## 4 Discussion  
### How Did Snow Drought Influence the Phenological Period of Forest and Grass Directly and Indirectly  
Increasingly more researchers have found that snow accumulation is decreasing, and the temperature is increasing in mid-high latitude areas because of the abnormal weather conditions caused by climate change [9; 40; 44]. In fact, snow drought events represent the extreme climate change within snow accumulation and air temperature. Our results indicated that low snowpack influenced SOS and EOS more for forest and grass phenological periods than snowy conditions. In addition, the forest and grass hold different changing phenological periods in the mid-high latitude area. For the forest, the phenological periods advanced with the reduction of snowfall in winter. Less snowfall means less deeply frozen snow, which needs to be thawed in the spring, giving rise to the warm temperature in the spring. For the grassland, the threshold divided the snowfall into two types: more than 80% represents less snowfall, and less than this amount means that relatively more snowfall remains. The changes in melted snowfall in the snow drought year bring plant germination ahead and early maturation [45]. The soil moisture reservoir is the pathway by which snowfall reaches upland ecosystems. Water stored by snowfall in winter could provide essential nutrients and water resources for spring vegetation growth [46]. Moreover, less snowpack appears to affect root growth and soil microorganism activities strongly and then influence the soil respiration rate [47]. This change in soil microbial physiological processes drives the variation in plant growing season.  
In fact, not only the snow accumulation decreased, but also the temperature was high in the snow drought period. Temperature is a key factor that affects the forest and grass phenological period while making them terminate the dormancy period earlier and begin spring-up time. The increasing temperature exerts rises in both air and soil temperature, and higher temperature accelerates the root and shoots growth. In addition, elevated temperature benefits plant root system nutrient acquisition from several soil layers, thereby increasing the vegetation growth rate [48]. Furthermore, with a warming climate, the leaf area allocation and plant nitrogen productivity are improved in a complex manner to affect terrestrial ecosystems [49].  
Moreover, the forest production precedes grass production for nearly 15 days, which is due to the diverse vegetation characteristics. The reason owes to the fact that the forest obtains nutrients from both root systems and twigs, which hold richer sources of nutrition than the grassland [50]. In addition, a forest with dense branches and leaves absorbs more light energy than grass. Compared with that in the grassland, the snow on branches disappears due to wind and sunlight, whereas the snow on the ground prefers to remain longer and delays the germination time [51]. Thus, the forest land phenological periods are ahead of those of grassland.  
[PERSON] et al. [52] found that the onset of the green-up was 4-9 days earlier on average in the warmer urban areas in the mid-high latitude and indicated well-defined plant growing patterns dependent on latitude. [PERSON] et al. [8] found that winter warming influenced the growth and abundance of flowers, and aboveground flower species affect buds in various degrees in the High Arctic area. [PERSON] et al. [53] investigated climate change and flowering phenology by experiment, and the conclusion was that there was advanced timing of flowering with the earlier snowmelt date and higher soil temperature in Gunnison County, Colorado. In the future, these climate change external environmental conditions will deeply affect natural and agricultural ecosystems.