The vernal window (the time between when air temperatures warm above freezing to initiate snowmelt and when forest canopies fully leaf out) is a key period for the functioning of seasonally snow-covered, forested ecosystems. In the northeastern United States, winter has warmed much more rapidly than the growing season, and shorter winters could lead to a lengthening of the entire vernal window, potentially altering energy, water, and carbon balances as ecosystems trend toward warmer and less snowy winters. Here, we explore historical and future changes in the timing and duration of the vernal window using remote sensing, modeling, and historical observations to assess ecosystem asynchronies that occur within the window, and discuss potential impacts on forest ecosystem function. Specifically, we calculate vernal window metrics timing using daily 500 m MODIS Snow Cover (MOD10A1 and MYD10A) for snow disappearance date) and Conterminous United States 1 km AVHRR Remote Sensing Phenology Data for canopy green-up date. The remote sensing results are then compared to the UNH Water Balance Model (WBM) forced with a high resolution (4 km), 29-member ensemble of statistically downscaled temperature and precipitation data under lower (RCP4.5) and higher (RCP8.5) climate scenarios.
Results/Conclusions
MODIS period of record mean (2001-2018) snow disappearance date broadly agrees with WBM snow disappearance date, with key differences likely arising from availability of MODIS data due to cloud cover, detection of snow cover under dense forest canopies, and parameterization of snow melt rates in WBM. Historically, snow disappearance data typically occurs between February 15th and April 1st, with a wider range of interannual and spatial variability than canopy green-up. By 2100, WBM projects snowmelt dates that advanced at a faster rate (1-2 months earlier) than canopy closure (2-3 weeks earlier) estimated using a thermal time phenology model for dominant deciduous broadleaf species in the region, suggesting an overall lengthening of the vernal window. The most extreme case is a shift into a snow-free regime in the southern part of the Northeastern United States domain, in which the vernal window ceases to exist and is replaced by a prolonged period of increased net radiation into forest ecosystems between senescence and leaf out that may fundamentally alter ecosystem function. Large uncertainties in future precipitation trends limit understanding of how future growing season climate will respond to an absence of seasonal snow cover in the Northeastern United States.