Spring temperature and snow cover co-regulate variations of forest phenology in Changbai Mountains, Northeast China

Forest phenology is undergoing significant changes as a consequence of climate warming, something which is further complicated by snow cover phenology. While previous research has shown tight links between climate factors, snow cover and forest phenology changes, the relative importance of climate and snow cover on forest phenology has not yet been fully quantified. Here, we analyzed the variations of forest phenology and the influences of different factors on the changes of forest phenology in Changbai Mountains from 2001 to 2019. We extracted forest phenology [start of growing season (SOS), length of growing season (LOS) and annual maximum NDVI (NDVI_max, indicating peak growth)] using MODIS NDVI and analyzed their spatio-temporal patterns. We found advanced SOS (− 0.4 days/year), prolonged LOS (0.84 days/year) and enhanced peak growth (0.002 year⁻¹) of forest at the regional scale. Then we analyzed the effects and relative importance of seasonal climatic factors and snow cover phenology on variations of forest phenology changes. The results indicated that spring temperature was the most important factor causing forest phenology changes. Snow cover duration and snow cover end day also played key roles on forest phenology changes. Snow cover end day showed positive influence on SOS and negative influence on LOS and peak growth. Spatially, the dominant factors affecting forest phenology varied among hydrothermal gradients. Our study provides useful insights on assessment and quantify the influences of climatic factors and snow on forest phenology, which are essential to predict the response of forests under global warming.