. Results of a biological survey of Mount Shasta, California. Natural history; Natural history. OCT., TIMBERLINE. 29 result is that this slope, nuless too steep or otherwise unsuited to tree growth, or within the track of avalanches, is commonly covered with â white-bark pines. As a rule the timbered area on these westerly slopes takes the form of a broadening tongue, beginning at the highest alti- tude attained by trees on the crest of the ridge and increasing iu width at lower altitudes until the bottom of the valley is reached and skirted, and the limit of avalanche movement passed,
. Results of a biological survey of Mount Shasta, California. Natural history; Natural history. OCT., TIMBERLINE. 29 result is that this slope, nuless too steep or otherwise unsuited to tree growth, or within the track of avalanches, is commonly covered with â white-bark pines. As a rule the timbered area on these westerly slopes takes the form of a broadening tongue, beginning at the highest alti- tude attained by trees on the crest of the ridge and increasing iu width at lower altitudes until the bottom of the valley is reached and skirted, and the limit of avalanche movement passed, when the trees again strike out boldly. The pines in the basins are much larger and more erect than those on the summits of the ridges; they decrease in size with iucrease in elevation. The long oblique line which on the west side of each ridge marks the lower limit of tree growth commonly. Fig. 14.âHigU timberline ridge, abowing eiiecta of slope exposure. The dark patches on the left (west) side of tiie ridge are dwarf wiute-bark pines. (Photographed John H. Sage.) marks also the upper limit of the area in shadow during the late afternoon. In local spots other factors may account for the absence of trees. Thus, they are always absent from the avalanche-swept bot- toms of the valleys, and from ground kept wet by springs or melting snow. Studied with these facts in view, comparatively few treeless areas will be found which can not be explained, and the position of true timberline may be fixed with some degree of certainty. This is really a very important matter inasmuch as it affects the zone position of a great many species. It is necessary to remember that the reason trees are absent from the cold east and northeast slopes of the ridges whose summits are. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original work.
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Keywords: ., bookcentury1800, bookdecade1890, bookp, booksubjectnaturalhistory
