Temporal And Spatial Variation Trend Of Atmospheric CO₂ Fertilization Effect In Forests Of Xiaoxing’an Mountains

Forest ecosystems are the mainstay of terrestrial ecosystem,accounting for about one third of the total area of terrestrial ecosystems and play a very important role in the global carbon cycle,and slow down greatly the trend of the global warming.Some believe that strong carbon sink effect of land is very likely to be related to the rise of carbon dioxide in the atmosphere,that is,there exists the fertilization effect（β）of carbon dioxide in the atmosphere.In recent years,the quantitative studies about the fertilization effect of carbon dioxide in the atmosphere have also gradually become a hot topic in global carbon cycle research.However,the spatial and temporal trends of the effect of atmospheric carbon dioxide fertilization in the regional scale are still not clear.Therefore,this research takes the forests of Xiaoxing’an Mountains as an object to study,applies tree-rings data and EC-LUE GPP datasets to quantify the temporal and spatial trends of forest atmospheric carbon dioxide fertilization effects in Xiaoxing’an Mountains from 1982 to 2015,and further compares and analyzes the differences in results between remote sensing GPP products and tree chronology data.According to the tree-rings data,this research firstly uses naturally growing tree-rings data combined with generalized additive mixture models（GAMMs）to remove the effects of tree age and other factors on tree radial growth（BAI）,and then uses a linear mixed models to quantify the fertilization effect of atmospheric carbon dioxide in the residual growth signal of BAI after eliminating the age trend.Finally,this research used a 15-year time window to respectively quantify the spatial and temporal trends of the atmospheric carbon dioxide fertilization effects estimated by the two pathways over the past 34 years.Based on the tree-rings pathways,the results show that there was a significant fertilization effect of carbon dioxide in the atmosphere of 87.5%existing in the study sample areas,and the mean value of all red pine sample areasβ_tree was 40.2±15.1cm²·100ppm^-1,and show a significant decreasing trend overall,the average annual downward trend was about2.1cm²·100ppm^-1y^-1.As the results based on EC-LUE GPP pathway show,89.8%research areas exist the significant atmospheric carbon dioxide fertilization effects,and the mean value ofβ_GPP was 29.3±13.5g C·m^-2·100ppm^-1 for the entire Xiaoxing’an Mountains.Similar to the results of tree-rings,it was also show a significant decreasing trend as a whole（P<0.01）,and an annual average decreasing trend of about 2g C·m^-2·100ppm^-1y^-1;Spatially,β_GPPdecreased most significantly in the southeast of the Xiaoxing’an Mountains,and the decreasing trend was between 2～6g C·m^-2·100ppm^-1y^-1,while a few areas in the northwest of Xiaoxing’an Mountains appear a slight upward trend,and the upward trend of most areas was between 0～5g C·m^-2·100ppm^-1y^-1.In addition,a comparative analysis of the quantitative results of the two pathways revealed a high degree of agreement（r=0.58,P<0.05）,indicating that rising atmospheric carbon dioxide had a significant contribution to both GPP and radial tree growth in forests of Xiaoxing’an Mountains during 1982-2015,however,these effects are gradually diminishing.