## 3 Results
### Phenology Includes SOS and EOS of the Forest over the Past 20 Years  
The main type of forest land in Sanjiang Plain contains coniferous forest, coniferous broadleaved mixed forest and deciduous broad-leaved mixed forest. The dominant species include pine trees, _Quercus monoglica_ forest, _Populus davidiana_, white birch, etc. The forest growth period start of the season (SOS) was calculated for each year and compared among the 16 phenological periods with meteorological data (Figure 5). As expected, 12 of the 16 phenological periods (2003-2005, 2003-2010, 2003-2015, 2003-2016, 2012-2005, 2014-2005, 2014-2010, 2014-2015, 2014-2016, 2019-2005, 2019-2010, and 2019-2016) had data that showed a trend of SOS ahead of the forest in snow drought years, with an average advance of 6 days and an advanced rate of 8.3%. Three of the 16 phenological periods (2012-2010, 2012-2015, and 2012-2016) had prolonged SOS in the snow drought year, with an average postponement of 6 days and a rate of 5.8%, and only one period (2019-2015) remained with unchanged SOS.  
The twelve advanced phenological periods represented 75% of the data, and in seven of them, the temperature was higher in the snow drought year. Thus, the temperature factor was not strongly affecting the forest SOS. The most advanced period was 2014-2005 with a 16 day early, maximum-advanced rate of 22.9%, and its snowfall differed 133.4 mm. One-day ahead was the most frequent date, and it occurred four times (2003-2015, 2012-2005, 2019-2010, and 2019-2016). The abnormal trend in the SOS of the forest included periods 2012-2010, 2012-2015, and 2012-2016, where the temperature in 2012 was the second-lowest in all selected years. It is shown that the postponed SOS in the forest may have been because of the temperature.  
Figure 4: The spatial distribution of snowfall in the Sanjiang Plain. Each picture is the average snowfall cover from last year’s December to this year’s February. P: snow accumulation (mm); T: average temperature (°C).  
The vegetation growth phenology indicators included the start of the season (SOS) and end of the season (EOS). We also analyzed the EOS of the forest in major years and interacted each one with two climate factors (Figure 6). Most of the results (14 phenological periods: 2003-2010, 2003-2015, 2003-2016, 2012-2010, 2012-2015, 2012-2016, 2014-2005, 2014-2010, 2014-2010, 2014-2015, 2014-2016, 2019-2005, 2019-2010, 2019-2015, and 2019-2016) showed a trend of advance in the EOS of the forest in snow drought years, with an average advance of 5 days and 7.1% in the ahead rate. The advanced date of EOS was one day shorter than the SOS for the forest. In one period (2012-2005), the end of the season remained unchanged, and in another period (2003-2005), EOS was postponed for just one day. In the 2019-2015 period, the EOS in 2019 was nine days earlier than 2015, which is the maximum advanced trend. The date of forest EOS was around the beginning of October. The EOS showed a similar advanced pattern to SOS for the forest, which reflected that the beginning of plant growth determines the end of the season.  
Figure 5: The forest land start of the season (SOS) date by year in the Sanjiang Plain. The left axis is the year, the right axis is the temperature (\({}^{\circ}\)C), the upper axis is the snowfall accumulation (mm), and the lower axis is the DOY of each year’s SOS. Rectangles represent the DOY of SOS, triangles represent the temperature, and lines represent the snowfall accumulation.