Direct and indirect effects of climatic factors on ecosystem carbon and water fluxes and water use efficiency across different climatic zones in China

Abstract

Understanding how climatic factors affect ecosystem carbon and water processes is essential for preserving ecosystem health and managing water resources effectively. In addition to their direct impacts, climatic factors also indirectly regulate these fluxes by modifying leaf area index (LAI) and root-zone soil moisture (SMroot). Therefore, this study employed structural equation modeling to evaluate the total (STE), direct (SDE), and indirect (SIE) effects of climatic factors on ecological indicators (EIs). The EIs included gross primary productivity (GPP), evapotranspiration (ET), and ecosystem water use efficiency (eWUE). The findings indicate that temperature is the key climatic driver of GPP, with its STEs ranging from 0.34 to 0.89. The LAI-mediated indirect effect (SIE_LAI) accounts for over 58.6% of the STE, suggesting that warming-induced increases in GPP are primarily due to enhanced LAI. Climatic factors affect ET mainly through direct impacts. In arid regions, STEs of precipitation on ET range from 0.28 to 0.56, whereas in humid regions, STEs range from 0.03 to 0.28, suggesting that precipitation has a more pronounced effect on ET under water-limited conditions. In contrast, under sufficient water availability in humid regions, temperature plays a crucial role in driving ET, with STEs exceeding 0.78. Notably, the indirect effect of precipitation on ET via replenishing SMroot (SIE_SMroot) accounts for more than 50.6% of the SIE in arid and semi-arid regions, indicating that SMroot is a critical mediator of precipitation’s impact on ET. Although wind speed and CO₂ have minimal effects on GPP and ET, they significantly reduce eWUE by lowering GPP and raising ET. As a result, they become key climatic drivers in most regions, particularly in agricultural lands, where STEs fall below −0.36. Warming can enhance eWUE in most regions. However, the effect of precipitation depends on local water availability. It tends to lower eWUE in arid and semi-arid regions and slightly increase it in humid and semi-humid regions.