Trees grow more rapidly at low altitudes than at higher and cooler elevations. Under similar conditions some species increase in size faster than others, but the rate of growth depends principally upon environment. The average increase at the stump in valley land is about 1 inch in 6 years. A Douglas fir growing along the stage road between the park boundary and Longmire's, at the age of 90 to 120 years may have a breast diameter of 20 inches and yield 700 feet of saw timber. But many of the trees of this size may be much older on account of having grown in the shade or under other adverse conditions. The trees between 200 and 300 years of age are often 40 to 50 inches in diameter and may yield an average of from 2,700 to 5,500 board feet. The largest Douglas firs are sometimes over 400 years old and 60 to 70 inches in diameter. Such trees when sound will produce over 8,000 feet of lumber.
The western red cedar has a shorter and more tapering trunk and its volume in board feet is proportionally smaller. A tree 50 inches in diameter and 175 feet high contains about 3,400 board feet.
The size of the trees decreases rapidly at higher elevations. In the subalpine forest the annual growth is very small. At elevations of 6,000 feet the white-bark pine requires 200 years to attain a diameter of 10 or 12 inches. The annual rings are so close together that they can not be distinguished without a magnifying glass.
DESCRIPTIONS OF SPECIES.
DOUGLAS FIR (PSEUDOTSUGA TAXIFOLIA).
The Douglas fir ([figs. 2], [3], [4], and [5]) is the best known and the most important timber tree of western North America. It is found from British Columbia southward to northern Mexico. The finest forests occur in Oregon and Washington at low elevations. The Douglas fir is common in the park up to 3,500 feet, sometimes in nearly pure stands, but more often mixed with other species. It grows in all situations. In the higher mountains it prefers warm southern exposures and is seldom found on wind-swept ridges. It seeds annually, but most profusely at intervals three or four years apart. The red squirrels gather and store large quantities of the cones in order to provide a supply of the seeds for their winter rations. The growth of the young tree is very rapid. As the tree becomes older the rate of growth varies with the situation and the character of the soil so that the size does not closely determine the age of the tree.
Fig. 2.—Douglas fir (Pseudotsuga taxifolia).