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Cloudiness regulates gross primary productivity of a poplar plantation under different environmental conditions

Xu, Hang, Zhang, Zhiqiang, Chen, Jiquan, Zhu, Mengxun, Kang, Manchun
Canadian journal of forest research = 2017 v.47 no.5 pp. 648-658
air temperature, carbon, climatic factors, cloud cover, eddy covariance, forest ecosystems, microclimate, photosynthesis, photosynthetically active radiation, primary productivity, soil water, vapor pressure, China
Cloud cover regulates the gross primary productivity (GPP) of forest ecosystems by changing the radiation component and other environmental factors. In this study, we used an open-path eddy covariance system and microclimate sensors installed over a poplar plantation in northern China to measure the carbon exchange and climate variables during the mid-growing seasons (June to August) in 2014 and 2015. The results indicated that the GPP of the plantation peaked when the clearness index (CI) was between 0.45 and 0.65, at which point diffuse photosynthetically active radiation (PARdᵢf) had reached its maximum. Cloudy skies increased the maximum ecosystem photosynthetic capacity (Pₘₐₓ) by 28% compared with clear skies. PARdᵢf and soil moisture were the most and the least crucial drivers for photosynthetic productivity of the plantation under cloudy skies, respectively. The ecosystem photosynthetic potential was higher under lower vapor pressure deficit (VPD < 1.5 kPa), lower air temperature (Tₐ < 30 °C), and nonstressed conditions (REW > 0.4) for cloudy skies due to effects of Tₐ and VPD on stoma. Overall, our research highlighted the importance of cloud-induced radiation component change and environmental variation in quantifying the GPP of forest ecosystems.