The picture at the top of this page was taken near Mary’s Lake in Estes Park. The mountain shown is Twin Sisters. Its slopes are a fine hunting ground for flowers.
SUB-ALPINE
Above 10,000 feet the pattern of life changes. Until timberline is reached at about 11,500 feet, this band of mountain country is called the Sub-Alpine or Hudsonian life zone. Soil and moisture conditions are almost as favorable as in the lower montane zone, but here the snows of winter stay late, especially on north slopes, and frost may come even in mid-summer. The race to ripen seed, before winter comes, is intense, and the seeds, when produced and scattered, face special problems of germination and survival.
The trees of this zone are largely Engelmann spruce, limber pine and alpine fir. Some thick forest stands exist, but the main pattern is small compact tree groups—one or more big seed-trees surrounded by younger offspring—with open patches of grass between. Perennial flowering plants, springing from woody root-crowns have special advantages here, though some annuals thrive, especially if they can get started in the fall and remain dormant under snow till spring. Melting snows in May, June and early July give natural irrigation to large areas of this zone. Competition with sedges and grasses and ability to stand light frost are problems for the plants that live here. Many typical alpine plants of the next higher zone work down into these sub-alpine meadows.
The picture at the top of this page was taken just west of the Poudre Lakes in Rocky Mountain National Park. Lake Irene is in the foreground.
ALPINE
From timberline (about 11,500 feet) up to our highest Colorado mountain summit (Mt. Elbert 14,431 feet) climate is too severe for any trees. This condition marks these areas off as a separate life zone called Alpine or Arctic. Soil forms only slowly on these rocky summits, but mosses, lichens, sedges and grasses have been here for ages of time, all of them patiently building humus. Erosion carries less soil away from the tops than it does from the lower hillsides. So in the spaces between the barren looking rocks, good soil exists, and water, though mainly falling as snow, and not quite as heavily as in the sub-alpine zone below, is adequate for plants. Here grasses, sedges, a few dwarf shrubs and herbaceous plants have all the sunlight to themselves without tree competition. The ever-present adverse condition is low temperature, frequently with strong wind.
It is a land of tough dwarf things. Perennials are the rule, though annuals are found. Low woody mats with basal leaves and flowers only a few inches high are a common pattern. Bulbs and tubers wedge themselves between rocks, out of reach of ground squirrels, if possible. When spring comes with a rush, usually late in June, these dormant plants burst into life in the days of longest sunshine. Shoots of new growth erupt from the ground with buds all formed ready to open. By the end of July the seed crop is largely mature, and by mid-August the browns and crimsons of fall colors in leaves and grasses spread a Persian carpet over these heights. Warm days from then till winter are days of germination for newly scattered seeds and, for established plants, preparation of buds for next year.