Forests on the Move: Tracking Climate Related Changes of Treelines in Montane Systems of the Northeastern United States

Background/Question/Methods

Montane treelines are controlled by a range of climatic and edaphic factors, and these treelines are anticipated to shift their location in response to anthropogenic climate change. As the regional climate in the northeastern United States has warmed over the past several decades (particularly in the winter), we hypothesized that treelines have advanced upslope at the expense of alpine ecosystems. We used current and historical high-resolution aerial imagery from the Presidential Range of New Hampshire and Katahdin in Maine to quantify the regional advance of treelines over the last four decades. Digitized and georeferenced historical images were overlain with contemporary false color near-infrared NAIP (National Agriculture Imagery Program) imagery and 2m digital elevation models in ArcGIS 10.8. Current and historical treeline locations were determined from classifications of changes in tree cover based on rasterized versions of acquired imagery. This approach was coupled with ground surveys of forest vegetation and topographical features to ground-truth treeline classification and to provide information on additional possible drivers of treeline location. Differences in treeline position changes between treeline forms (based on species demography) were examined. We used multiple linear regression models to examine the importance of both topographic and climate variables on treeline advance.

Results/Conclusions

RMean regional treeline positions have shifted upslope (vertically) significantly over the past several decades for the Presidentials and Katahdin (3 m/decade for both). Diffuse treeline forms (low density forest edge structured by temperature-related growth limitation) experienced significantly greater upslope shifts than other forms, indicating that increasing temperatures are impacting treeline recruitment and growth, and that tree demography can be used to predict the spatial dynamics of treeline advance. Results from multiple linear regression models revealed that both topographical features (slope and aspect) as well as climate (accumulated growing degree days - AGDD) explained significant variation in the magnitude of treeline advance (R2 = 0.29). The direction of treeline shifts was consistent with the hypothesis that climate change induces upward movement of treeline elevation in mountains via increasing temperatures. This was supported by greater observed upslope shifts in areas with lower AGDD (cooler climate proposed to be more sensitive to warming), particularly in areas with diffuse treelines. Our findings suggest that cooler, high elevation diffuse treelines may be more sensitive to increasing temperatures than other areas and can serve as key indicators of rapid climatic change in the future.