Spatial-temporal variation and driving mechanism of vegetation carbon and water use efficiency in Heilongjiang Province, China.

Carbon use efficiency (CUE) can reflect carbon assimilation capacity and carbon sequestration potential of organisms. Water use efficiency (WUE) can reflect the energy conversion efficiency during plant growth. Both indices can represent the carbon and water cycle mechanism of terrestrial ecosystems. Based on MODIS data pro-ducts, we calculated the annual CUE and WUE of vegetation in Heilongjiang Province from 2001 to 2020, and analyzed the variations and driving mechanism of carbon and water use efficiency in cropland, woodland, shrubland and grassland by using Sen-MK method, Hurst index analysis, partial correlation analysis and residual analysis. The results showed that the mean value of CUE and WUE in Heilongjiang Province was 0.550 and 0.869 g C·m^-2·mm^-1, respectively, with high values in north and south and low values in east and west. During 2001-2020, 54.8% of vegetation CUE in the study area showed an increasing trend, and 59.6% of vegetation WUE showed an increasing trend. The increases of both CUE and WUE were highest in woodland and lowest in cropland. Vegetation CUE and WUE were negatively correlated with temperature and positively correlated with precipitation. The contribution rates of climate change and human activities to vegetation CUE were 46.6% and 53.4%, and the contribution rates to vegetation WUE were 42.3% and 57.7%, respectively.