Phenological implications of sequential 2020 spring freezes for deciduous forests of the Interior East, USA

Abstract:

Warming winters may increase the likelihood of spring freezes that disrupt forest phenology. Forest species are known to vary in their frost sensitivity, yet little is known of how that biological vulnerability relates to local topographic factors. Microclimatic impacts have been associated with both lower slope sites affected by cold air drainage and higher elevations that are more likely to freeze later in the spring, despite greening up later. This research compares phenological behavior as tracked by NDVI time series during two successive freeze events using high frequency 250m MODIS satellite imagery and high resolution 10m Sentinel 2 imagery. A freeze in mid-April 2020 resulted in widespread damage to freshly emerged leaves across Tennessee and Kentucky in lower and mid slope positions, but the cold spared the canopy of upper slopes. A month later, sub-freezing cold struck forests of West Virginia, Virginia and Kentucky, impacting ridgeline forests well into summer. Percent phenological progression maps quantify the status and track subsequent recovery. These maps contextualize the site-specific phenological timing of the freeze, which suggests vulnerability. That local phenological status, as much as the species present, is likely important for understanding freeze responses such as the persistence of NDVI impacts into the growing season. Microclimate vulnerability shifted over the course of the spring to include at first lower slopes, then upper slopes. A similar local response was observed during hard freezes during the earlier-than-normal springs of 2007 and 2012 in the region, and together, these four freeze events suggest commonalities and differences. Patterned vegetation NDVI responses to spring cold suggest that the hazard from freezes with a warming climate will likely be mediated by local topographic and vegetational factors in complex ways.