COS 132-6 - Understory plant phenological windows shift asynchronously across the Appalachian Trail corridor with warming springs

Abstract: Warming associated with climate change will likely impact the onset of spring flowering and other phenological stages for forest plants across the eastern United States. In particular, understory forbs and spring ephemerals which fix a disproportionate amount of carbon before canopy closure (i.e., phenological windows of opportunity) may be negatively impacted by earlier tree leaf-out caused by warming. However, detailed information on the spatial patterns of phenological change for these taxa is still lacking. We synthesized phenological 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 ( >2 million observations pared down to ~116,000) across the full length of the Appalachian Trail (AT) corridor (34°N-46°N latitude) in order to assess the potential for shifts in spring phenological windows. We used hierarchical Bayesian modeling to examine the sensitivity of Day of Year (DOY) of flowering for 11 understory forb species and DOY of leaf-out for 16 canopy tree species to mean spring temperature (April-June). Analyses were carried out across the AT corridor, and further separated by region (South, Mid-Atlantic, and North). We also included observations of 6 alpine plant species as a separate group of interest.

As expected, spring phenology for both understory plants and canopy trees advanced with warming (~7 days/°C and ~5 days/°C, respectively). However, warming sensitivity between each group varied by latitude, with phenology advancing to a greater degree in the mid-Atlantic region (~10 days/°C) than the southern or northern regions (~5 days/°C) of the AT. While we find evidence that phenological windows are remaining stable in southern and mid-Atlantic portions of the AT corridor, greater understory plant temperature sensitivity compared to trees in the northern AT suggests an expansion of the spring phenological window. Alpine plant flowering advance in the region is more subtle (~1 day/°C) and seemingly decoupled from spring temperature. Our analyses suggest an asynchronous response of forest plant phenology to potential warming across a wide latitudinal gradient in the eastern United States. Further, evidence for an expansion of the spring phenological window suggests a potential beneficial effect on understory plants in some regions from warming. This information allows us to better explore spatial and temporal trends in spring phenology as it relates to warming and may help us better anticipate future threats to understory plants.