Variable Warming Effects on Flowering Phenology of Cherry Trees Across a Latitudinal Gradient in Japan

Rising global temperatures are affecting plant phenology. Spring warming can lead to early flowering due to accelerated heat accumulation after dormancy. Winter warming, however, may counteract this acceleration by extending the chilling period required for endodormancy release. We tested this hypothesis with historical bloom data of a flowering cherry (Prunus x yedoensis) collected from multiple locations in Japan across a latitudinal gradient. We determined differential chilling and forcing temperature effects based on advances (-) or delays (+) of bloom dates per degree Celsius of change (temperature sensitivity, ST). We applied a chilling-forcing modeling approach to reveal and disentangle the differential temperature effects during and after dormancy on spring phenology across different latitudes over time. Our analysis showed that the effects of chilling temperatures during dormancy were variable along the latitudinal gradient, while the effect of forcing temperatures after dormancy was more consistent regardless of latitude. Notably, the elevated temperatures, and thereby reduced chilling, during endodormancy delayed the bloom dates at lower latitudes, and this trend was more pronounced in recent years. Our results support the hypothesis that the effect of winter warming during endodormancy can offset or even outweigh the effect of spring warming by extending the dormancy period, and this can manifest as an overall delay in bloom dates. While the delay in flowering phenology due to winter warming is observable only in lower latitudes currently, it is likely to expand to higher latitudes in the future climate if the warming trend continues. Our findings also highlight the value of concerted long-term phenological records that can provide insights into the mechanisms underlying plasticity in tree physiology with a unique opportunity to predict their responses in a changing climate.