Estimation Of Gross Primary Production Using Sun-induced Chlorophyll Fluorescence In China

Sun-induced Chlorophyll Fluorescence(SIF)is a co-product of photosynthesis.Compared with the vegetation index based on reflectance,it can provide a more intuitive reflection of information related to vegetation photosynthesis,and provide a more direct measurement method for photosynthetic carbon sequestration.The Near-infrared Reflectance of Vegetation(NIRv)is obtained through the Near-infrared Reflectance(NIR_T)and the Normalized Differential Vegetation Index(NDVI),SIF_TOTALis obtained through its ability to eliminate the effects of canopy structure.The relationship between SIF_TOTALand Gross Primary Productivity(GPP)was established based on this data differentiating C3/C4 vegetation as a way to estimate vegetation GPP from 2001 to 2018.Analyzing the spatial distribution characteristics and temporal variability of vegetation GPP in China,and quantifying the relative contributions of three climate factors-temperature,precipitation and solar radiation-to inter-annual variability(IAV)of GPP,the main conclusions were as follows:(1)By zero-intercept fitting of the GPP observations at the vorticity flux sites with the SIF_TOTALdata on the corresponding raster divided into C3/C4 vegetation,the ratio coefficient of GPP_FLUXto SIF_TOTALfor C3 vegetation was 127.06(R²=0.70,P<0.01),and the ratio coefficient of GPP_FLUXto SIF_TOTALfor C4 vegetation was 277.16(R²=0.65,P<0.01).The GPP observation data in the two vegetation types are highly correlated with SIF_TOTAL,and both have passed the significance test of P<0.01,indicating that the ratio coefficient can be used for the estimation of Chinese vegetation GPP.(2)From 2001 to 2018,the spatial distribution of monthly GPP in China was quite different.From January to July,it showed a gradual increase from southeast to northwest;from August to December,it showed a gradual decrease from northwest to southeast.The spatial distribution of GPP in the year shows a downward trend from southeast to northwest.The annual average value of GPP is 790.25 g C·m^-2·a^-1,and the total amount is 7.19 Pg C·a-1.The overall change during the year showed a unimodal distribution with an increase first and then a decrease,with the highest value of GPP in July and the lowest value in January.From 2001 to 2018,the change trend of GPP from the month of 2001 to 2018 showed a fluctuating upward trend.The change rate of GPP was the fastest in May,and the change rate of GPP was the slowest in February.In the year GPP showed a fluctuating upward trend at a rate of 4.77 g C·m^-2·a^-1.It shows that China's vegetation status is showing a good development trend.(3)Among the 32 provinces(autonomous regions and municipalities directly under the central government)in China from 2001 to 2018,Hainan Province had the highest mean value of GPP,and Xinjiang Uygur Autonomous Region had the lowest.In terms of change trends,all provincial-level administrative regions show a fluctuating upward trend in GPP,with Shanxi Province rising the fastest;Tibet Autonomous Region rising the slowest,indicating that the ecological condition of vegetation has improved greatly in all of China's provincial-level administrative regions.(4)The mean values of correlation coefficients between Chinese vegetation GPP and temperature,precipitation,and solar radiation are 0.11,0.10,and-0.06,respectively.GPP has the strongest correlation with temperature,followed by precipitation and solar radiation.Vegetation GPP IAV has the strongest sensitivity to temperature IAV,with an average sensitivity coefficient of 28.83;followed by precipitation(0.16)and solar radiation(0.005).(5)The mean relative contribution of temperature to GPP IAV was 7.99%,the mean relative contribution of precipitation to GPP IAV was 10.09%and the mean relative contribution of solar radiation to GPP IAV was 7.32%,with the mean total relative contribution of temperature,precipitation and solar radiation to GPP IAV being25.42%,with the areas showing a positive contribution to GPP IAV accounting for97.79%and 2.21%of the areas with negative contribution to GPP IAV.This indicates that precipitation makes the greatest contribution to vegetation GPP IAV,followed by air temperature and solar radiation.Factors other than temperature,precipitation and solar radiation should not be neglected.