| Solar-Induced Chlorophyll Fluorescence(SIF)is rich in photosynthetic information and is considered to be a direct indicator of the photosynthetic intensity of vegetation.Gross Primary Production(GPP)is the total amount of carbon fixed by vegetation through photosynthesis,which is important for quantifying global carbon balance and assessing the mitigation effect of ecosystems on climate change.The strong carbon sink potential of tropical and subtropical forests is receiving widespread attention due to their near-evergreen nature throughout the year.As subtropical forests in China are widespread in southwest China and are an important part of the carbon sink in East Asia,accurate estimation of GPP in subtropical evergreen forests is important for assessing forest carbon dynamics in East Asia.The development of SIF in recent years has provided new opportunities to quantify photosynthetic productivity GPP,but there is still a lack of tower-based fluorescence observation studies for evergreen forests(such as osmanthus fragrans forest)in southwest China.Since 2020,a synergistic observation of tower-based fluorescence and carbon flux has been carried out at the Chongqing Jinfo Mountain Karst Ecosystem National Observation and Research Station for subtropical evergreen osmanthus fragrans forest,which provides a feasibility to study SIF and its response relationship with GPP.The SIFGPP coupling relationship can be affected by biotic and abiotic factors such as environmental factors,canopy structure,plant functional types,and growth cycles.Based on this,this study obtained tower-based SIF observations from the Beibei site in Chongqing,combined with eddy flux and environmental meteorological data to analyze the ability of SIF in monitoring photosynthetic potential of osmanthus fragrans forest,to elucidate the differences in SIF-GPP coupling relationships at different time scales,and to further clarify the influence of environmental factors on the SIF-GPP relationship.At the same time,this study addresses the uncertainty of satellite-based SIF products under cloudy and rainy weather characteristics in southwest China,and the lack of effective real-world data verification in southwest China,and carries out the authenticity check of remote sensing products based on tower-based SIF data.The main research findings are as follows.(1)Multi-temporal scale SIF-GPP correlation variation patterns.This study demonstrated that SIF and GPP in osmanthus fragrans forest showed similar trends on seasonal and diurnal scales,but SIF peaked earlier than GPP on 30-minute,daily and monthly scales.SIF had great potential to track GPP changes,but the SIF-GPP relationship varied with time scale,from 30-minute(R2=0.42)to daily average(R2=0.51)scales,the SIF-GPP correlation increased significantly in the aggregation of time scales,this finding that could be further extended to different seasons and weather conditions.At the same time,the SIF-GPP relationship always remained strongly nonlinear.The SIF showed a significant midday depression in summer due to the influence of stronger light,higher vapor pressure deficit(VPD)and solar observer geometry in the corresponding season.(2)Analysis of the influence of environmental factors on the SIF-GPP relationship.In this study,the nonlinearity of SIF-GPP at half-hourly and daily scales could be explained based on environmental factors;the nonlinearity at 30-minute scale might be due to a different mechanism of variation between the linear SIF and the saturation effect of GPP under bright light conditions,while the nonlinearity at daily scale could be due to the weak environmental stress during the study period.It was also found that SIF decoupled from GPP as,for example,VPD increased,indicating that environmental factors influenced SIF and GPP to different degrees.In addition,the correlation and slope of the SIF-GPP relationship were stronger under cloudy conditions than under sunny conditions.SIF was dominated by APAR,unlike the stronger SIF-GPP relationship,the quantum yield of SIF SIFyield(SIF/APAR)and light use efficiency(LUE)showed weaker correlations,leading to a weaker SIF-GPP relationship than APAR-SIF.(3)Analysis of the applicability of satellite-based scale SIF data at BB site.In this study,the daily-scale Tropospheric Monitoring Instrument SIF(TROPO-SIF)product and the eight-day-scale global Orbiting Carbon Observatory-2 based SIF(GOSIF)were selected for authenticity testing work.Both data could match the SIF values at towerbased site to some extent.GOSIF was more strongly correlated with ground-based SIF(r=0.91)and performed significantly better than TROPO-SIF(r=0.66,RMSE=2.036)in monitoring photosynthesis GPP.There is some difficulty in tracking the flux GPP using TROPO-SIF,probably due to the fact that GOSIF is an eight-day synthetic product of fused meteorological data,reducing the uncertainty.In summary,this study addressed the key issues of the lack of continuous towerbased fluorescence observations in osmanthus fragrans forest,the different patterns of SIF-GPP variation at different time scales,and the complexity of the influence of environmental factors on the SIF-GPP relationship,based on synergistic observations of ground-based SIF and flux GPP,combined with environmental data,to elucidate the multi-timescale coupling pattern of SIF-GPP relationship in osmanthus fragrans forest,and to further test the authenticity of satellite-based SIF products.This study conducted a continuous tower-based fluorescence observation study for osmanthus fragrans forest,which provided a theoretical foundation for better understanding the ability of SIF to track photosynthetic changes in osmanthus fragrans forest in southwest China,the patterns and influencing factors of SIF-GPP relationship changes at different time scales,and the applicability of satellite-based SIF products under special weather conditions in southwest China,the study also provided a feasible idea for developing more stable and reliable GPP products. |
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