SITKA SPRUCE (Picea sitchensis)
Although found in the moister mountain regions, this exceedingly valuable tree seldom occurs to a commercially important extent except along the coast, where it is common on swales and fertile benches and in river bottoms often forms pure stands of great density. Yields of 100,000 feet an acre are not unusual and the trees are very large. It is also common, although of small size, in swamps.
This spruce reproduces readily in openings, whether made by fire or cutting. Unthrifty specimens may be found under shade, but considerable light is necessary for successful development. Even then, height growth in youth averages slower than that of fir or hemlock. The leader shoot is likely to die, so that hardly more than 25 per cent of the young trees establish a regular form of growth before a height of 20 or 30 feet is reached. After this stage spruce grows uniformly and rapidly, still somewhat slower than fir in height but exceeding it in diameter. The branches are slow to die, however, so that the tree remains bushy for most of its length until it reaches 60 or 80 feet in height, and even afterward a dense stand is required to clear it. In many pure spruce forests the larger trees have been able to withstand the pruning influences and remain limby, while the smaller ones, being pushed in height growth to reach sufficient light for survival, have cleared themselves with remarkable rapidity.
The natural occurrence of Sitka spruce, except in Alaska, is probably limited chiefly to situations where it escapes competition, in youth at least, with the more hardy and rapid-growing species. It has the greatest advantage over these on river bottoms and flats where there is a dense growth of deciduous brush and where the soil is very wet in spring. In considering it as a possible second crop, the same competition must be remembered. Whether seeding is natural or artificial, the extent to which it will hold its own with any considerable quantity of other species is doubtful. If such are present and the situation is adapted to them, any expensive effort to get spruce merely by modifying methods of logging or handling the slash is certainly likely to be disappointing. Under the conditions mentioned as peculiarly favorable for spruce, gradual natural restocking may be expected if some seed supply is preserved, but since the growth is rather slow and a thin stand will remain limby, it may pay to hasten returns by supplementary artificial planting. Some authorities question the financial practicability of this on the ground that since spruce is of slower growth it will pay better to use the ground for fir, but the latter is unlikely to be true of bottom land.
After summing all its advantages, the peculiar merits of spruce for certain purposes should be weighed, for sufficiently higher stumpage value will compensate for delay in harvesting the crop. Moreover, Sitka spruce has not been as thoroughly studied by foresters as the more prominent Western trees, and while the foregoing notes represent general present opinion, further figures on rate of height growth may be more encouraging. There is no doubt that diameter increase is rapid from the start. Most of the disadvantages mentioned also decrease toward the southern limit of the spruce range, the growth on the Oregon Coast being rapid.
WESTERN YELLOW PINE (Pinus ponderosa)
In this species we have the important western conifer which most often permits the selection system of management. With certain exceptions in which the entire stand is mature, the object of conservative logging should be to remove trees past the age of rapid growth and foster those that remain for a later cut. When comprising the entire stand, or at least clearly dominating it, with all ages fairly evenly represented, successful in reproduction, and not so dense as to present mechanical difficulties, it is ideally adapted to this form of management. The important underlying principle is that, since for a period of its life the normal individual tree increases in wood production and then declines, it is bad economy to cut it while it is still growing rapidly or to allow it, after slowing down, to occupy ground which might be used by a tree still in the vigor of production. For example, if at 100 years old it contains 500 board feet, it has averaged an addition of 5 feet, a year throughout its life. If at 125 years old it contains but 560 feet, the average increment will be but 4-1/2 feet a year. It will not give equal return for the soil, moisture and light it monopolizes during these 25 years. At the same time, probably there are young trees nearby which hitherto have averaged below the maximum, but if released from its competition will forge ahead for a period at the end of which they will give a greater annual return than if cut at present. It would be as bad economy to cut these today as to spare the over-mature tree. In short, the production of the forest is not only sustained, but actually increased, by removing the oldest trees at just the proper time; and is decreased by taking out young trees either not yet at the natural age of greatest mean annual increment or capable of artificial stimulation by thinning.
By studying the relation of age to production in the particular locality, the proportion of different age classes, and also finding the approximate average diameter which corresponds to the age at which he desires to cut, the professional forester can make a very accurate selection of the trees which can be removed to best advantage at present and also fix the time and yield of the next cutting. Fortunately, however, commercial and silvicultural considerations accidentally coincide so nearly under average yellow pine conditions as to make certain rough rules which can be laid down entirely consistent with logging methods now in practice. Diameter is far from exact indication of age, for the location of the forest and the situation of the individual tree, especially as it affects the relation between height and diameter growth, are potent factors, but as a rule merchantability for saw-material is not far from maturity.
In a great majority of cases the approximate minimum diameter for cutting which would be fixed by it forester would be somewhere between 16 and 30 inches, but say it were 18 inches, for example, it would not arbitrarily apply throughout the stand. Most trees with yellow, smooth bark and small heavy-limbed tops, perhaps partially dead, are mature regardless of their size. If small, they have been crowded or stunted and may as well be cut. Trees with large, healthy crowns composed of many comparatively small branches, and with rough dark bark showing no flat scaling, are sure to be growing rapidly, even if quite large. They are also less desired by the lumberman, who often calls them black pine or black jack, so may often be spared, without much sacrifice, for seed trees or in order to continue their rapid wood production.
The seed tree problem in such a pine forest and under such a system as has been described is comparatively simple, for there are likely to be enough young trees of fruiting age left to fill up the blanks between existing seedlings. The density of the latter determines to a large extent the number and location of seed trees necessary, but there should always be two to four to the acre, even if this requires leaving some that would otherwise be logged.