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Description: (a) The first species to colonize the bare Earth following retreat of the glaciers are small species such as cyanobacteria, moss, and lichens. (b) Mountain avens (Dryas drummondii) is a flower common in the Dryas seral stage. (c) Soil nitrogen and litterfall increase rapidly as alder (Alnus sinuata) invade. Note also the appearance of a few spruce trees higher up the valley. (d) Spruce (Picea sitchensis) and hemlock (Tsuga heterophylla) trees comprise a climax spruce-hemlock forest at Glacier Bay, with moss carpeting the ground. Two hundred years ago, glaciers occupied this spot.
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