Water use efficiency (WUE) is a measure used for evaluating the trade-off between carbon sequestration and water consumption of terrestrial ecosystems. Much attention has been directed on how and to what extent WUE varies with climate variability and land-use changes. In this study, the Carnegie-Ames-Stanford Approach model and remote sensing data are utilized to analyze the response of WUE to climate variability and land-use changes in the mountainous area of North China from 2000 to 2020. The results show that the annual WUE follows an increasing trend at a rate of 0.012 gC/mm·m²·yr with an average value of 1.027 gC/mm·m². Spatially, significant heterogeneity was detected in WUE with gradients decreasing from the southeast to the northwest, while significant increases were found in Beijing, Tianjin, and some regions of Hebei. The annual average WUE of different vegetation types decreased following the order of mixed forest > needleleaf forest > broadleaf forest > grassland > shrubland > cropland. Among various meteorologic and vegetation factors, WUE was mainly influenced by temperature, precipitation, and LAI. The cumulative effect of these three factors explained 79.43% of the variations of WUE. Finally, when precipitation was 490–510 mm, or temperature was 13 ℃, or LAI was 1.27 m²m^-2, WUE reached the threshold value, i.e., vegetation had the highest degree of water use. These results have implications for the management of water resources and regional ecosystem optimization in regions with limited water resources.
 

Climate variability,Land-use changes,WUE variation,Threshold value,North China