismanaged woodlots than to describe the work done in a few concrete cases.
A burned-over stand of hard-woods may be taken as an example. The species represented in the stand were chestnut, red, white and yellow oak, with scattering white-wood, white ash, sweet birch and beech. Most of the trees were of sprout origin. The stand ran about eighteen cords to the acre. Fire had been allowed to run through the lot a few seasons before. Many of the chestnuts were badly scorched about the base, and were dying back in the crown. The other trees had also suffered to a considerable extent. There was very little seedling reproduction on the ground. It was evidently im possible to do anything with any but the best of the existing trees ; it would have been a waste of land to allow the others to cumber it. The stand, therefore, was severely thinned, about one-third of the volume being removed. The thinning was done in the winter ; as spring came on, the tops and larger limbs were piled and burned, in order to prepare the ground for planting. Then the whole was underplanted to white pine. Two-year-old seedling stock was used, the distance being six feet each way. The planting cost about six dollars an acre. Ninety-seven per cent of the plants were alive the following spring. The wood was sold the winter following for three dollars a cord on the pile, which insured a net profit of over a dollar a cord on the thinning operation. It was removed while the snow was on the ground, and hence there was no injury to the young pines. The result of this treatment will be a pine stand with a mixture of hard-woods. A part of the hard-woods will be removed when the pine is thinned, but the re mainder will remain until the final crop is gathered.