Detecting latitudinal range shifts of forest trees in response to recent climate change is difficult because of slow demographic rates and limited dispersal but may be facilitated by spatially compressed climatic zones along elevation gradients in montane environments. We resurveyed forest plots established in 1964 along elevation transects in the Green Mountains (Vermont) to examine whether a shift had occurred in the location of the northern hardwood-boreal forest ecotone (NBE) from 1964 to 2004. We found a 19% increase in dominance of northern hardwoods from 70% in 1964 to 89% in 2004 in the lower half of the NBE. This shift was driven by a decrease (up to 76%) in boreal and increase (up to 16%) in northern hardwood basal area within the lower portions of the ecotone. We used aerial photographs and satellite imagery to estimate a 91-to 119-m upslope shift in the upper limits of the NBE from 1962 to 2005. The upward shift is consistent with regional climatic change during the same period; interpolating climate data to the NBE showed a 1.1°C increase in annual temperature, which would predict a 208-m upslope movement of the ecotone, along with a 34% increase in precipitation. The rapid upward movement of the NBE indicates little inertia to climatically induced range shifts in montane forests; the upslope shift may have been accelerated by high turnover in canopy trees that provided opportunities for ingrowth of lower elevation species. Our results indicate that high-elevation forests may be jeopardized by climate change sooner than anticipated.climate change ͉ range shift G lobal climate is currently warming at an unprecedented rate with potentially profound and widespread effects on the distributions of ecological communities. Mean global temperature rose by 0.6°C over the past century, the rate of warming since 1976 has been greater than any other period during the last 1,000 years, and the decade 1990-1999 was the hottest in recorded history (1, 2). Recent climate change has been driven primarily by anthropogenic emissions of greenhouse gases (GHG), and warming is likely to continue at the same or an accelerated rate for the foreseeable future (3-6): global temperatures are predicted to rise by another 1.4-5.8°C by the year 2100 (7). Climate is an important determinant of species' ranges, and rising temperatures associated with GHG emissions are predicted to lead to species' migrations poleward or upward in elevation (8-10). Climate-linked range shifts have already been observed in some taxa (11,12). Although forest composition and geographic distributions of canopy trees are expected to shift with global warming, it is not clear what level of inertia, or time lag, forests will display to climatic forcing nor how strong the relationship will be between warming and tree line rise (13-15). Historical reconstructions and models of forest response to climate change suggest that the natural pace of tree recruitment and canopy turnover result in century-scale responses of ecotones to climate change, which could ma...