Abnormal windfall losses in the natural forest were found on some areas during the field examination. Most serious were those in the provinces of Malleco and Cautin, where, on the areas sampled, 2 percent of the total sawtimber stand had been blown down in a single year. The average annual windfall loss on sampled areas in the provinces of Valdivia and Osorno was 1.4 percent of total stand; average loss over a long period of time is undoubtedly somewhat lower, but the samples indicate that wind can be a serious agency of depletion. No abnormal windfall losses were found in the other provinces.

No epidemic insect or disease mortality was observed in the natural forest. As previously explained, the volume of damage caused by heart rot and other fungi was deducted in the compilation of timber volume; the inventory figures show content of sound wood only. Allowance for endemic increase in volume of decay is included in the estimates of natural depletion given in tables 19 and 20.

FOREST GROWTH

A forest, unlike a mine, is a replaceable resource the individual trees that compose it are living organisms. The volume of wood added to a single tree during one year is not large, but the total amount added by all the trees in an extensive forest may be substantial. Thus, a portion of the tremendous depletion suffered by the forests of Chile is being replaced by tree growth.

This annual growth was found to total 730 million cubic feet, or 1,959 million board feet, or 1.1 percent of the total solid wood and sawtimber stand, respectively. The rate of growth varies greatly between the different species, depending on tree size, age and vigor. For example, Alerce, which occurs mostly in large, old, overmature trees of poor vigor, is growing at the rate of only 0.3 percent annually.

In

Canelo, which occurs principally in relatively small, young trees of good vigor, is adding volume at the rate of 2.0 percent annually. The exceptionally young, vigorous and fast-growing plantation stands are annually adding wood at a rate equivalent to 14 percent of their present solid wood volume.

Forest growth by zone, species, and province group is given in tables 21 and 22.

Of the total solid wood growth, 85 percent is being added to trees in the natural forest zone, 13 percent to those in planted forests, and about 2 percent to woodland zone species.

Of the 619 million cubic feet of growth occurring in the natural forest, 92 percent is added to mature stands of sawtimber size, 8 percent to second-growth forests.

Annual solid wood volume growth averages 122 cubic feet per hectare in mature sawtimber forest, 256 cubic feet in natural secondgrowth, and 653 cubic feet per hectare in the forest plantations. The very high growth rate on planted areas is now at approximately its maximum possible level, but in contrast, the natural forest zone is capable of growing wood much more rapidly than it is at present. The latter is now occupied principally by mature and overmature forests, which are capable of utilizing only a fraction of the productive capacity of the land. If well covered with a balanced, thrifty growing stock of native hardwood species, an average annual growth rate of 250 cubic feet per hectare eventually could be achieved in the natural forest zone. This possible rate of growth is considerably above the average for the hardwood forests of North America.

Potential annual growth (figure 11) is 1,304 million cubic feet. This is not the theoretical maximum but a level of productivity that might be anticipated in Chile under reasonably good management. The oretical maximum values have been substantially discounted to allow for a degree of understocking believed inevitable in the administration of extensive forest areas. As here used, potential growth presupposes an average annual growth rate of 200 cubic feet per hectare on the 5.44 million hectares of commercial natural forest land, and that growth in forest plantations be increased from 94 to 196 million cubic feet annually, or approximately what existing forest tree nursery capacity is capable of maintaining. Growth in the woodland zone is assumed to increase only nominally, from a total of 18 to 20 million cubic feet annually.