Spatio-temporal characteristics of vegetation water use efficiency and their relationships with climatic factors in alpine and subalpine area of southwestern China

Aims Water use efficiency (WUE) is an important indicator for understanding the coupling and trade-off relationships between ecosystem water and carbon cycles. In order to reveal the response and adaptation characteristics of different vegetation types to climate change regionally, we examined the trends, altitudinal distributions, and spatial variations of WUE in nine vegetation types in the alpine and subalpine area of southwestern China during the period of 2000-2014. Methods We estimated the vegetation WUE using Moderate Resolution Imaging Spectroradiometer (MODIS) gross primary production (GPP) and evapotranspiration (ET) products, and the gridded climate data interpolated from Aunsplin. Trend analysis and correlation analysis were conducted to examine the relationships between vegetation WUE and other factors, including air temperature, precipitation, and elevation. Important findings The results showed that: (1) The average annual WUE in the study region was 0.95 g·m^-2·mm^-1 with a significantly increasing trend at 0.011 g·m^-2·mm^-1·a^-1 from 2000 to 2014. The average WUE showed a pattern of higher in the southeast region but lower in the northwest region, and the WUE was found increased in 85.84% of study area. (2) The average WUEs were increasing in all vegetation types, and the WUEs showed a gradient of evergreen needleleaf forest > savannas > evergreen broadleaf forest > woody savannas > cropland > deciduous broadleaf forest > mixed forest > closed shrublands > grassland. (3) The WUE was positively correlated with air temperature in 89.56% of the study area, but it was negatively correlated with precipitation in 92.54% of the study area. The highest correlation between WUE and air temperature was found in grassland, while the highest correlation between WUE and precipitation was identified in woody savannas. (4) As the regional climax vegetation type in southwestern China, the evergreen needleleaf forest’s WUE has high adaptation abilities to both altitudinal change and climate change.