Scientists and other observers came to Glacier Bay to see the great glaciers and found here the ideal natural laboratory for the study of plant succession. How do plants recover a raw landscape? What happens when nature wipes the slate clean and starts over from scratch?
Today a trip up the bay mimics glacial retreat and rolls back plant succession, from the mature forest at Bartlett Cove to the naked Earth structure at the fjords' farthest reaches. Biological succession produces profound change here in a mere decade. Early, long-range studies of plant succession began in Glacier Bay in 1916, with the work of Professor William S. Cooper. His plant studies were continued in 1941 by Professor Donald Lawrence and others.
Plant recovery may begin here with no more than "black crust", a mostly algal, felt-like nap that stabilizes the silt and retains water. Moss will begin to add more conspicuous tufts. Next come horsetail and fireweed, dryas, willows, alder, then spruce, and finally hemlock forest. In many areas the final or climax stage of plant succession may be the boggy muskeg, but this may take hundreds of years to develop, after the establishment of hemlock-spruce forest. Where plant's seeds happen to land can be critical.
The chaotic rock-and-rubble aftermath of a glacial romp is deficient in nitrogen. Alder and dryas are important pioneers because they improve the soil by adding nitrogen to it. Much of northern Europe and America were pioneered by dryas when the last Ice Age ended. Sitka alder begins to form dense entanglements that are the bane of hikers. Spruce takes hold and eventually shades out the alder. A forest community is begun. Each successive plant community creates new conditions. The theory holds that plant competition modifies the environment -- light and moisture availability, and soil nutrients -- so that plant populations also change. Over time, successive plant communities will occupy the environment, hence plant succession. The time from naked rock to revegetation is not necessarily long.
The patterns by which animals re-inhabit the land after glaciers retreat are not as neat as with plant succession. There are no true pioneer species paving the way for succeeding species. Land mammals must either walk or swim. They cannot, as plant seeds and spores do, hitch rides on wind, waves, or with birds. Extensive water, ice, or mountains loom as impassable barriers. Low mountain passes are often the conduits through which land mammals begin to repopulate the park. Usually they will live off this young terrain only part of the year at first. Resident populations may gradually build. The process of colonization at Glacier Bay and throughout Southeast Alaska is somewhat hindered by the fact that mammals in general have not had enough time since the Wisconsin Ice Age wound down to recolonize the land.
Did You Know?
Sitka Alder is a nitrogen fixer. Like members of the bean family, they enrich the soil, making them an important contributor to soil development as plants recolonize land recently exposed from under ice.