Seasonal Variations Of Maximum Carboxylation Rate And Its Effects On Terrestrial Gross Primary Production Simulation

Terrestiral gross primary productivity(GPP)is a measure of photosynthesis rate of vegetation,an important component of carbon cycling in terrestrial ecosystems,which significantly affects global carbon balance.Process based ecological models is an effective means to estimate the temporal and spatial characteristics of regional and global terrestrial GPP.The maximum carboxymethyl rate(Vcmax)of the leaves is an indispensable parameter to characterize the photosynthetic ability of vegetation,which is the rate of the important speed-limiting reaction in the process of photosynthesis,and plays a decisive role in ecological models.Obtaining seasonal variation characteristics of Vcmax is of great significance for GPP simulationIn this study,the Ensemble Kalman filter method combined with flux data was used to optimize the Vcmax firstly,and temporal and spatial variation of Vcmax and its relationships with environmental factors was analyzed,and then correlations between chlorophyll index and Vcmax was analyzed,finally Vcmax with seasonal variations and annual averaged Vcmax were used to drive BEPS model to simulate GPP in China during 2007-2010,respectively.The results provide a basis for the acquisition of seasonal and spatial variation information of Vcmax,and lay the foundation for improving the accuracy of regional GPP simulation.The contents and conclusions are as follows:(1)Temporal and spatial variation of Vcmax and its relationship with environmental factorsThere were obvious seasonal variations in Vcmax,which varied from 18-32 ?mol CO2 m-2 s'1.There was a positive correlation between Vcmax and environmental factors such as temperature and radiation.(2)Correlations between chlorophyll index and VcmaxThere were linear relationships between chlorophyll index and Vcmx.The R2 between MTCI(Medium Resolution Imaging Spectrometer Terrestrial Chlorophyll Index)and Vcmax changed from 0.19(leaf scale)to 0.32(canopy scale).Correlations between chlorophyll index MTCI and TCARI(Transformed Chlorophyll Absorption in Reflectance Index)with Vcmx was improved during 2006-2010 at UMBS site.(3)Effects of seasonal variations of Vcmax on GPP simulationGPP distributed spatially variations in China,and the overall trend is increasing from northwest to southeast,and the average annual GPP of China region in 2007-2010 is 6.69 Pg C yr-1.Montly total GPP varied from 0.08 Pg C month-1 to 1.25 Pg C month-1.There were strong positive correlations between simulated GPP with BEPS model and GPP MPI and SIF(Sun-induced chlorophyll fluorescence)with high R2 values of 0.98 and 0.95,respectively.Compared with annal averaged Vcmax,relationships between GPP simulated with seasonal variation of Vcmax and GPP MPI and SIF increased.R2 between GPP and GPP MPI in the spring,summer,autumn and winter,increased 0.03,0.01,0.01 and 0.03 respectively,and R2 of vegetation type ENF(Evergreen needleleaf forest),EBF(Evergreen broadleaf forest),DNF(Deciduous needleleaf forest),S(Savannas)and Crop were increased obviously in the seasons;The determination coefficients R2 between GPP and SIF in the spring,summer,autumn and winter,increased 0.02,0.01,0.001 and 0.02 respectively,and the R2 of vegetation type ENF,DNF,DBF and Crop were improved.