Divergent responses of canopy and ecosystem water use efficiency to environmental conditions over a decade in a shrubland ecosystem dominated by Artemisia ordosica

Canopy and ecosystem water use efficiency (WUET and WUEE) measure the trade-off between carbon gain and water loss at canopy and ecosystem scale respectively, and provide vital information for water resource management in drylands. But the assessment of environmental controls on WUET and WUEE on the decadal scale in drylands remains limited. Here, using eddy covariance (2014-2023) and sap-flow measurements (2022-2023), we investigated the environmental controls of WUET, WUEE, and their components (i.e. gross ecosystem production GPP, evapotranspiration ET, and transpiration T) in a shrubland ecosystem dominated by Artemisia ordosica of northern China. Results showed that WUEE (variation coefficient, CV=12.5 %) exhibited a larger interannual variation than WUET (CV=8.2 %), due to similar responses of GPP and T to environmental factors. As a result, the interannual variation in WUET seems to be independent of both canopy greenness (i.e. normalized difference vegetation index NDVI) and climatic factors (soil water content SWC and vapor pressure deficit VPD). While interannual variation in WUEE was mainly controlled by NDVI through changing the ratio of T to ET. The interannual variation in GPP and ET was significantly affected by SWC in spring, which impacted canopy greenness in summer (NDVI_summer). We further found that a decrease in SWC is not universally associated with reductions in GPP and ET. By contrast, the sensitivity of GPP and ET to an increase of VPD is always negative across the full SWC range. Our results suggested that the response of WUEE to environmental factors does not necessarily reflect changes in canopy water use efficiency. Despite the predominant VPD stress on GPP and ET, SWC is a key factor in determining interannual variation in GPP and ET and need to be taken into account when studying long term trends in GPP or ET.