Divergent responses of spring phenology to daytime and nighttime warming

Abstract

Spring phenology (i.e., start of season, SOS) of plants in temperate regions has shifted earlier in response to increasing temperature. However, the respective influences of daytime and nighttime warming on the changes in SOS remain poorly understood although an ongoing asymmetric diurnal warming has been observed. In this study, we characterized the responses of satellite-derived SOS to daily minimum temperature (Tmin) and maximum temperature (Tmax) across Appalachian Trail regions in the Eastern United States between 2001 and 2013 using a partial correlation analysis. We found that the partial correlation coefficients between SOS and Tmin(RSOS−Tmin) are opposite in sign compared to that between SOS and Tmax(RSOS−Tmax) in 81.5% of study area. Furthermore, we found a significant decrease in RSOS−Tmin and an increase in RSOS−Tmax from cold to warm regions (P < 0.001). These results suggest that daytime and nighttime warmings play distinct or even contrasting roles in spring phenological changes, which should be considered in phenology models. Thus, we proposed a new framework utilizing both Tmin and Tmax, instead of daily average temperature (Tavg), in modeling phenology, and tested this framework using modified CMIP temperatures projections by 2100 with the consideration of changes in diurnal temperature range. The SOS advancement was less pronounced in TmaxTmin–based projection using this new framework at the mild and warm zones, compared to original Tavg –based projection, and such discrepancy between these two projections increased with time. This study disentangled phenological responses to daytime warming from nighttime warming across a wide range of temperature conditions. Our findings suggest that phenology models should incorporate such divergent phenology responses to improve future phenology projection in light of asymmetric diurnal warming, for an improved representation of land–atmosphere interactions in Earth system models.