Tree encroachment in alpine environments of Northeastern North America: Evidence from multiple approaches

Alpine zones in northeastern North America (NNA) are rare and support diverse ecological communities distinct from lower elevation forests. Global change drivers can alter the composition and functioning of these plant communities. The forest-alpine ecotone (treeline) is also influenced by a myriad of environmental drivers and is anticipated to encroach into alpine systems. We sought to determine the extent and drivers of treeline advance in NNA using multiple methodologies, including aerial and satellite-based imagery and long-term in-situ observation. For our approach, we (1) compared current and historical high-resolution aerial imagery of two ranges to quantify the advance of treeline over the last four decades. Vegetation delineation of aerial images were coupled with in-situ surveys to ground-truth treeline classifications. We used mixed effects models to examine the importance of both topographic and climatic variables on treeline advance. (2) Greening trends (NDVI) were modeled at 35 alpine and treeline ecotone sites in the Adirondacks (New York), New England, and Quebec’s Gaspé Peninsula using Landsat 5-8 imagery (1984-2024). (3) Permanent alpine vegetation point-intercept transects (with associated environmental data) in the Adirondacks and White Mountains (New Hampshire) were periodically sampled to quantify changes in alpine species composition and monitored for increased tree abundance. NNA treelines have shifted upslope on average by 3 m/decade since the 1970’s. Diffuse treelines (low tree density) displayed significantly greater upslope shifts (5 m/decade) compared to denser treelines, suggesting that both warming and 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). Most sites (88% total alpine area) have experienced significant greening via annual NDVImax. Greening occurs in the alpine zone interior in addition to some ecotone positions, potentially revealing both increased alpine vegetation productivity and tree establishment. Greening trends are moderated by landscape position and vary among plant community type (herbaceous vs. shrub-dominated). Non-metric multidimensional scaling (NMDS) illustrates an increase in shrub and tree species abundance over the past four decades, with a decrease in arctic specialist species abundance. These changes were highly positively correlated with site-level temperature and negatively correlated with anthropogenic atmospheric nitrogen (N) loading. Tree encroachment (and shrubbification) of NNA alpine challenges the future character and functioning of these rare systems. Development of remote sensing-based monitoring programs for NNA alpine will provide methodologically-consistent regional-scale information on how such ecosystems respond to environmental change, better informing stewardship and management activities.