Environment and performance of white spruce below and above the forest limit across the boreal forest in Alaska

The study was conducted in three latitudinally spaced mountain ranges in Alaska, Chugach Mountains in the South, White Mountains in the Interior and the Brooks Range in the North. In each mountain range, the study was conducted at three paired treeline and forest sites in separate watersheds. Growing season length and mean growing season soil temperature decreased with increasing latitude but not between forest and adjacent treeline. Average and maximum wind speeds decreased with latitude and increased with elevation. Tree leader and crown damage decreased from south to north and showed strong elevational increase in the South only. Maximum needle longevity was much lower in the southernmost mountains and there treeline trees suffered significantly greater premature needle loss than trees in the more northern mountains. Non-structural carbohydrate (NSC) pools for an entire branch decreased with latitude and the influence of elevation decreased with latitude. Branch growth decreased with latitude with Chugach trees growing three times more per year than the northern trees and the frequency of an elevational decline in growth was greater there than in the trees further north. In the Chugach Mountains, trees with cones had twice as many cones as the more northern trees and in all the mountain ranges, treeline trees had half as many cones as forest trees. The highest germination success was in seeds from interior Alaska trees while trees from northern Alaska had the lowest success. Treeline trees had lower germination success and this elevational pattern was strongest in the southernmost mountains.