Understanding increases in tree growth by hydropower development using tree-ring carbon and oxygen stable isotopes

Rapid and intensive hydropower development in dry valleys in southwest China, the world's most hydropower-rich region, is likely to have a significant impact on the growth and landscape of adjacent plants. Our preliminary study found that there was a significant increase in the growth of terrestrial trees closing to the bottom of the valleys within three years after the impounding of artificial reservoir, and followed by a rapid decline thereafter. However, it’s unclear about the mechanism of this growth increase-decrease. In this study, we collected tree rings from four trees and analyzed the carbon and oxygen stable isotope ratios (δ13C and δ18O) of their α-cellulose. We found that δ13C and δ18O did not correlate well with tree-ring widths, but intrinsic water use efficiency (WUE) had a significantly negative correlation with tree whorl widths. δ13C was in better agreement with each other than that for δ18O, and the latter was better correlated with source water (i.e., precipitation). Analysis on data in the three years after impoundment showed that δ13C and δ18O changes well with tree growth, but WUE did not show this accelerating effect. These may indicate that hydropower development on local climate may not promote plant growth and that there are more complex mechanisms for plant growth changes around reservoirs. This study is important for understanding the effects of human activities on plant growth and the mechanisms of ecological restoration.