## 4 Discussion  
### Spatial Heterogeneity of WUE  
IM, spanning 19 degrees longitude from east to west, is located in the transition zone from a humid climate to semi-arid and arid climates. The interaction of climate, terrain and land coverage resulted in the zonal distribution of vegetation types [56]. In this study, there was a spatial distribution pattern of high WUE in the east but low in the west of IM, decreasing with decreasing precipitation from northeast to southwest and increasing with decreasing temperature from south to north (Figure 3). This was similar to the existing research results [5; 26; 41]. There were great differences in WUE among different vegetation types, which reflected differences in the carbon-water coupling of vegetation in different regions and under different climatic conditions. We found that the WUE was relatively high in forest and swamp in the study area, but relatively low in meadow. According to the definition and calculation formula, WUE is the product of the ratio between carbon assimilation and plant transpiration (GPP/T) as well as the ratio between the plant transpiration and gross evapotranspiration (T/ET) [3]. GPP/T mainly reflects the adaption of vegetation to environment, while T/ET represents redistribution of water in the physical and biological processes of an ecosystem [7; 57], and its variation is mainly dominated by LAI [11]. The annual mean LAI in IM was between 0\(\sim\)2.64, which had a similar spatial pattern with WUE. This was similar to the result in the humid regions of China, where vegetation indices can explain more than 80% of the spatial variations in WUE [58]. In terms of LAI values in different vegetation types, swamp showed the highest, followed by forest, shrub, cultivated vegetation, and meadow, successively. Forest was the dominant vegetation in the east of study area. Under moisture conditions, a higher LAI resulted in stronger photosynthesis and a higher T/ET [3]. Due to the high canopy cover of forest, plant-transpiration-induced water losses and vegetation interception evaporation were intensified. However, it also intercepted more solar radiation and decreased soil evaporation, which could offset the increase in gross ET caused by plant-transpiration-induced water losses [7; 11]. Additionally, the fractional vegetation coverage showed a linear growth with LAI when LAI was lower than 3 [59]. The high fractional vegetation coverage of forest in the northeast might weaken the runoff caused by precipitation, thus decreasing the nutrient loss in soil. Hence, forest can absorb groundwater and nutrients to support growth through the developed root system, finally resulting in a relatively higher WUE [60]. Although, there was sufficient water in the growing environment of swamp, which could lead to a higher ET. The ET process was restricted by the relatively lower temperature in northeastern IM [61]. Moreover, the relatively high WUE in northeastern IM may have been due to the high GPP value of swamp. In western IM, meadow was the dominant vegetation type, in which there was low precipitation and high soil evaporation, thus making water difficult to be fully used. In addition, the relatively high altitude restricted the growth of vegetation [12; 16] in western IM, resulting in the low T/ET and WUE. The WUE of cultivated vegetation was higher than that of surrounding vegetation as there were sufficient nutrients in its growing environment, and water input from irrigation activities ensured effective plant growth [7].