Elevation, substrate, and the potential for climate-induced tree migration in the White Mountains, New Hampshire, USA

We assessed the potential for climate-induced migration of tree species along elevation gradients in the White Mountains of New Hampshire. To do so, we determined the extent to which tree species abundances were associated with elevation, site, and substrate-related variables over a range of elevations (530-880 m) that included the transition from northern hardwoods to spruce-fir forest in the White Mountain National Forest (USA). One-hundred and ten, 400 m2 plots were established along three elevational transects; transects were separated from each other by at least 9 km. In each plot, site and substrate characteristics were measured and all stems ≥2.5 cm dbh were tallied. Species importance values were calculated and those of the five most abundant tree species - balsam fir, red spruce, sugar maple, American beech, and yellow birch - were regressed on elevation and site-substrate characteristics. Plots were ordinated using detrended correspondence analysis and their first and second axis scores were regressed on elevation and site-substrate characteristics. Both elevation and site-substrate characteristics - parent material type in particular - were important predictors of importance value. Balsam fir and red spruce abundance increased with elevation and, at all elevations, reached greatest abundance on shallow-to-rock parent materials. Fir showed greater abundance on north-facing than on south-facing slopes. Sugar maple and American beech declined with elevation and both, but especially sugar maple, were associated with fine and compact tills. Yellow birch, which did not show any association with substrate characteristics, increased to about 770 m, then declined. The frequency of different parent material types changed with elevation, with deep, fine and compact tills becoming less frequent - and shallow soils (rock within 50 cm of soil surface) becoming more frequent - with elevation. If the tree species-substrate associations described here are causal, then the elevational patterns of species abundance observed today are a consequence of both edaphic and climatic factors. As a consequence, vegetation response to climatic warming may be complex. While warming may result in upward migration of yellow birch and American beech, sugar maple, confronted with reduced availability of suitable substrate at high elevations, will likely show little upward response. Red spruce and balsam fir may persist on thin soils at lower elevations unless displaced by eastern hemlock. Thus, climatic warming will likely alter traditionally recognized tree assemblages in this region.