Cloudy Sky Conditions Promote Net Ecosystem CO2 Exchange in a Subtropical Coniferous Plantation Across Seasons

Dynamic changes in solar radiation have an important influence on ecosystem carbon sequestration, but the effects of changes caused by sky conditions on net ecosystem CO2 exchange (NEE) are unclear. This study analyzed the effects of sunny, cloudy, and overcast sky conditions on NEE using carbon flux and meteorological data for a subtropical coniferous plantation in 2012. Based on one-year data, we found no seasonal variation in the light response curve under various sky conditions. Compared with sunny sky conditions, the apparent quantum yield (α) and potential photosynthetic rate at a light intensity of 150 and 750 W m−2 (P150 and P750) under cloudy sky conditions increased by an average of 82.3%, 217.7%, and 22.5%; α and P150 under overcast sky conditions increased by 118.5% and 301% on average. Moderate radiation conditions were more favorable for maximum NEE, while low radiation conditions inhibited NEE. In most cases, when the sunny NEE was used as a baseline for comparison, the relative change in NEE (%NEE) was positive under cloudy sky conditions and negative under overcast sky conditions. The average maximal %NEE under cloudy sky conditions was 42.4% in spring, 34.1% in summer, 1.6% in autumn and –87.3% in winter. This study indicates that cloudy sky conditions promote photosynthetic rates and NEE in subtropical coniferous plantations.