Impacts Of Solar Radiation On Net Ecosystem Carbon And Water Exchange In A Mixed Plantation In The Xiaolangdi Area

Solar radiation is one of the main environmental factors that control carbon dioxide exchange between forest ecosystem and the atmosphere. As a natural climate variable, clouds strongly influence environmental conditions on the surface of the ground and the proportion of diffuse radiation among solar radiation that reaches the Earth’s surface. In this study, the effects of cloudiness intensity on the net ecosystem exchange of net ecosystem exchange of carbon dioxide (NEE), evapotranspiration (ET), light use efficiency (LUE), and water use efficiency (WUE) were analyzed in a plantation in Xiao langdi forest ecosystem station, Henan province, based on30-min flux and meteorology data obtained for the middle growing season (June-August) during2006-2008. The results showed that NEE reached its maximum when the clearness index (kt) ranged between0.4and0.5(average value is0.44). NEE decreased when the value of k, was higher than0.5. NEE was more negative under cloudy skies than that under clear skies. It was indicated that net carbon uptake increased under cloudy sky conditions. Compared with that under clear skies, the light-saturated maximum photosynthetic rate (Pmax for the plantation under cloudy skies during the mid-growing seasons increased by38%,58%and55%in2006,2007and2008, respectively, and the ecosystem apparent quantum yield(a) is2.6,19and2.2times more than that under clear skies. ET showed a single peak seasonal variation, which was low in fall and winter and relatively high in spring and summer. The LUE was higher under cloudy than under clear sky conditions. The results showed that WUE reached its maximum when the clearness index (k,) ranged about0.35.When the sky conditions change from clear to cloud, the proportion of diffuse radiation to total solar radiation and environmental factors (air temperature, vapor pressure deficit etc.) also change, which influence carbon exchange between forest ecosystem and the atmosphere. During cloudy days, the increase in diffuse radiation received by forest ecosystem was more easily absorbed by shaded leaves for photosynthesis. VPD is an important factor that affects stomatal conductance. VPD is less under cloudy sky conditions than that under clear sky conditions. On general, photosynthetic rates of plants decrease with an increases of VPD. Therefore, the decrease in VPD associated with cloudy conditions can enhance canopy photosynthesis. Temperature is a controlling factor in ecosystem respiration processes. Air temperature decreased linearly with decreasing k, in the plantation. It indicated that a reduction in air temperature could cause a decrease in ecosystem respiration. When k, was more than0.5, diffuse photosynthesis active radiation (PAR(?)) decreased. The increasing diffuse PAR and decreasing VPD and air temperature under cloudy skies improved ecosystem photosynthesis and reduced ecosystem respiration in the middle growing season in the plantation. Consequently, net carbon and WUE uptake by the plantation ecosystem increased.