Abstract: Aims Cloud and aerosol variations influence the total amount of solar radiation as well as the proportion of diffuse and direct radiation, which can potentially alter the microclimate of forest ecosystems. The clearness index (CI), a crucial parameter for assessing sky conditions, reflects changes in solar radiation. However, it remains unclear how clouds and aerosols impact the dynamics of carbon exchange in light-sensitive tropical montane rainforests. Methods Based on carbon flux data and meteorological data during the wet (June to October) and dry seasons of 2013–2017, we compare the difference in gross primary productivity (GPP), ecosystem respiration (ER), and net ecosystem productivity (NEP) under clear and cloudy conditions. The light response model was derived using a rectangular hyperbolic curve. Meanwhile, we utilized partial correlation analysis and a structural equation model to assess the influence of diffuse photosynthetically active radiation (PARf), direct photosynthetically active radiation (PARd), air temperature (Ta), vapor pressure deficit (VPD), and volumetric soil water content (VWC) on GPP, ER, and NEP. Important findings Cloudy skies improve the efficiency in utilizing photosynthetically active radiation, with the canopy quantum efficiency (α) increased by 45–88%. Additionally, cloudy skies enhanced canopy photosynthesis and net carbon uptake while reducing ER. GPP and NEP increased by 6–8% and 17–21%, while ER decreased by almost 2%. PARd decreased dramatically following the decline in CI, while PARf changed only slightly. Since PARf directly enhanced NEP, it offset the suppression caused by the decline in total PAR. PARf and PARd are the major influencing factors of GPP, controlling its variation under clear and cloudy skies, respectively. Ta was the most controlling factor of ER, determining its variation under clear skies, while Ta, VPD, and VWC jointly controlled ER under cloudy skies. GPP and NEP peaked at moderate radiation (PAR=1 300–2 000μmol·m-2·s-1) and moderate CI (=0.3–0.5), while ER was maximized at high radiation levels (PAR=2 300μmol·m-2·s-1) and comparatively high CI (=>0.5). Overall, CI regulates both the quantity and quality of solar radiation. The net effects of diffuse radiation can compensate for the loss of total PAR quality, while intermediate levels of PAR and CI can enhance carbon exchange. This study emphasizes the crucial role of radiation and environmental effects induced by clouds and aerosols, offering insights for advancing our understanding of how tropical forests respond to climate change.

Key words: net ecosystem productivity, clearness index, diffuse radiation, tropical montane rainforest, eddy covariance