Assessing the elevational synchronization in vegetation phenology across Northern Hemisphere mountain ecosystems under global warming

In recent decades, changes in vegetation phenology have exhibited a tight coupling with global warming. Mountain ecosystems are particularly susceptible to climate change, with high elevations warming faster than lower areas. The inconsistent rate of temperature change across elevations may lead to synchronization of phenological patterns, reducing the differences in phenology timing between high and low elevations. In this study, we employ MODIS data spanning from 2001 to 2022 to analyze the spatiotemporal changes in the start of the growing season (SOS) and the end of the growing season (EOS) as a function of elevation in the Northern Hemisphere mountain ecosystems. We find that 87 % of high-elevation areas experience a later SOS and 71 % have an earlier EOS than low-elevation areas. And our analysis reveals a mixed pattern of elevational gradients for the temporal trends in both SOS and EOS, with approximately half of the study areas showing elevational synchronization, while the other half exhibiting asynchronization. Our findings suggest that temperature is the primary driver of spatial patterns of these elevation gradients, but the complex interplay between temperature and precipitation, combined with diverse responses to these changes for different land cover types, led to the observed mixed patterns of elevational synchronization in phenology. Future climate change is likely to further shift phenological patterns across elevations, affecting the structure and function of mountain ecosystems. Our findings on the variations in vegetation phenology across elevations are crucial for understanding mountain ecosystem responses to climate change.