NATURE AS A MODEL
We thus see that if economic reasons suggest it, we may use the selection system as a basis for artificially managing the shade bearing species such as hemlock, white fir, cedar, spruce, and even Western yellow pine. We may cut the largest and oldest trees and still have a well started second crop. If there is not much young growth to leave, even a little is valuable. It may be decidedly best to leave medium sized trees, which otherwise we would cut, because they are still growing rapidly.
On the other hand, we see that this method would not be of any advantage at all in insuring a second crop of Douglas fir, for there is no young growth of this species to protect. The small and medium sized trees, instead of being immature, are merely stunted specimens of the same age as their larger brothers and unlikely to gain in size if left. Selection cutting here would save for future use only such understory of shade-bearing species as may exist. Unless this is an object, the best plan is to cut clean and get all we can. If we leave any fir at all it is for the purpose of reseeding, not to secure better utilization of the trees themselves, and whether we do so depends, theoretically at least, upon whether it is better than artificial seeding or planting. In short, selection cutting harvests the ripest trees of a perpetual forest, while clean cutting destroys the forest in order to start an entirely new and more rapid growing one.
Clean cutting is therefore necessary as well as natural in dealing with intolerant trees. But it does not follow that the selection system, although natural to tolerant species, is the only one adaptable to them. While the one class demands light, the other does not demand shade. It is merely capable of enduring it. Indeed, except for the greater susceptibility of some species to extreme heat and dryness when very young, as a rule shade bearing trees grow much better if they do have ample light supply. Consequently clean cutting may be the best system for these also under certain economic conditions.
Besides its influence upon the occurrence of species in the forest, light practically governs the physical form of the individual tree. If grown in an opening and not artificially pruned, a tree will have a conical trunk and living branches almost down to the ground. The denser and consequently darker the forest, the more cylindrical the trunk, the smaller the crown of branches and the greater the clear length. The individual tree has no object in assuming a desirable commercial form and does so only when deprived of side light by numerous neighbors. Then it sacrifices diameter growth to height growth in reaching for the top light necessary for its life. At the same time the lower branches are killed by shade and drop off, the scars being healed and eventually buried. The pin knots near the center of a big clear log are the remains of branches which when living were at the top of the young tree.
This is why, if it is to produce good timber, any forest must be dense enough to cover the ground throughout the early part of its life at least. When we see an excellent clear stand of mature Douglas fir, for example, we may know that it consists of the comparatively few survivors of a close sapling growth in which the weak were gradually killed out after serving their office of pruning and forcing the vigorous. Had only the trees we now see been on the ground they would be worthless except for firewood. For the same reason artificial forest planting must be thick, although the fillers or nurse trees may be of inferior species if not of so rapid growth as to gain the mastery.
Nature teaches many lessons which we must recognize in artificial management or fail, but she is no more the best grower of forest crops than she is of agricultural crops. We have to study natural methods of forest perpetuation to see how they may be improved upon as much as to adopt them as models. As a rule the virgin forest is exceedingly wasteful of ground. The possibilities under intelligent care are not indicated by nature's average, but by her accidental best, and usually they far exceed even this. A fair comparison is that of scientific farming with unsystematic gleaning from wild and untended fields. The foregoing general principles of forest growth have been purposely outlined very briefly so as to serve as a mere introduction to their application or modification in concrete cases.