Ecology of Alpine Plants
During most of my adult life I have been a student of wild vegetation and of the kinds of plants that make it up. I have never been able to find the harmony that is assumed to be there. I think it is merely another expression of the so-called balance of nature, which has been a millstone tied to ecology for over a century. All my experience of wild nature tells me that it is (and always has been )ina state of imbalance, disharmony and uncertainty.
Hugh M. Raup Beware the Conventional Wisdom
The Environment
The alpine environment is that region above the tree-line where the predominant visible elements are rocks, ice and snow—in sharp contrast with the zone of continuous vegetation below tree-line. Many factors interact to determine whether trees will be absent or present. Among them are temperature, precipitation, wind, and soil. The single most critical factor, however, in determining the presence of trees is low summer temperature. A frost-free season of about two months is a requirement for existence of even the most hardy conifers (Arno and Hammerly 1984). It is the same with herbaceous plants. Some can resist even the bitterest of winter temperatures but they obviously cannot live without a growing season, brief as this season may be. The length of the time of growth varies among different species — the minimum length for the hardiest plants may be a month or less.
The lowest temperatures of winter in the area are not necessarily always found in the alpine zone. Though there is usually a gradual decrease in temperature with increasing elevation (about three degrees Fahrenheit per each thousand feet) inversions cause some mountainous areas to be actually warmer, on the average, than surrounding forested valleys. So, although alpine plants must be able to resist the rigors of high-elevation winters, it is not that resistance alone that enables them to live where they do.
W. D. Billings and H. A Mooney (1968) write that "the uniqueness of polar and alpine plants lies in the fact that they are the only plants adapted to metabolizing, growing, and reproducing at low temperatures." In other words, it is the ability of the plants to survive and thrive in summer alpine conditions that enables them to live where they do.
Along with atmospheric cold, alpine regions are characterized by aridity through much of the year. Aridity may not be always obvious, but at some elevations moisture does not reach plants for as many as eight to nine months of the year. Winter snow is unavailable as moisture for many months and in some summers, atmospheric drought prevails for long periods.
Two characteristics of alpine regions everywhere are intense insolation (solar radiation received by earth usually inducing drying) and snow cover for at least half the year. Generally speaking, as elevation increases the air becomes thinner. Higher elevations receive more snow than low elevations and thinner air leads to increased insolation. Aridity, therefore, increases proportionately with elevation especially if the snow is removed by wind. Another characteristic
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