A selection forest is one in which all ages of trees exist, from seedling to maturity. It is the natural growth of species which are tolerant of shade. In a natural state, undisturbed by cutting, it maintains much the same aspect continuously, for as the oldest trees die, their place is taken by younger ones. Obviously such a forest must be composed of species, whether one or several, which can grow beneath its own shade. The understories of varying ages are as dense as their light requirements and the density of the overwood permit.
The common hardwood forests of the East illustrate one type of the natural selection forest. On the Pacific slope an example is afforded by hemlock, either practically pure or mixed with white fir, but probably the most typical is the ordinary Western yellow pine under certain conditions. At its best this tree composes a forest so dense that all young growth is shaded out, but everyone is familiar with the frequent opener stand containing all ages. The younger trees are often called blackjack.
EVEN-AGED FORESTS
On the other hand, trees extremely intolerant of shade occur only in what the forester calls even-aged forests. Being unable to start in the darkness of an existing stand of any considerable density, they must seize opportunities to recover openings. The Douglas fir of the Northwest, more commonly called red or yellow fir, is an excellent illustration. In the interior states this species reproduces under cover to some extent, because there is a stronger light average throughout the year and because the stand is not so dense. In the typical Douglas fir forests of Oregon and Washington, discussed in this booklet, it never does so. While hemlock, cedar and white fir undergrowth may be abundant, Douglas fir seedlings are seldom seen except in burns, slashings, roads, or open spots in the woods. And the fir trees composing the dominant stand are of nearly the same age.
How, then, did this even-aged fir forest begin? Close scrutiny will practically always find the answer in fragments of charred wood. Long ago another similar forest occupied the ground until lightning or an Indian's fire started a new cycle. Possibly recurring burns swept the area many times before wind-blown seeds began to start advance groups of fir, which, when fifteen or twenty years old, themselves fruited and filled the blanks between them. Perhaps destruction was not so complete and surviving trees made the process a swifter one. Except in the very oldest forests, where remains of the original stand have entirely rotted away, the history in either case may be read in ancient snags and fallen logs.
Suppose, however, that fire had not come to aid the fir in perpetuating itself? This, too, we can answer from the signs today. Every Northwestern woodsman knows tracts of varying size (usually small because fire has been almost universal) covered with big old hemlock, white fir and cedar, with here and there a dying giant fir, perhaps, but mainly showing fir occupancy only by rotting stumps and logs. No sign of fire is seen. When this fir forest was approaching middle age, the shade bearing species began to appear beneath it. As the firs began to crowd themselves out, the later comers shot up with the increased light and filled the open places. At last the even-aged fir forest was completely transformed into a selection forest of other trees, which will remain until some accident again gives fir a chance if any survives near enough to reach the spot with seed.
Douglas fir is not the only Western tree which usually grows in even-aged stands. Lodgepole pine has the same habit, often supplanting yellow pine after fire or logging. Western white pine is perhaps more tolerant than Douglas fir, hence more likely to hold its own without artificial aid, but is also more certain to compete successfully if it has such aid. The same is true of tamarack.
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.