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Liu Zhigang Ma Qinyan(Beijing Forestry University,Beijing,100083,China) Pan Xiangli(Suburbs Forestry Bureau,Nanning,Guangxi) Xie Zhengming(Sichuan Forestry Cadres School,Chengdu)  
The leaf growth course of a Larix principis rupprechtii plantation located in North China is from the last ten day period of April to the first ten day period of June. Based on the data of actual measurement of six day typical sunny days in the period of leaf growth,the dynamic processes of light attenuation and light absorption in the different layers of forest canopy were determined and analyzed. In sunny days,the observed daily mean values of PAR transmitted through the canopy range from 30.53% to 3.25% in transmittance,from 6.48% to 4.12% in reflectance,from 65.33% to 92.93% in absorptance and from 0.2787 to 0.4640 in extinction coefficient. During the deciduous period,approximate 65% of PAR energy of incidence is absorbed by the branches and boles of stand. The dynamic status of light distribution in the forest canopy is not only related closely to the increase of leaf quantity,but also affected remarkably by the unfolding extent of the bunches of leaves. During the leaf growth,the radiation absorption equation of the canopy can be formulated as follow:ΔQ ct =25.36170 exp (0.10715SAI), r=0.9821in which, ΔQ ct is the total daily absorption amount of PAR (molm -2 d -1 )transmitted through the canopy and SAI is the surface area index of stand(m 2/m 2). The main absorption surface(i.e.acting surface) of the forest canopy and the contribution of the canopy below the averagy stand contact height to solar radiation absorption are closely related to the leaf quantity of stand.At the end of the leaf growth,most of incident light energy(over 78%) is absorbed by the forest canopy above two thirds of the average stand height. Only 8% or less of the energy,however,is absorbed by the canopy below the average stand contact height. The extinction coefficient is the lowest in the central canopy but the highest in the lower canopy.
【Fund】: 国家自然科学基金
【CateGory Index】: S791.22
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