Meta-analytic and experimental evidence that warmer climate leads to shift from advanced to delayed spring phenology

In association with past global warming, advanced spring phenological events have been observed worldwide. The global average temperature will continuously increase approximately 2 degrees C to 4 degrees C within this century. However, whether spring phenology will continue to advance in a future warmer world remains largely un-certain. Through a comprehensive analysis of global experimental data, we show that temperature sensitivities of spring leaf-out and flowering, expressed in days advance of spring phenological events per degrees C warming, decreased with the increase in warming extent. In particular, several species at higher elevations showed delayed spring phenology when warming exceeded 4 degrees C. Using whole-tree-chamber experiment involving nine tree species, we also observed a significant decrease in the temperature sensitivity of spring budbreak in response to warming in both winter and spring. Especially in winter, although there was a slight advance in budbreak under +2 degrees C, a delay was observed under +4 degrees C. Using common garden experiments of 220 Arabidopsis accessions worldwide, we similarly found a warming-induced delayed flowering. Combining multi-experiment data, our study provides robust experimental evidence that spring phenology may move from being advanced to being delayed under future climate warming scenarios.