[Illustration: {uncaptioned}]
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.