Sensitivity of vegetation water use efficiency to climate and vegetation index in Loess Plateau, China

Abstract: Water use efficiency (WUE) refers to the amount of CO2 or dry matter produced by per unit consumed water by plants, which is an important indicator of carbon-water coupling. A comprehensive understanding of spatial and temporal distribution and influencing factors of WUE is crucial for predicting land surface-atmosphere interactions and future dynamics of terrestrial ecosystems in response to future climate warming. In this study, we analyzed the response of WUE to precipitation, air temperature and vegetation index in Loess Plateau. Data were obtained from well-established products from moderate resolution imaging spectroradiometer (MODIS). The data of gross primary productivity (GPP) and evapotranspiration (ET) were from the MOD17A3 and MOD16A3, respectively. The data had been validated. WUE was the ratio of GPP to ET. Gridded precipitation and air temperature data were interpolated by ANUSPLIN software, and the satellite-derived normalized difference vegetation index (NDVI) was from Advanced Very High Resolution Radiometer (AVHRR)/ National Oceanographic and Atmospheric Agency (NOAA), all the data in this article were resampled to 1 km resolution. The duration was from the year of 2000 to 2014. Land coverage was classified by CMG products (MCD12Q1) with resolution of 1 km. The analysis focused mainly on 3 natural vegetation groups because cultivated vegetation had experienced intensive human management in Loess plateau. The growing season was consequently defined as the duration from March to October. All the variables were detrended by linear fitting before performing the ridge regression using MATLAB. When applying ridge regression to explore the sensitivity of WUE to temperature, precipitation and NDVI, WUE was dependent variable, and temperature, precipitation and NDVI were dependent variables. The results showed that: 1) GPP, ET and WUE distribution had an obvious spatial and temporal pattern in Loess Plateau. The GPP and ET gradually reduced with increasing precipitation. Forest had the highest GPP and ET and shrubland had the lowest GPP and ET. The lowest WUE was found in the southwest of Loess plateau, which was an higher altitude area. Mean annual WUE decreased with increasing precipitation. The interannual variation of GPP, ET and WUE were also obvious, the same variation trend were found in WUE and GPP, which were significantly higher in the year of 2012, 2013 and 2014 than the other years. But the ET didn't showed obvious variation regulation. 2) The sensitivity of WUE to air temperature, precipitation and NDVI showed significant spatial divergence. the sensitivity of WUE to temperature was significantly positive in the whole Loess plateau. The sensitivity of WUE to precipitation and NDVI presented a threshold effect, i.e., WUE increased with precipitation and NDVI when precipitation was less than 500 mm, WUE decreased with precipitation and NDVI when precipitation was over 550 mm. 3) The sensitivity of WUE to air temperature, precipitation and NDVI were related to vegetation type. The NDVI was positively correlated with the WUE of grassland and shrubland, and negatively correlated with the WUE of forest. The sensitivity of WUE of shrubland to temperature and precipitation was significantly higher than that of forest and grassland. This research is helpful in understanding the climate adaptation mechanism of ecosystem under the background of future climate and land use cover change in Loess Plateau.