Contributory factors of the secular trends to changes in ecosystem water-use efficiency in China

Abstract

Ecosystem water-use efficiency (WUE) is an effective indicator of ecosystem functioning, which is widely used to characterize the coupling relationship between terrestrial carbon and the water cycle. Although the linear trends in WUE have been well reported, the nonlinearity of the secular trends in WUE and their relative contribution to the overall changes were still poorly investigated. In this study, we used ensemble empirical mode decomposition (EEMD) to extract the secular trends of the WUE and their relative contribution (RC) to the WUE changes in China, which represents the stability of the ecosystem functioning. We found that although increasing trends dominated the secular trends of the WUE, nearly half of the study areas had experienced trend shifts. Except that deciduous forests and mixed forests were dominated by trend shifts, most of the land cover types were dominated by increasing trends. The RC exhibited a bimodal pattern along the elevation gradient, with peaks at 20 m and 5000 m. Similarly, the RC peaked at the east and south aspects, and was the lowest for the west and north aspects. In comparison, the RC exhibited a U-shaped pattern with the slope, and was the lowest at 80°. For the monotonically increasing trends and decreasing-to-increasing trends, the high RC mainly occurred on the North China Plain and Loess Plateau, indicating ecosystem improvement and strong stability. Meanwhile, it was low in northeastern China and southwestern China, indicating unsustainableecosystem improvement. The RC was always very low for increasing-to-decreasing trends and monotonically decreasing trends, indicating low ecosystem stability with a high possibility for improvement. Temperature and the atmospheric carbon dioxide (CO2) concentration were the two main climatic factors driving the WUE changes. The climatic mean state rather than the climatic variability affected the ecosystem stability. The results of this study highlight the relative contribution of the secular trends to WUE changes and provide a better understanding of the structure and the stability of ecosystem functioning.