## 3 Results  
### Trends in WUE  
Figure 3 illustrates the spatial distribution of the 20-year average WUE and its trends in the study area. Over the past two decades, the mean WUE in the FPENC has fluctuated between 0.52 and 2.60 gC kgH\({}_{2}\)O\({}^{-1}\), with most areas showing WUE values ranging from 0.92 to 1.76 gC kgH\({}_{2}\)O\({}^{-1}\) (Figure 3a). Nearly all the area exhibited a non-significantlydegraded trend over the FPENC, only the area in the west of Liaoning, north of Hebei and Shanxi, central Inner Mongolia was shown to be non-significantly improved, and some areas in Ningxia and Gansu showed a significant improvement (Figures 3b and S1). The mean monthly WUE of the vegetation types of FPENC during the study period exhibited higher values mainly occurring from May to September, and the highest value was observed in June (2.55 gC H\({}_{2}\)O\({}^{-1}\)), followed by May (2.33 gC H\({}_{2}\)O\({}^{-1}\)), September (2.24 gC H\({}_{2}\)O\({}^{-1}\)), July (1.98 gC H\({}_{2}\)O\({}^{-1}\)), and August (1.94 gC H\({}_{2}\)O\({}^{-1}\)) (Figures 4, 5 and S4). The average WUE was highest in summer (with WUE higher than 2 gC H\({}_{2}\)O\({}^{-1}\) in most areas) (Figure S5). To evaluate the reliability of the numerical results, statistical tests were performed. The annual averaged WUE of the whole FPENC has exhibited a notable decreasing trend over the twenty years (R2 = 0.29, \(p\) < 0.01). Specifically, it demonstrated a significant decline during the first ten years (R2 = 0.39, \(p\) < 0.05), while in the last ten years, it showed a slight upward trend (2013-2022) (R2 = 0.16, \(p\) = 0.14) (Figures S2 and S3). To gain deeper insights into how various vegetation types respond to climatic and biological factors, we examined the WUE trends for each vegetation type group. It showed an overall tendency of decrease Figure 6a,b). The annual WUE all showed a sharp decrease in 2010, and it showed an increasing trend afterwards (Figure 6b). In terms of ten vegetation types, the annual WUE of evergreen needle forest exhibited the highest value (1.87 \(\pm\) 0.12 gC H\({}_{2}\)O\({}^{-1}\)), with the WUE of mixed forest ranking second afterwards (1.61 \(\pm\) 0.1 gC H\({}_{2}\)O\({}^{-1}\)), and the open shrubland showing the lowest WUE (1.08 \(\pm\) 0.14 gC H\({}_{2}\)O\({}^{-1}\)). Especially, the grassland, which occupied the majority area in the FPENC, had a relatively low value of WUE (1.16 \(\pm\) 0.11 gC H\({}_{2}\)O\({}^{-1}\)) (Figure 6a, Table 2). Figure 3: (**a**) The annual average water use efficiency (WUE) and (**b**) the spatial distribution of its trend during 2003–2022 over the study area. Figure 4: Monthly means of water use efficiency (WUE) from 2003 to 2022 over the study period. Figure 5: Boxplot of the mean values of monthly water use efficiency (WUE) over the FPENC from 2003 to 2022. Figure 6: (**a**) Inter-annual variations of water use efficiency (WUE) in different vegetation types over the study area and (**b**) the monthly averages of water use efficiency and its trends from 2003 to 2010, 2011 to 2022, and 2003 to 2022 in the farming–pastoral ecotone of Northern China. ENF: evergreen needle forest, DNF: deciduous needle forest, DBF: deciduous broadleaf forest, MF: mixed forest, CSH: closed shrubland, OSH: open shrubland, WSAs: woody savannas, SAVs: savannas, GRA: grassland, and CRO: cropland. Figure 7: Spatial distribution of annual averages for the biophysical variables, (**a**) enhanced vegetation index (EVI), (**b**) leaf area index (LAI), (**c**) precipitation (PPT), (**d**) net radiation (Rn), (**e**) temperature (Temp), (**f**) vapor pressure deficit (VPD), and (**g**) soil volumetric water content (VSWC) at 7 cm below the ground during the study period of 2003–2022.