## 4 Discussion
### Relationship between SWUE, Climate, Geography, and Phenology Factors  
To date, few studies on SWUE and its driving forces were conducted. Similar to WUE, SWUE reflects the coupling of the carbon-water cycle of terrestrial ecosystems and may be affected by environmental and biotic factors such as climate factors, soil features, and phenological factors [9]. In this current study, we found that the growing season SWUE tended to increase with the increase of NDVI, LST, and precipitation in northeast China. Since April, vegetation gradually developed from seedling to maturity, while NDVI and LST values kept increasing, and precipitation also increased. Good hydrothermal conditions during the summer were favorable for vegetation growth. Some previous studies indicated that, when leaf area of vegetation becomes larger, vegetation will reduce the loss of soil moisture by preventing evaporation, thereby potentially increasing SWUE [9; 35]. The SWUE showed relatively weak and positive correlations with precipitation in the study area, which was related to the decreasing water supply from precipitation due to the surface runoff. Some similar researches also showed that high WUE is related to high temperature, high precipitation, and high NDVI [3; 36]. At the biome level, SWUE showed large differences. In our study, higher SWUE in the growing season was observed in coniferous and broadleaf mixed forests, broadleaf forests, and coniferous forests in northeast China. The cold temperate coniferous forests are mainly distributed in the north of the Great Khingan Mountains, including evergreen or deciduous coniferous forests with frost resistance. Although there is lower precipitation and evaporation, the coniferous forests with cold and drought tolerance can grow better than other vegetation. The warm temperate deciduous broadleaf forests are mainly distributed in the south of Liaoning Province. The coniferous and broadleaf mixed forests are composed of evergreen coniferous, deciduous coniferous, and deciduous broadleaf forests, which are distributed in the south of the Great Khingan Mountains and the Lesser KhinganMountains and the Changbai Mountains. The wide leaf area of broadleaf forest could reduce soil moisture loss; thus, SWUE was relatively high. [PERSON] et al. found that evergreen broadleaf forests had the highest mean annual SWUE, while shrubs had the lowest value at a global scale for the period of 2000-2014 [9]. Both findings suggested that forests could generate higher productivity per unit of soil water consumption. The differences in SWUE values might be because the researches were conducted at a global scale and regional scale, respectively, and the classification schemes of forests were somewhat different. The SWUE distribution varied among soil types. Coniferous and broadleaf mixed forests are mainly distributed on dark-brown forest soils, where the highest SWUE values in the growing season were observed. Lowest SWUE was identified for solonetz in arid and semi-arid areas in northeast China, which was in agreement with [PERSON] et al. 's result of lower annual SWUE in arid ecosystems. Topography directly affects the material flow and energy transformation of the land surface. During the study period, vegetation SWUE showed a certain regularity with elevation change in northeast China. Below 1000 m, SWUE presented a rising trend. The cropland was mainly distributed below 400 m, where SWUE was marginally associated with the greater anthropogenic influence. Coniferous forests and broadleaf forests were mainly distributed between 800 and 1000 m, where vegetation SWUE reached the maximum. When the elevation was more than 1000 m, the SWUE decreased gradually as elevation increased. This was related to the decrease of GPP in vegetation with the elevation increasing. The variations of growing season length have important effects on vegetation growth. The results of this study show that SWUE increased with earlier SOS and later EOS in northeast China. Similarly, [PERSON] et al.