Photoperiod decelerates the advance of spring phenology of six deciduous tree species under climate warming

Vegetation phenology in spring has substantially advanced under climate warming, consequently shifting the seasonality of ecosystem process and altering biosphere–atmosphere feedbacks. However, whether and to what extent photoperiod (i.e., daylength) affects the phenological advancement is unclear, leading to large uncertainties in projecting future phenological changes. Here we examined the photoperiod effect on spring phenology at a regional scale using in situ observation of six deciduous tree species from the Pan European Phenological Network during 1980–2016. We disentangled the photoperiod effect from the temperature effect (i.e., forcing and chilling) by utilizing the unique topography of the northern Alps of Europe (i.e., varying daylength but uniform temperature distribution across latitudes) and examining phenological changes across latitudes. We found prominent photoperiod-induced shifts in spring leaf-out across latitudes (up to 1.7 days per latitudinal degree). Photoperiod regulates spring phenology by delaying early leaf-out and advancing late leaf-out caused by temperature variations. Based on these findings, we proposed two phenological models that consider the photoperiod effect through different mechanisms and compared them with a chilling model. We found that photoperiod regulation would slow down the advance in spring leaf-out under projected climate warming and thus mitigate the increasing frost risk in spring that deciduous forests will face in the future. Our findings identify photoperiod as a critical but understudied factor influencing spring phenology, suggesting that the responses of terrestrial ecosystem processes to climate warming are likely to be overestimated without adequately considering the photoperiod effect

Divergent responses of spring phenology to daytime and nighttime warming

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 \u3c 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