Changes in the alpine forest‐tundra ecotone commensurate with recent warming in southcentral Alaska: Evidence from orthophotos and field plots

Abstract

The complex response of the forest‐tundra ecotone (FT) to climate change may not generalize well geographically. We document FT changes in a nonpermafrost region of southcentral Alaska during a known warming period. Using 1951 and 1996 orthophotos overlain on digital elevation models across 800 km2 of the west Kenai Mountains, we identified cover classes and topography for 978 random points and the highest closed‐canopy conifer patches along 205 random altitudinal gradients. Results show 29% of FT area increased in woodiness, with closed‐canopy forest expanding 14%/decade and shrubs 4%/decade; unvegetated areas decreased 17.4%/decade and tundra 5%/decade. Area of open woodland remained constant but changed location. Timberline, estimated using both the 205 altitudinal gradients and the upper quartile elevations of closed‐canopy forest among the 978 points, rose very little. Tree line, identified using upper quartiles of open woodland, rose ∼50 m on cool, northerly aspects, but not on other aspects. Dendrochronology on high‐elevation seedlings showed a congruence between decadal recruitment and regional changes in climate from 1945 to 2005. Patterns observed in the climatic FT of the Kenai Mountains corroborate other studies that show regional and landscape specificity of the structural response of FT to climate change. FT shifted upwards on cooler, presumably more mesic aspects near seed sources; however, on warm aspects the density of shrubs and trees increased, but FT did not rise. If current conditions continue for the next 50–100 years, the Kenai FT will markedly change to a far woodier landscape with less tundra and more closed‐canopy forest.