Modeling Effects Of Changes In Diffuse Radiation On Light Use Efficiency In Forest Ecosystem

The changes of diffuse radiation fraction in global solar radiaton can affect light use efficiency(LUE)and carbon sink function of forest ecosystem.In this study,we used 30-minute flux data and routine meteorological data observed at broad leaved Korean pine forest of Changbai Mountain(CBS),subtropical plantation coniferous forest at Qianyanzhou(QYZ),and subtropical coniferous and broad-leaved mixed forest in Dinghushan(DHS)from 2003 to 2008.Applying the modified light response model,we analyzed the initial quantum efficiency and the capacity of a canopy to resist photosynthetic saturation at high levels of photosynthetically active radiation(PAR)under direct PAR and diffuse PAR in order to clarify the effects of diffuse PAR on photosynthetic characteristics of vegetation.Furthermore,based on 30min flux data,routine meteorological data,and vegetation and soil parameters,we used Soil-Plant-Atmosphere Continuum(SPAC)model to simulate LUE in different layers of canopy and analyze the impacts of changes in diffuse radiation on LUE in CBS and QYZ from June to August from 2006 to 2008.The results showed that:(1)The LUE in the ratio range of diffuse radiation to global solar radiation(Sf/S0)between 70%and 85%was more than that in the Sf/S0 range between 55%and 70%by averaged 14.0%?10.1%and 16.0%at CBS,QYZ,and DHS,respectively during mid-growing season(June to August)from 2003 to 2008.(2)The initial quantum efficiency under diffuse PAR condition(af)is higher than that(?r)under direct PAR condition by averaged 0.29g CO2 mol-1 photon in CBS from June to August from 2003 to 2008.?f was higher than ?r by averaged 0.26g CO2 mol-1 photon in mid-growing season without drought stress in QYZ.In DHS,?f was higher than ?r by averaged 0.3 1g CO2 mol-1 photon.The results indicated that the diffuse PAR could enhance LUE than the direct PAR under weak light intensity conditions without air temperature and water stress.Compared with diffuse PAR condition,the capacity of canopy to resist photosynthetic saturation at high levels of PAR(?f)is higher than that(?r)under direct PAR condition in CBS,QYZ,and DHS by averaged about 0.4mg CO2 m-2s-1 during study period.The results verified that the diffuse PAR could improve the photosynthetic capacity of forest ecosystem,comparing with direct PAR.(3)Based on the SPAC model simulation,the maximum LUE of CBS and QYZ occured at the 6th or 7th layer in canopy without condition control.When the other environmental factors kept normal daily variation and the diffuse radiation increased by 10%,the change of LUE in ten layers of canopy in CBS and QYZ showd a gradual increasing trend.Moreover,the increase in LUE is the largest in the tenth layer of canopy.The increase in LUE in the tenth layer of canopy can reach 19.2%and 14.3%at CBS and QYZ,respectively.For the entire canopy,the gross primary productivity(GPP)and LUE increase respectively by average of 3.4%and 16.4%in CBS and increase respectively by average of 2.8%and 11.4%in QYZ.The results indicated that the increase of the diffuse radiation enhanced LUE at CBS and QYZ.