Vegetation optimal temperature modulates global vegetation season onset shifts in response to warming climate

The warming climate strongly impacts vegetation phenology, altering the terrestrial carbon cycle, biodiversity, and food production. Here, we analyzed satellite observations from 1982 to 2015 and found that approximately one-third of vegetated areas experienced a significant shift in the timing of the start of the growing season (SOS), with 65.5% exhibiting an earlier trend and 34.5% a delay. We revealed that these changes in SOS were significantly associated with temperature variations, with a response depending on the differences between seasonal temperature and optimal growing temperature for vegetation (Delta T). When Delta T < 0 (seasonal temperature below optimal temperature), warming accelerates the photosynthetic process, leading to an earlier SOS. Conversely, when Delta T > 0, vegetation may experience photosynthetic inhibition induced by rising temperature, delaying SOS. Based on these findings, trends of earlier SOS can be expected to gradually decelerate and even potentially transition into delayed shifts with additional warming in the future.