Abstract
Abstract. Clouds can significantly affect carbon exchange process between forest ecosystems and the atmosphere by influencing the quantity and quality of solar radiation received by ecosystem's surface and other environmental factors. In this study, we analyzed the effects of cloudiness on net ecosystem exchange of carbon dioxide (NEE) in a temperate broad-leaved Korean pine mixed forest at Changbaishan (CBS) and a subtropical evergreen broad-leaved forest at Dinghushan (DHS), based on the flux data obtained during June–August from 2003 to 2006. The results showed that the response of NEE of forest ecosystems to photosynthetically active radiation (PAR) differed under clear skies and cloudy skies. Compared with clear skies, the light-saturated maximum photosynthetic rate (Pec,max) at CBS under cloudy skies during mid-growing season (from June to August) increased by 34%, 25%, 4% and 11% in 2003, 2004, 2005 and 2006, respectively. In contrast, Pec,max of the forest ecosystem at DHS was higher under clear skies than under cloudy skies from 2004 to 2006. When the clearness index (kt) ranged between 0.4 and 0.6, the NEE reached its maximum at both CBS and DHS. However, the NEE decreased more dramatically at CBS than at DHS when kt exceeded 0.6. The results indicate that cloudy sky conditions are beneficial to net carbon uptake in the temperate forest ecosystem and the subtropical forest ecosystem. Under clear skies, vapor pressure deficit (VPD) and air temperature increased due to strong light. These environmental conditions led to greater decrease in gross ecosystem photosynthesis (GEP) and greater increase in ecosystem respiration (Re) at CBS than at DHS. As a result, clear sky conditions caused more reduction of NEE in the temperate forest ecosystem than in the subtropical forest ecosystem. The response of NEE of different forest ecosystems to the changes in cloudiness is an important factor that should be included in evaluating regional carbon budgets under climate change conditions.
Highlights
Solar radiation, temperature and moisture are the main environmental factors that control carbon dioxide exchange between terrestrial ecosystems and the atmosphere (Law et al, 2002; Baldocchi, 2008)
Many studies have shown that increase in diffuse radiation received by ecosystem’s surface can significantly enhance net ecosystem exchange of carbon dioxide (NEE) (Goulden et al, 1997; Gu et al, 1999, 2003; Law et al, 2002) and light use efficiency (LUE) of forest ecosystems (Gu et al, 2002; Alton et al, 2007; Farquhar and Roderick, 2008)
When the value of kt exceeded 0.6, NEE of the temperate forest at CBS reduced more with increasing kt than that at DHS. This suggests that NEE of the temperate forest at CBS was more sensitive to strong solar radiation under clear sky conditions, compared to the subtropical forest at DHS
Summary
Temperature and moisture are the main environmental factors that control carbon dioxide exchange between terrestrial ecosystems and the atmosphere (Law et al, 2002; Baldocchi, 2008). Other environmental variables (temperature, vapor pressure deficit (VPD), etc.) can change (Gu et al, 1999; Urban et al, 2007) Changes in these environmental variables can strongly affect carbon exchange between terrestrial ecosystems and the atmosphere (Letts et al, 2005; Urban et al, 2007). Volcanic eruptions and air pollution have changed atmospheric aerosol content (Niyogi et al, 2007; Farquhar and Roderick, 2008). These changes in cloudiness and atmospheric aerosol content will likely affect the carbon sink function of terrestrial ecosystems in the future
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