A rapid upward shift of a forest ecotone during 40 years of warming in the Green Mountains of Vermont

Abstract

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...

Constraints to and conservation implications for climate change adaptation in plants

“…1b). Indeed, shifts in species distributions in line with temperature shifts have already been observed across a range of plant taxa (Sturm et al 2001;Parmesan and Yohe 2003;Jump and Peñuelas 2005;Hickling et al 2006;Beckage et al 2008).…”

The altitude-for-latitude disparity in the range retractions of woody species

“…Recent changes in the distribution of plant species in mountains have been reported for various plant species, from tropical epiphytes [3], alpine plant species [17] and forest understory plants [18], to shrubs and trees [3,4,12,26,27]. Mountain treelines have shifted upwards, commonly by up to 130 m, over the past 50 years [8].…”

“…However, an increasing number of reports demonstrate that retraction of the trailing edge of the distribution of various woody species is now underway in mountains worldwide [3,4,12,[26][27][28][29]. Tree species, including the European beech (Fagus sylvatica) [12,29], trembling aspen (Populus tremuloides) [28], Ponderosa pine (Pinus ponderosa) [4] and red spruce (Picea rubens) [26] are declining at their lower range limits, driven by elevated mortality and reproductive decline, linked to increasing temperatures and drought stress [12,30,31]. The few studies that quantify altitudinal range retractions or entire range shifts indicate range retractions of up to 140 m in species ranging from boreal forest trees to drought-tolerant evergreen shrubs during the past 50 years [26,27].…”

“…Elevated temperature and drought can directly elevate mortality [4,27,30,31,48,53,55,56], increase the susceptibility of species to attack by pests and pathogens [5,28,49,57], or favour outcompetition owing to reduced reproduction and relative competitive ability [12,26,29,55].…”

“…The result is a rapid shift in the boundary or ecotone between species or vegetation zones [4,12,26,29].…”