Distinct latitudinal patterns of shifting spring phenology across the Appalachian Trail Corridor

Warming associated with climate change will likely continue to advance the onset of spring phenology for many forest plants across the eastern United States. Understory forbs and spring ephemerals which fix a disproportionate amount of carbon during spring may be negatively affected by earlier canopy closure (i.e., phenological windows), however, information on the spatial patterns of phenological change for these communities is still lacking. To assess the potential for changes in spring phenological windows we synthesized observations from the Appalachian Mountain Club′s (AMC) Mountain Watch (MW) project, the National Phenology Network (NPN), and AMC′s iNaturalist projects between 2004 and 2022 (n = 118,250) across the length of the Appalachian Trail (AT) Corridor (34°N-46°N latitude). We used hierarchical Bayesian modeling to examine the sensitivity of day of year of flowering and leaf-out for 11 understory species and 14 canopy tree species to mean spring temperature (April-June). We conducted analyses across the AT Corridor, partitioned by regions of 4° latitude (South, Mid-Atlantic, and North). Spring phenologies for both understory plants and canopy trees advanced with warming (~6 days/°C and ~3 days/°C, respectively). However, sensitivity of each group varied by latitude, with phenology of trees and understory plants advancing to a greater degree in the mid-Atlantic region (~10 days/°C) than the southern or northern regions (~5 days/°C). While we find evidence that phenological windows remain stable in southern and mid-Atlantic portions of the AT, we observed an expansion of the spring phenological window in the north where there was greater understory temperature sensitivity compared to trees (~1.6 days/°C). Our analyses indicate differential sensitivity of forest plant phenology to potential warming across a large latitudinal gradient in the eastern United States. Further, evidence for a temperature-driven expansion of the spring phenological window suggests a potential beneficial effect for understory plants, although phenological mismatch with potential pollinators is possible. Using various extensive citizen-science derived datasets allows us to synthesize regional- and continental-scale data to explore spatial and temporal trends in spring phenology related to warming. Such data can help to standardize approaches in phenological research and its application to forest climate resiliency. ### Competing Interest Statement The authors have declared no competing interest.