Impacts Of Changes In Solar Radiation On Terrestrial Gross Primary Productivity In China

Terrestrial ecosystem carbon cycle is one of the hot spots in global change studies currently.Gross primary productivity?GPP?,which is an important component in global terrestrial carbon cycle,indicates the ability for plants to fix carbon or energy by photosynthesis.Solar radiation at the Earth's surface,as the primary energy source of photosynthesis,is a crucial environmental factor that affects terrestrial GPP.To better estimate GPP at regional or global scales,all kinds of climate or non-climate factors should be considered comprehensively.In this study,China's radiation datasets,meteorological data,leaf area index data and land cover data were used to driven the two-leaf light use efficiency model?TL-LUE?to investigate the spatial distribution of China's terrestrial GPP and the impacts of solar radiation change to China's terrestrial GPP from 1981 to 2012.The TL-LUE model was first globally calibrated and validated using carbon and meteorological observation at 58 typical flux sites.Then 3 times simulations were executed by utilizing TL-LUE model from 1981 to 2012.Finally,the spatial and temporal variation of GPP in China's terrestrial ecosystem was analyzed and the contribution of radiation change to the variation of GPP is identified.The main conclusions could be drawn as followings:?1?The ability of TL-LUE model in simulating GPP for different vegetation types at multi time scalesIn this study,the MOD 17 algorithm,TL-LUE model and nonlinear two leaf light use efficiency model?TL-LUEn?were calibrated by using carbon flux data,meteorological data,the Moderate Resolution Imaging Spectroradiometer?MODIS?LAI data and land cover data at 85 site years.Then the calibrated parameters were validated at other 58 site years.The results suggest that,TL-LUEn model performed slightly better than TL-LUE at half-hourly and daily scale,and poorer than TL-LUE at 8-day scale.TL-LUE model showed better performance than MOD17 algorithm at all the three time scales and different vegetation types,but the difference between the simulated GPP by TL-LUE and MOD 17 decreased as the time scale increased.Both TL-LUEn and TL-LUE reduced the sensitivity of the error in simulating GPP to the radiation.Thus,it indicates that stratifying the canopy into sunlit and shaded leaves can effectively improve the ability of LUE models in simulating GPP,especially at short time scales?half-hourly and daily time scales?.?2?The parameterizing results and regional applicability of TL-LUE modelThe value of optimized maximum light use efficiency?LUE?of shaded leaves(?_msh)is 2.6-5 times larger than that of optimized maximum LUE of sunlit leaves(?_msu)for TL-LUE model.The value of optimized maximum LUE(?_msu)for MOD 17 model is smaller than ?_msu but larger than ?_msu,As for specific vegetation type,?_msu,?_msu and?_msu change little in different time scales and sites,while the optimized parameters for TL-LUEn??m and ??change a lot in different time scales and sites.Therefore,TL-LUEn is more applicable at individual sites at the half-hourly scale while TL-LUE could be regionally used at half-hourly,daily and 8-day scales.MOD 17 is also an applicable option at 8-day scale.?3?The spatiotemporal variability of solar radiation in China from 1981 to 2012The average value of nationwide annual global solar radiation and diffuse radiation are 5258,16 MJ m^-2 yr^-1 and 2496.40 MJ m^-2 yr^-1,respectively.In general,the global solar radiation is lower in the west,and higher in the east.The south of Qinghai-Tibet Plateau is high value area,while the Sichuan Basin is a low value center across China.The diffuse radiation is lower in the north,and higher in the south and west.The south of Qinghai-Tibet Plateau and southwest of Yunnan-Guizhou Plateau are the high value center across China,while the northeast China and Sichuan Basin have relatively low values of diffuse radiation.The diffuse fraction is lower in the west,and higher in the east.The high value area of diffuse fraction is in Sichuan Basin.The nationwide annual global solar radiation has a significant decrease trend(2.18 MJ m^-2 yr^-1,p<0.05)from 1981 to 2012,decreased by 1.3%in the 32 years.The decrease mainly happened in Beijing,Tianjin,Hebei,and Qinghai-Tibet Plateau.The nationwide annual diffuse radiation increased significantly?p<0.05?from 1981^-2012,and the rising speed is 0.2%10a^-1.The increase happened in the same area as the decrease of the nationwide annual global solar radiation happened.The nationwide annual diffuse radiation showed a certain degree of upward trend.?4?Impacts of radiation change to GPP of terrestrial ecosystem in China from 1981 to 2012The averaged annually national total GPP in China estimated by TL-LUE is 6.78 Pg C yr^-1,of which the contribution made by diffuse radiation is 5.05 Pg C yr^-1.The averaged annually GPP is decreasing from southeast to northwest and from the coast to inland.The high value areas are Southeast China and some part of Northeast China,in which the averaged annually GPP is higher than 1000 g C m^-2 yr^-1.The low value areas are northwest China,in which the averaged GPP is lower than 200 g C m^-2 yr^-1.The simulation results indicate that,the decrease of global solar radiation from 1981 to 2012 causes that the averaged annually GPP in China reduced by 0.13 Pg C yr^-1,while the increase of diffuse radiation fraction from 1981 to 2012 causes that the averaged annually GPP in China rised by 0.06 Pg C yr^-1.Therefore,the increase of canopy LUE under higher diffuse radiation fraction can indeed make up the loss of GPP under lower global solar radiation to some extent.