Temperatures are rising globally and affecting plant phenology. Spring phenology of temperate trees is mainly determined by chilling conditions in winter for breaking dormancy and forcing conditions in spring for bud-break and flowering. It has been generally hypothesized that warming in winter may extend the dormancy period leading to later bud development, while warming in spring will result in early flowering due to accelerated heat accumulation. We tested this hypothesis using temperature data and Yoshino cherry (Prunus × yedoensis) blossom records along a latitude gradient of locations in Japan (1952-2019) and Washington D.C. in the United States (1937-2019). Four contrastive winter and spring mean temperature combinations, cold winter-cold spring (CC), cold winter-warm spring (CW), warm winter-cold spring (WC) and warm winter-warm spring (WW) were classified from temperature data. Winter and spring warming effects were tested by advance (-) or delay (+) of bloom dates per degree changes of winter and spring mean temperature. We also implemented partial least square (PLS) regression to identify chilling and forcing periods, and calculated chilling requirements (CR) and heat requirements (HR) with Dynamic Model and Growing Degree Hours Model, respectively.
Results/Conclusions
The results showed that both the overall temperature and spring temperature effects were significant in these four combinations from lower to higher latitudes. However, winter temperature effect was significant only at lower latitude locations. All responses of winter warming, spring warming and net effect (winter + spring responses) were shown descending trends from lower to higher latitudes. CRs were increasing and HRs were decreasing along latitude gradients. We found that warmer spring temperatures advanced flowering time at all latitudes, but warmer winter temperatures caused a significant delay only at lower latitudes. Overall, the spring warming effect is stronger than the winter warming effect. Spring warming effect for advancing flowering time could be compensated by winter warming at lower latitudes. In conclusion, warmer temperatures in winter and spring shift Yoshino cherry blossom dates differentially and exert varying effects on bloom time across different latitudes. Slowing down of phenological shifts that were caused by warmer winters only manifest at lower latitudes in current climate might be extended to higher latitudes in the future with climate change.