Combined Effects of Precipitation and Temperature on the Responses of Forest Spring Phenology to Winter Snow Cover Dynamics in Northeast China

The start date of vegetation growing season (SOS) is generally considered as an essential indicator to reflect vegetation growth condition. To date, relatively little research has explored the combined effects of temperature and precipitation on the responses of forest spring phenology to snow cover in detail. To investigate this, we applied the developed plant phenology index (PPI) derived from Moderate Resolution Image Spectroradiometer (MODIS) to estimate SOS over Northeast China from 2004 to 2018, and explored the relationship between SOS and climate variables, such as temperature, precipitation and snow cover. Our results indicated that winter snow cover dynamics had a significant effect on the forest spring growth in the following year. SOS showed a negative correlation with snow cover duration (SCD) and the ending date of snow cover (SCED), whereas had a positive correlation with the onset date of snow cover (SCOD). It implied that a longer SCD, later SCED and earlier SCOD would promote forest growth. Furthermore, we first revealed that SOS was more closely associated with the preseason temperature than winter temperature. Most regions exhibited a significant positive correlation with increasing preseason temperature, but the winter temperature showed an opposite pattern except for the cool-temperate needleleaf forest region. Meanwhile, SOS had a negative relationship with precipitation, especially for preseason precipitation. Furthermore, with increasing of temperature and precipitation in winter, the responses of SOS to snow cover phenology in temperate needleleaf and broadleaf mixed-forest region are contrary to that in the other three regions. During the period from SCED to SOS, the responses of SOS to snow cover phenology varied among different vegetation zones and the gradients of temperature and precipitation.