Trade-off between watershed water yield and ecosystem productivity along elevation gradients on a complex terrain in southwestern China
Understanding the tradeoffs between water yield and ecosystem productivity is important for developing strategies for large scale ecological restoration worldwide. This study focused on a national forest protection project in the Upper Yangtze River Basin where a logging ban was implemented in 1998. We used a hydrologic model and remote sensing data to study the interactions between water and carbon cycles along elevation gradients in the Minjiang watershed (MJ), where extensive deforestation and reforestation have occurred in the past seven decades. Average annual evapotranspiration (ET), water yield, and gross primary productivity (GPP) from 2000 to 2015 were estimated as 429 mm yr−1, 555 mm yr−1, and 1002 g C m−2 yr−1, respectively. ET decreased sharply and consistently with increasing elevation, whereas GPP only decreased significantly in high elevation areas (i.e.,>3,000 m), resulting in divergent trends of water use efficiency (WUE) with elevation. Evergreen needleleaf forests (ENF) contributed 28% of water yield and 37% of GPP at the watershed scale, while grassland (GRA) also contributed 28% of water yield, but only 20% of total watershed GPP. Moreover, runoff coefficients showed strong negative correlations with GPP, suggesting a general trade-off relationship between water yield and ecosystem productivity in MJ. Our results suggest that vegetation composition and elevation played a key role in determining the relative ecological benefits for carbon and water in the study watershed with a complex terrain.