Long‐term trends in greenness in the alpine zone of northeastern North America from remote sensing

Alpine zones above treeline in the Appalachian Mountains and adjacent ranges of northeastern North America support distinctive ecological communities that thrive where conditions are too harsh for trees to grow. These communities differ from alpine zones elsewhere on the continent and are a regionally important source of floristic biodiversity, but are subject to overlapping forces of environmental change including rising temperatures, CO 2 fertilization, varying air quality and atmospheric nutrient deposition, declining snow cover, invasive species, and intense recreational use. Using 41 years of Landsat imagery, greening trends (represented by the normalized difference vegetation index, NDVI) and phenological cycles were modeled at 35 alpine sites in the Adirondacks, New England, and Quebec's Gaspé Peninsula. Most sites (69%, representing 88% of the region's alpine area) have experienced significant increases in alpine zone NDVI max . Most sites also appear to have shifted towards earlier phenological peak greenness, but rates of greening are spatially and temporally heterogeneous and phenological shifts have not yet reached significance due to the sparse Landsat observational record. Notably, significant greening occurs disproportionately in the interior of alpine zones, not only where treeline is advancing. Greening trends are moderated by landscape position factors related to wind exposure and insolation and vary among seven alpine community types studied, with cliff, sedge meadow, and cushion‐tussock communities experiencing some of the largest changes. Methodological improvements such as an adaptive phenological model used for seasonality adjustment of observations reduced the noise level in estimated trends and effectively minimized the impact of observation frequency bias. Development of remote sensing monitoring programs for alpine zones will provide methodologically consistent regional‐scale information on how alpine ecosystems respond to environmental change, complementing the local‐scale work of field ecologists and better informing alpine zone stewardship and management activities.