The pattern of primary succession at Glacier Bay, Alaska.

(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.