Forests on the move: Tracking climate‐related treeline changes in mountains of the northeastern United States

Abstract

AimAlpine treeline ecotones are influenced by environmental drivers and are anticipated to shift their locations in response to changing climate. Our goal was to determine the extent of recent climate‐induced treeline advance in the northeastern United States, and we hypothesized that treelines have advanced upslope in complex ways depending on treeline structure and environmental conditions.LocationWhite Mountain National Forest (New Hampshire) and Baxter State Park (Maine), USA.TaxonHigh‐elevation tree species—Abies balsamea, Picea mariana and Betula cordata.MethodsWe compared current and historical high‐resolution aerial imagery to quantify the advance of treelines over the last four decades, and link treeline changes to treeline form (demography) and environmental drivers. Spatial analyses of the aerial images were coupled with ground surveys of forest vegetation and topographical features to ground‐truth treeline classification and provide information on treeline demography and additional potential drivers of treeline locations. We used multiple linear regression models to examine the importance of both topographic and climatic variables on treeline advance.ResultsRegional treelines have significantly shifted upslope over the past several decades (on average by 3 m/decade). Gradual diffuse treelines (characterized by declining tree density) showed significantly greater upslope shifts (5 m/decade) compared to other treeline forms, suggesting that both climate warming and treeline demography are important correlates of treeline shifts. Topographical features (slope, aspect) as well as climate (accumulated growing degree days, AGDD) explained significant variation in the magnitude of treeline advance (R2 = 0.32).Main ConclusionsThe observed advance of treelines is consistent with the hypothesis that climate warming induces upslope treeline shifts. Overall, our findings suggest that gradual diffuse treelines at high elevations may be indicative of climate warming more than other alpine treeline ecotones and thus they can inform us about past and ongoing climatic changes.

Climatic determinants of the Carpathian treeline and its projected upward shifts in response to climate change

“…This variable is more readily estimated and included in the standardized set of bioclimatic variables under the BIO10 code (Hijmans et al 2005). Our ndings align with the study of treeline changes in the northeastern United States, where AGDD was found to signi cantly account for variations in the extent of treeline advancement (Tourville et al 2023). We do not, however, assert the universal applicability of these results.…”

“…Diffuse treelines are more often limited by climatic factors, unlike abrupt and krummholz treelines, which are more likely to be shaped by anthropogenic and natural disturbances (Harsch et al 2009;Hansson et al 2021). It is indirectly con rmed by gradual diffuse treelines being more responsive to climate warming (Tourville et al 2023). We manually delineated treeline ecotones on high-resolution Bing images.…”