Novel effects are here and there seen in long hedges of wind-trimmed trees. These are aligned by the wind. They precisely parallel the wind-current and have grown to leeward from the shelter of a boulder or other wind-break. Apparently an adventurous tree makes a successful stand behind the boulder; then its seeds or those of other trees proceed to form a crowding line to the leeward in the shelter thus afforded. Some of these hedges are a few hundred feet in length; rarely are they more than a few feet high or wide. At the front the sand-blasts trim this hedge to the height and width of the wind-break. Though there may be in some a slight, gradual increase in height from the front toward the rear, the wind trims off adventurous twigs on the side-lines and keeps the width almost uniform throughout.
During the wildest of winds I sometimes deliberately spent a day or a night in the most exposed places at timber-line, protected in an elkskin sleeping-bag. Wildly, grandly, the surging gusts boomed, ripped, roared, and exploded, as they struck or swept on. The experience was somewhat like lying in a diver's dress on a beach during a storm. At times I was struck almost breathless by an airy breaker, or tumbled and kicked indifferently about by the unbelievable violence of the wind. At other times I was dashed with sand and vigorously pelted with sticks and gravel.
This was always at some distance from tree, boulder, or ledge, for I took no risks of being tossed against trees or rocks. Many times, however, I have lain securely anchored and shielded beneath matted tree-growths, where in safety I heard the tempestuous booms and the wildest of rocket-like swishes of the impassioned and invisible ocean of air. The general sound-effect was a prolonged roar, with an interplay of rippings and tumultuous cheerings. There were explosions and silences. There were hours of Niagara. In the midst of these distant roarings the fearful approach of an advancing gale could be heard before the unseen breaker rolled down on me from the heights.
The most marked result of cold and snow is the extreme shortness of the growing-season which they allow the trees. Many inclined trees are broken off by snow, while others are prostrated. Though the trees are flattened upon the earth with a heavy load for months, the snow cover affords the trees much protection, from both the wracking violence of the winds and their devitalizing dryness. I know of a few instances of the winter snows piling so deeply that the covered trees were not uncovered by the warmth of the following summer. The trees suspended in this enforced hibernating sleep lost a summer's fun and failed to envelop themselves in the telltale ring of annual growth.
Snow and wind combined produce acres of closely matted growth that nowhere rises more than three feet above the earth. This growth is kept well groomed by the gale-flung sand, which clips persistent twigs and keeps it closely trimmed into an enormous bristle brush. In places the surface of this will support a pedestrian, but commonly it is too weak for this; and, as John Muir says, in getting through, over, under, or across growths of this kind, one loses all of his temper and most of his clothing!
Timber-line is largely determined by climatic limitations, by temperature and moisture. In the Rocky Mountains the dry winds are more deadly, and therefore more determining, than the high winds. During droughty winters these dry winds absorb the vital juices of hundreds of timber-line trees, whose withered standing skeletons frequently testify to the widespread depredations of this dry blight. A permanent advance, too, is made from time to time. Here and there is a grove, a permanent settlement ahead of and above the main ranks. In advance of these are a few lone trees, heroes scouting in the lead. In moist, sheltered places are seedlings and promising young trees growing up in front of the battle-scarred old guard. Advances on dry, wind-swept ridges are more difficult and much less frequent; on a few dry ridges these trees have met with a repulse and in some places have lost a little territory, but along most of its front the timber-line is slowly advancing into the heights.
With this environment it would be natural for these trees to evolve more hardiness than the present trees have. This would mean trees better fitted to contend with, and more likely to triumph over, the harsh conditions. Evolutionary development is the triumphing factor at the timber-line.
The highest timber-line in the world is probably on Mount Orizaba, Mexico. Frank M. Chapman says that there are short-leaved pines (Pinus Montezumæ) from thirty to forty feet high, on the southern exposure of this peak at an altitude of about 13,800 feet. In Switzerland, along the steep and snowy Alps, it is sixty-four hundred; on Mt. Washington, about forty-five hundred feet. In the mountains of Colorado and California it is of approximately equal altitude, between eleven and twelve thousand feet. Advancing northward from California along the timber-line, one enters regions of heavy snow-fall as well as of restricting latitude. Combined, these speedily lower the altitude of timber-line, until on Mt. Rainier it is below eight thousand feet. There is a noticeable dwarfing of the forest as one approaches the Land of the Midnight Sun, and in its more northerly reaches it comes down to sea-level to form the Land of Little Sticks. It frays out at its Farthest North just within the Arctic Circle. Most of the Arctic Ocean's icy waves break on treeless shores.
Everywhere at timber-line the temperature is low, and on Long's Peak the daily average is two degrees below the freezing-point. At timber-line snow may fall any day of the year, and wintry conditions annually prevail from nine to ten months. The hardy trees which maintain this line have adjusted themselves to the extremely short growing-season, and now and then mature and scatter fertile seeds. The trees that do heroic service on all latitudinal and altitudinal timber-lines of the earth are members of the pine, spruce, fir, birch, willow, and aspen families. At timber-line on the Rocky Mountains there are three members each from the deciduous trees and the evergreens. These are the Engelmann spruce, limber pine, alpine fir, arctic willow, black birch, and quaking aspen.
A few timber-line trees live a thousand years, but half this time is a ripe old age for most timber-line veterans. The age of these trees cannot be judged by their size, nor by general appearance. There may be centuries of difference in the ages of two arm-in-arm trees of similar size. I examined two trees that were growing within a few yards of each other in the shelter of a crag. One was fourteen feet high and sixteen inches in diameter, and had three hundred and thirty-seven annual rings. The other was seven feet high and five inches in diameter, and had lived four hundred and ninety-two years!