The spatio-temporally varying relationship between water yield and carbon storage services of larch plantation in the Liupan Mountains of northwest China

The water-carbon relationship in forests is complex and plays a crucial role in the provision of important ecosystem services, such as water yield and carbon storage. Understanding this intricate relationship is imperative for effectively managing the multiple services of forests. Therefore, a study was conducted to investigate the spatio-temporal variation of water yield and carbon storage as well as their relationship in larch plantations located in the semi-humid Liupan Mountains of northwestern China. The water yield data during growing season were calculated using the water budget principle, which involved subtracting evapotranspiration determined through long-term monitoring of hydrological processes from precipitation. Carbon density data were obtained by summing up vegetation carbon, humus carbon, and soil organic carbon (SOC) of 0-100 cm. These datasets were utilized to develop models that integrated the effects of key factors such as tree density, tree age, and elevation. Subsequently, these models were employed for predicting variations in water yield and carbon density while analyzing their relationship. The results indicate that the water yield initially exhibits a rapid decrease followed by a gradual decrease as tree density increases; additionally, it decreases initially and then increases with rising tree age and elevation. The carbon density shows an initial increase with rising tree density until a threshold, after which it declines; however, the limit of maximum tree density prevents stands with higher tree ages from reaching this threshold on more favorable sites. On other hand, carbon density consistently increases with rising tree age, and exhibits an initial increase followed by a decrease with rising elevation. Consequently, due to the combined effects of carbon density and water yield, their relationship displays a highly complex spatio-temporal variation pattern characterized by three distinct features: 1) A general variation mode exists in which there is a tradeoff relationship between decreasing water yield and increasing carbon density with rising tree density until a threshold, followed by a synergistic relationship where both services decrease; 2) Within this variation mode, the peak carbon density increases with rising tree age and site quality, while the range of water yield variation initially decreases and then increases with rising elevation; 3) However, on stands with higher age and better sites, the peak carbon density associated with the tree density threshold cannot be reached, resulting in only a tradeoff relationship existing. The multifunctional forest management aiming to achieve a balanced supply of dominant service (e.g., water yield in dryland regions) and other important services (e.g., carbon sequestration) should be improved by considering the complex water-carbon relationship.