Geomorphic and climatic influences on white spruce growth near the forest-tundra ecotone in Southwestern Alaska

Abstract

Three types of treelines occur in Alaska: a latitudinal treeline running east-west along the Brooks Range, alpine treelines in mountainous regions, and a longitudinal treeline running northsouth along the Bering Sea coast. Latitudinal and alpine treelines in Alaska have been extensively studied; however, little is known about longitudinal treeline in western Alaska. Here I describe the associations between a longitudinal treeline in southwestern Alaska and geomorphology, soils, and climate. This diffuse, lowland treeline is dominated by white spruce (Picea glauca (Moench) Voss) and is presently expanding rapidly westward. Tree age and stand structure vary markedly according to geomorphic position and soil characteristics but generally fall into four vegetation-landscape associations. I cored spruce growing in these four associations to determine limiting germination dates and compare tree growth with climate records. Results show that timing and rate of establishment has varied between vegetation-landscape associations; however, once established, white spruce growth responds positively to warmer summer temperatures with minor variations between sites. Unlike drought-stressed white spruce in Interior Alaska, under likely near-term temperatures, spruce in southwestern Alaska will probably continue to respond positively to warming temperatures. My data suggest this treeline will continue to move westward across varying topographic features and soil conditions, resulting in a complex spatial mosaic of forested and nonforested communities behind the expanding forest margin.