Effects of clouds and aerosols on ecosystem exchange, water and light use efficiency in a humid region orchard

Abstract

Investigating the patterns of water and carbon dynamics in agro-ecosystems in response to clouds and aerosols can shed new insights in understanding the biophysical impacts of climate change on crop productivity and water consumption. In this study, the effects of clouds and aerosols as well as other environmental factors on ecosystem water and carbon fluxes were examined based on three-year eddy covariance measurements under different sky conditions (quantified as the clearness index, K_t, i.e., the ratio of global solar radiation to extraterrestrial solar radiation) in a kiwifruit plantation in the humid Sichuan Basin of China. Results showed that evapotranspiration (ET) and canopy transpiration (T_c, measured by sap flow sensors) increased, while ecosystem light use efficiency (eLUE) and ecosystem water use efficiency (eWUE) decreased with increasing K_t. GPP presented a parabolic relationship with increasing K_t. The path analysis revealed that surface conductance (G_s) and canopy conductance (G_c) were the most dominant variables directly regulated carbon (GPP) and water (ET and T_c) fluxes. The effect path of K_t on ET and T_c was converted from through diffuse photosynthetic active radiation (PAR_dif) to direct PAR (PAR_dir) when the sky became clearer. The effect path of K_t on GPP was primarily through PAR_dif under different sky conditions. The declined eWUE with increasing K_t was caused by the different responses of GPP and ET to PAR_dir under clear skies. The declined eLUE resulted from the sharp decrease in GPP/PAR_dir, which surpassed the slight increase of GPP/PAR_dif with increasing PAR. The Priestley-Taylor Jet Propulsion Laboratory ET model (PT-JPL) incorporating K_t with an exponential function produced more reliable T_c estimates but minor improvement in ET. Further, the LUE-GPP model incorporating K_t with a linear function obtained much better GPP estimates. Our study shed light on how sky conditions modulate water and carbon dynamics between the biosphere and atmosphere, highlighting the necessity of the inclusion of sky conditions for better modeling regional water and carbon budgets.

Keywords: Canopy transpiration; Clouds and aerosols; Ecosystem light use efficiency; Ecosystem water use efficiency; Evapotranspiration; Gross primary productivity.