Glacier Bay provides the premier example of how vegetation returns to a landscape following deglaciation, allowing visitors to observe over 250 years of plant succession in the course of just one trip to the glaciers. Near the mouth of the bay, lush spruce and hemlock forests dominate the landscape. These lands began emerging from the ice around 300 years ago and have had the most time to recover from the effects of the glaciers. Traveling toward the glaciers, one travels into a landscape more recently revealed from the ice. The more mature spruce and hemlock forests give way to fast-growing deciduous forests of cottonwood and alder, which give way to shrub lands then tundra until up near the glaciers nothing grows at all. Mature vegetation of the Glacier Bay region can be subdivided into eight categories:
Near the shore, a few salt-tolerant species form salt marshes, which are among the most productive maritime plant communities. Grasses, sedges and wildflowers such as silverweed and lupine thrive here. The band of beach grass along the shore is often inundated by higher tides.
At and above extreme high water, a lush, diverse beach meadow is often present. Such areas can be dominated by grasses, wildflowers and large umbellifers such as cow parsnip. Beach meadows are a distinctive feature of the Glacier Bay region, where post-glacial isostatic rebound is causing the land to rise up out of the sea. As the land emerges, beach meadow vegetation creeps forward to claim it. These biotically important meadows are often backed by a narrow band of alder and then the forest.
Lowland forests are dominated by Sitka spruce and western hemlock, plus cedar near the outer coast and toward the south. Moss, ferns, evergreen herbs and shrub species such as blueberry and devils club cloak the ground in a thick, luxurious blanket. In order to have such lushness, sunlight must be able to penetrate the tree branches and reach the forest floor. In stands where the trees are the same age, they may form a uniform canopy which admits too little light to support undergrowth.
With increasing elevation, western hemlock gives way to mountain hemlock, which can better survive snow and wind conditions. Forested lands generally form an unbroken cloak on southeast Alaska landscapes unless interrupted by disturbance, wetness or altitude. Disturbance can take many forms, such as avalanche, snow creep, flooding, disease, insect infestation, glacial advance and wind throw. Infrequent or small scale disturbance does not erase the forest. In fact, it increases diversity by creating stands of different ages, which admit light to the forest floor. Such areas make up some of the region's best wildlife habitat.
Once disturbed, most trees take a long time to reestablish so if disturbance is too frequent or severe the forest will be replaced by shrub land. The hearty plants of this community can generally stand more punishment and bounce back faster if erased. Alder, salmonberry and copper bush withstand the deep snows of the subalpine, and can extend far downhill in avalanche chutes, where they are joined by elderberry, devils club and currants. Willow and alder are prominent in river valleys frequently disturbed by flooding.
Over time some places become too wet to sustain good tree growth. Bog communities are encouraged to develop by relatively level topography where there is little runoff or in areas with impervious sediments such as silty glacial sediments or raised marine deposits. Soil hardpans and acidic, spongy peat moss may also prevent good drainage leading to boggy conditions. Here stunted trees and sparse heath shrubs eke out a living on an ever-thickening mantle of mossy peat. As it thickens, the peat insulates the plants from access to minerals in the underlying rocks and sediments making life tough. At peat land edges where groundwater has contact with the substrate, or in developing wetlands more productive sedge-dominated low-land areas or “fens” may form.
With increasing altitude, tree growth is first impeded, then halted by low summer temperatures, wind, and damage from snow creep or avalanche. Often a zone of brush marks the transition between the forest and sub-alpine meadows similar to those just above the tide.
Farther up, where summers are brief and winter winds tend to blow away protective mantles of snow, tundra mats of prostrate shrubs, tiny herbs, mosses and lichens predominate. Even higher, bare rock and ice reign supreme.