辐射变化对中亚热带杉木人工林净CO2交换的影响

太阳总辐射是影响森林生态系统碳交换的重要因子。为认识辐射变化对杉木人工林碳交换的影响,本研究利用开路式涡度相关系统和气象梯度观测系统测得的CO₂通量和气象因子长期定位监测数据,用晴空指数(k_t)表示太阳辐射情况,分析了k_t对中亚热带杉木人工林生长季(4—10月)净CO₂交换(NEE)的影响。结果表明: 晴天时的太阳总辐射通常上午高于下午,杉木林NEE则上午低于下午,NEE的这种差异在太阳高度角为50°左右时达到峰值;在中等k_t(0.42～0.52)时,杉木人工林生态系统的碳吸收最强,不同高度角区间内10年平均最大NEE相对变化范围为11.0%～29.4%,其中最小和最大临界值分别出现在太阳高度角区间35°～40°和45°～50°;有云存在的中等k_t时,杉木林生态系统碳吸收和漫射光合有效辐射达到最大,后者可能是前者产生的主要原因。有云存在时的中等辐射条件对杉木人工林的NEE有促进作用,使碳吸收达到最大。

Effects of radiation changes on net ecosystem exchange of carbon dioxide in a middle subtropical Chinese fir plantation

Total solar radiation is an important factor affecting carbon exchange in forest ecosystem.In order to understand the effects of radiation change on carbon exchange in Chinese fir plantation,long-term monitoring data of carbon dioxide flux and meteorological factors measured by open eddy covariance system and meteorological gradient observation system were used in this study.The clearness index(k_t) was used to represent the condition of solar radiation.We analyzed the effects of k_t on net ecosystem exchange of carbon dioxide(NEE) in the central subtropical Chinese fir plantation during the growing season(from April to October).The results showed that total solar radiation in clear sky was usually higher in the morning than that in the afternoon,and that NEE was lower in the morning than in the afternoon.Such difference in NEE reached the maximum when the solar elevation angle was about 50°.At the medium k_t(0.42-0.52),carbon absorption of Chinese fir plantation was the strongest.The ave-rage maximum relative change of NEE in 10 years in different solar elevation angles ranged from 11.0% to 29.4%,while the minimum and maximum critical values appeared at 35°-40° and 45°-50°,respectively.When k_t was at the moderate degree due to the existence of clouds,carbon absorption and diffuse photosynthetically active radiation of Chinese fir plantation reached the maximum,and the latter might be the main reason for the former.Moderate radiation condition with the presence of cloud clould promote NEE of Chinese fir plantation and lead to largest carbon absorption.