As is the case throughout much of Southeast Alaska, temperate rainforest dominates the southern part of Glacier Bay National Park. The "high biological productivity" or ability of many plants to live in this coastal area is due to the mild, moist climate that has developed in the region over the past 200-300 years.
This is an old growth forest with massive evergreen trees like western hemlock and Sitka spruce that drip with lichens and mosses, and a thick layer of vegetation such as blueberries, fungi, liverworts and wildflowers that blankets the forest floor. The sheer quantity of things living or that once lived but are now decaying means that this type of forest produces some of the largest accumulations of organic material on earth.
As the forest matures or ages, trees grow taller and their branches form a canopy that shades the ground beneath. The soil becomes more acidic and swampy, favoring the growth of western hemlock. Spruce does almost as well in these conditions. Yellow cedar grows in the park's southwestern half on wet, sloping sites and peatland fringes. Mountain hemlock is common at higher elevations.
Over time, the forest canopy tends to become more open as trees of different ages and sizes survive. Some trees die from insects. Others are snapped off in wind storms. More light can reach the ground allowing herbs and shrubs to grow. Downed wood accumulates on the forest floor. Rotting tree trunks become "nurse logs" to the young vegetation, providing them with support and nutrients. Old-growth conditions like these could go on for centuries if there are no fires or disease.
Mixed in among the forest stands are open areas of ancient peatlands. These marshy areas are too wet for large trees. Plants like sedges, willows, and alders are common here.
The qualities of the forest changes as you move into Glacier Bay or explore the park's outer coast. In these places, natural forces like glaciers, erosion and river deposition cause significant forest disturbances. Just as something starts to grow, a catastrophic event like an advancing glacier or massive beach erosion wipes the slate clean and plants must start over. In these places, the forest rarely reaches the old-growth stage. Meadows and shrublands are common. Ancient peatlands are nonexistent.