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The Role of Natural Disturbance in the Range Expansion of Sitka Spruce

by Kaarin Tae, Alaska Quaternary Center, University of Alaska Museum

Sitka spruce is expanding its range southwestward on Kodiak Island and the Alaska Peninsula, invading dense shrub-tundra vegetation. Range expansion is occurring in response to postglacial warming which melted the ice sheets on the north-Pacific coast 14,000 years ago, not in response to recent climate changes. The range limit of Sitka spruce is out of equilibrium with climate, as indicated by the survival of Sitka spruce plantations on several Aleutian Islands and the lack of correlation between annual ring growth and climate on Kodiak Island. The range expansion of Sitka spruce thus provides a valuable analog to the situation predicted for many boreal tree species in response to future global warming. Species' potential ranges will shift rapidly with climate change, but little is known of the processes that control the range expansions of trees following these shifts.

On Kodiak Island and the Alaska Peninsula, Sitka spruce are invading a landscape dominated by dense alder shrubs and grass. Seedlings of Sitka spruce are excluded from densely vegetated sites, but are abundant on sites which are disturbed or are not densely vegetated. Interspecific competition limits Sitka spruce seedling establishment on Kodiak Island, and disturbances to the existing vegetation cover are of great importance to Sitka spruce seedling establishment.

A large-scale disturbance of the landscape can have a dramatic and long-lasting effect on Sitka spruce population growth and range expansion. The eruption of Mount Novarupta in 1912 deposited volcanic tephra up to 30 cm deep on eastern Kodiak Island, and 50 cm deep on the coast of the Alaska Peninsula, burying all herbaceous plants and crippling alder shrubs, but having little effect on Sitka spruce trees. The surface of the tephra formed a competition-free environment for Sitka spruce seedling establishment. Recruitment of Sitka spruce seedlings on Kodiak Island and the coast of the Alaska Peninsula increased dramatically following the eruption due to the elimination of interspecific competition.

The expansion of Sitka spruce has been occurring for 14,000 years, ever since the deglaciation of the north-Pacific coast of North America. Sitka spruce and Alnus expanded quickly from south of the ice sheet into the freshly deglaciated landscape. They expanded at similar rates as far as Yakutat, Alaska, where Alnus arrived by 10,200 years BP, and Sitka spruce arrived by 9,560 years BP. Beyond Yakutat, the expansion of Sitka spruce was slowed by barriers to establishment and dispersal, including the Malaspina Glacier and the Bering Glacier. Alnus expanded from the north as well as the south, and occupied the area northwest of Yakutat many thousands of years before the arrival of Sitka spruce.

The timing of Sitka spruce arrival relative to the arrival of its competitors is very important in determining the rate of advance of Sitka spruce. Sitka spruce advance was rapid where it followed closely behind Alnus in the Puget Sound region, Fraser Lowland, Graham Island and Lituya Bay. At Icy Cape, Prince William Sound, and Kodiak Island, however, Alnus became established many thousands of years before the arrival of Sitka spruce. The rate of Sitka spruce advance west of Icy Cape continued at a slow rate, probably because establishment of individual trees and expansion of individual forest stands was limited by competition with the well-established shrub-tundra vegetation.

Interspecific competition can cause inertia in the range expansion of tree species, masking the effects of recent climate change. Increasing frequency and severity of distur- bance regimes have been predicted to accompany global warming. Increased disturbance will likely increase the invasibility of plant communities for tree species, and possi- bly aid in the natural readjustment of their geographic ranges.

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