Mixed three-dimensional zonal variations in vegetation phenology based on global site observations

Terrestrial climate and vegetation exhibit distinct zonal patterns in relation to longitude, latitude, and altitude, a phenomenon known as three-dimensional zonality. Accelerated warming rates since the industrial revolution, along with changes in latitude and altitude, have the potential to influence the geographical distribution of vegetative phenology. To comprehend and accurately predict changes in terrestrial ecosystems, it is critical to understand the three-dimensional zonal variations in global vegetation phenology. Using the PEP725, USA-NPN, and CPON phenological network datasets, we examined and compared the spatiotemporal dynamics of longitudinal, latitudinal, and elevational gradients at the beginning (SOS) and end (EOS) of the growing season over the last forty years in different regions and identified the potential mechanisms underlying these variations. It is revealed that the changes in latitudinal, longitudinal, and altitudinal gradients of SOS vary by location. Between 1980-1990 and 2008-2018, the latitudinal gradient of SOS in Europe decreased threefold, from 1.38 days/degree to 0.37 days/degree. In China, the longitudinal, latitudinal, and altitudinal gradients of SOS have all decreased, indicating that SOS is becoming more synchronized across longitude, latitude, and elevation. Unlike SOS, the latitudinal, longitudinal, and altitudinal gradients of EOS differ from species. For example, in Europe, the correlation between EOS and latitude has weakened for Aesculus hippocastanum and Betula pendula, indicating a reduced latitudinal gradient in EOS for these two tree species. In contrast, the correlation between latitude and EOS for Fagus sylvatica has strengthened, suggesting an increased latitudinal gradient in EOS for this species. In North America, due to the limited observation period, the changes in latitudinal gradients of plant phenological periods are not yet clear. These findings of mixed three-dimensional zonal variations in global vegetation phenology pose challenges for mitigating the possible adverse impacts of climate change.