Under this system recurring cuts may be made at periods of perhaps 30 or 40 years, taking out each time the trees which have passed the minimum diameter since the last previous cut. It is obvious, however, that if the process is to continue indefinitely, protection must be absolute. Destruction of young growth will stop the rotation at the time the surviving older material is harvested. At each cut the brush should be disposed of with this end in view. If the stand is very thin it may not add much to the danger of fire and, especially if reproduction is difficult and requires shelter, may best be left spread on the ground at some distance from remaining trees. Otherwise, and this is the rule, it should be piled and usually burned. In this process and in logging every effort should be made to protect existing young growth from injury. Ground fires should be prevented now and always hereafter.
So far, however, we have been considering how to make the most of a stand of many ages, due to constant reproduction permitted by the light supply in a fairly open forest. On the other hand, yellow pine sometimes produces a mature stand so heavy that there is little young growth beneath it, or even a thin old stand with either little reproduction or an invasion of lodge-pole pine. Such conditions are usually due to fire at some period. In the first of these cases, usually the dense stand has resulted from a fire which destroyed its predecessor not so completely as to remove the seed supply, but sufficiently to afford light for a more uniformly dense crop of seedlings than would occur in the normal forest. These have been thinned out as the stand grew old, but never to a degree which allowed much reproduction beneath them. The natural cycle will be begun again in time, for toward the end of the life of this unusually heavy stand, seedlings will begin to appear gradually as individual old trees die and admit more and more light. The other exceptions described are due to more recent ground fires which have destroyed only the less hardy young growth and perhaps also encouraged the lodge pole which, within its range, is always quick to take burned ground.
The same result is almost sure to follow the "Indian" method of forest protection sometimes advocated, which consists of purposely running ground fires frequently in order to prevent accumulation of sufficient debris to make an accidental fire fatal to timber of commercial size. While such immunity may be secured, and perhaps without sacrifice in stands so heavy as to have no reproduction or when the latter has already been destroyed, it is obviously at the expense of young growth if any exists. The counter argument that a small proportion escaping will be sufficient for the second crop is fallacious, because good timber will not be produced from these scattering seedlings subjected to strong light by later logging. Other means are necessary if the forest is to be reproduced.
This brings us to the possible management of yellow pine as an even-aged forest. Thoughtful foresters are beginning to suspect that while the "Indian" system of fire protection will usually be fatal if ordinary logging practice is followed, it may serve as an adjunct to a system which, if carefully applied, will be better than selection cutting for some of our pine areas. This plan is suggested where there is little young growth worth protecting and consists of depending upon seed trees almost entirely for reproduction, protecting carefully until the resultant even-aged second growth is large enough to stand Blight fire, and then burning periodically at such a season and with such safeguards as will prevent the fire from being injuriously severe.
Not only are there many existing forests where absence of small trees will permit clean cutting without sacrifice, but the same condition is likely to occur eventually in stands following selective logging if the second cut is long delayed. Although a good representation of all ages under the diameter limit remains, the density of this may become too great to allow further reproduction, and in time the dominant trees will shade out all smaller growth. To allow this purposely, choosing heavy cuts at intervals long enough to mature the crop from seed rather than frequent light cuts of a constantly replenishing stand, thus reducing the necessity of fire prevention, is the aim of those who favor clean cutting as the most practicable system. They assume that additional investment in seed trees, or planting to insure prompt starting of a new crop after cutting, will be unnecessary or at least offset by the smaller fire charge and greater economy of logging.
Theoretically, such practice with a species adapted to the selective method is uneconomical, for the ground is not fully utilized. Accidental open places in the stand are not occupied by young trees which would otherwise fill them. Time is lost by not starting the second crop until after logging, for were there no fire previously there would be considerable seedling growth which, although perhaps dormant because of shade, would begin to amount to something much quicker than that supplied by seed trees afterward. Nor is the system feasible where there is much fir or other species less fire-resisting than pine. It is dangerous in practice except where there is very little combustible matter on the ground and fire is generally easy of control, and exceedingly dangerous to advocate because serves as a pretext and example for indiscriminate carelessness with fire under all conditions. Finally, the alleged immunity of pine from injury by ground fires is exaggerated. As a matter of fact, while the whole stand is seldom perceptibly hurt, the immediate or gradual death of a good tree here and there thins the stand very considerably in a few years and it is such a thinning process in the past which makes many pine tracts bear but 5,000 feet to the acre where otherwise they would yield two or three times as much. Scorching also retards the growth of trees not actually injured otherwise.
The technical objections given above may sometimes be offset by practical advantages and the system is likely to receive expert approval for certain conditions provided it is not used as a cloak without taking sincere steps to replace the destroyed second growth by adequate seed trees or artificial seeding. The latter danger may easily warrant public alarm manifested by restrictive laws. Universal ground burning of green timber will distinctly reduce the prospect of unassisted natural reforestation on the great area of potential timber land in which, as a resource, regardless of ownership, the public is vitally interested. Under present conditions at least, a large proportion of this is likely to be logged without any view to a future crop. It is questionable whether any state should, or will, legally approve ground burning except under stipulation of proper management thereafter.
Unfortunately, it is necessary, in concluding this discussion of yellow pine, to admit that while an attempt has been made to outline the methods which will insure a second crop, the promise of satisfactory financial return is more doubtful than that offered by some other species. Compared with the typical coast trees, such as Douglas fir, spruce and hemlock, the growth is slow and the yield small. The chief circumstances in its favor are low land values, lesser fire risk, cheapness and certainty of reproduction and excellent market prospects. Less investment compensates somewhat for longer rotation and smaller yield. Low taxation, however, is an absolute essential.
WESTERN WHITE PINE (P. Monlicola)
Although as a distinct forest type this valuable tree is limited chiefly to Idaho, it occurs occasionally in mixture or small tracts over a wide range, and no reason appears why its commercial importance should not be extended by planting on cut-over lands. Its high value, rapid growth and heavy yield make it a particularly promising species for growing under forestry principles. Its chief requirements for success are fairly good moist land, access by the seed to mineral soil and ample light for the young seedlings.