Montane forest ecotones moved downslope in northeastern USA in spite of warming between 1984 and 2011

Abstract

Ecotones are transition zones that form, in forests, where distinct forest types meet across a climatic gradient. In mountains, ecotones are compressed and act as potential harbingers of species shifts that accompany climate change. As the climate warms in New England, USA, high-elevation boreal forests are expected to recede upslope, with northern hardwood species moving up behind. Yet recent empirical studies present conflicting findings on this dynamic, reporting both rapid upward ecotonal shifts and concurrent increases in boreal species within the region. These discrepancies may result from the limited spatial extent of observations. We developed a method to model and map the montane forest ecotone using Landsat imagery to observe change at scales not possible for plot-based studies, covering mountain peaks over 39 000 km(2) . Our results show that ecotones shifted downward or stayed stable on most mountains between 1991 and 2010, but also shifted upward in some cases (13-15% slopes). On average, upper ecotone boundaries moved down -1.5 m yr(-1) in the Green Mountains, VT, and -1.3 m yr(-1) in the White Mountains, NH. These changes agree with remeasured forest inventory data from Hubbard Brook Experimental Forest, NH, and suggest that processes of boreal forest recovery from prior red spruce decline, or human land use and disturbance, may swamp out any signal of climate-mediated migration in this ecosystem. This approach represents a powerful framework for evaluating similar ecotonal dynamics in other mountainous regions of the globe.

Tree demography suggests multiple directions and drivers for species range shifts in mountains of Northeastern United States

“…Instead, species exhibited individualistic demographic responses related to their life-history strategies that stemmed from their unique relationships with climate, land-use legacies, and soil characteristics, corroborating the individualistic character of plant communities (Gleason, 1926)a critical concept for accurate predictions of species migrations across landscapes under changing climate (cf., Iverson et al, 2008) that is sometimes overlooked in studies anticipating consistent synchronous species responses to global warming (cf., Parmesan & Yohe, 2003;Breshears et al, 2008). Importantly, our work suggested that land-use legacies can rival climate change effects and cause the unexpected downslope demographic shifts that we consistently documented for red spruce and sugar maple (and balsam fir on some mountains), ultimately leading at times to the downslope ecotone shifts observed in our region (Foster & D'Amato, 2015) and appearing more common in other studies of species range shifts (Lenoir et al, 2010). Future studies of species range shifts need to more explicitly test additional drivers of species distributions that are regionally important (e.g., fire effects in Schwilk & Keeley, 2012;mycorrhizal associations in Lankau et al, 2015) rather than focusing purely on a global warming effect.…”

“…For example, historic logging in low elevations may have in some cases moved lower boundaries of species distributions to higher elevations (cf., discussion in Foster & D'Amato, 2015), while shallow infertile soils in high elevations may limit upper range margins of some species to relatively lower elevations (Lee et al, 2005). Some boundaries between low-elevation temperate northern hardwood forest and high-elevation conifer forest in the northeastern United States have moved upslope over the last 30 years, while the majority moved downslope inconsistently with the regional warming trend (Foster & D'Amato, 2015), suggesting that drivers other than climate affected ecotone locations across this broad region.…”

The impacts of increasing drought on forest dynamics, structure, and biodiversity in the United States

“…Western redcedar ( Thuja plicata ) and western hemlock ( Tsuga heterophylla ) may expand, whereas ponderosa pine, lodgepole pine, grand fir ( Abies grandis ) and noble fir ( Abies procera ) may contract (Coops et al ., ). In the Green Mountains of Vermont, some work has indicated northern hardwoods have invaded the lower boundary of boreal forest in several locations over the last half century (Beckage et al ., ), whereas broader patterns for this region suggest downslope migration of boreal species (Foster & D'Amato, ). In this location, the ecotone is sharp, concentrated within 200 m of elevation.…”

Vulnerability of eastern US tree species to climate change

“…The latter does not appear to be keeping pace with climate warming, which is only projected to intensify (Corlett & Westcott, 2013). Conflicting results are likely due to divergent climate vectors (Crimmins et al, 2011;Dobrowski et al, 2013), land use and pollution history (Foster & D'Amato, 2015), interspecific competition (Lenoir et al, 2010;Foster & D'Amato, 2015), longevity of trees relative to the observed record (Fisichelli et al, 2014), small signal-tonoise ratios for the 20th century, and methodological differences. Conflicting results are likely due to divergent climate vectors (Crimmins et al, 2011;Dobrowski et al, 2013), land use and pollution history (Foster & D'Amato, 2015), interspecific competition (Lenoir et al, 2010;Foster & D'Amato, 2015), longevity of trees relative to the observed record (Fisichelli et al, 2014), small signal-tonoise ratios for the 20th century, and methodological differences.…”