Multi-scale Coupling Characteristics And Influencing Factors Of Solar-induced Chlorophyll Fluorescence And Photosynthetic Rate In Vegetation Canopy

The material and energy provided by plant photosynthesis are the key factors for human survival.Solar-induced chlorophyll fluorescence?SIF?is a by-product of plant photosynthesis and is closely related to plant photosynthesis.The study on SIF and its relationship with gross primary productivity?GPP?are of great significance for understanding the mechanism of photosynthesis.The relationship between SIF and GPP is affected by environmental conditions,canopy structure,vegetation physiological and biochemical status,vegetation growth stage,plant function type,and other factors,resulting in the complexity of the relationship between SIF and GPP.In this paper,the high-frequency SIF and corresponding eddy-covariance observation of different ecosystems were carried out at multiple sites.Based on the SIF,meteorological,and eddy covariance data,we illuminated the coupling rule of the SIF-GPP relationship at multi-time scales and analyzed the similarities and differences of the relationship at different ecosystems.Further,we clarified the characteristics of canopy structure and environmental conditions on the SIF-GPP relationship.On this basis,methods on how to correct and mitigate these effects are proposed.The main findings and conclusions of the paper are as follows:?1?Variation of the coupling relationship between SIF and GPP at different time scales.The SIF-GPP relationship is different at different time scales.On the daily scale and the 8-day scale,SIF-GPP showed strong linear correlations in both sunny and cloudy conditions but showed significant nonlinear correlation on the half-hour scale.The study also shows that there is a difference in the response pattern of SIF to PAR on sunny days.SIF exhibits a significant hysteresis response to PAR on sunny days,which may result in possible deviations when calculating the daily average SIF using instantaneous satellite-based SIF and correction factors.However,this hysteretic response is not observed on cloudy days.In addition,SIF and GPP had generally positive correlation under different ecosystem types though there are still some differences.?2?Vegetation growth stage and canopy structure affect the SIF-GPP relationship significantly.Results show that the SIF-GPP correlation varies with the growth stage in a maize ecosystem?the slope of the regression model is different?,indicating that the canopy structure affects the seasonal change of canopy SIF and its relationship with GPP.More importantly,results show that the significant correlation between SIF_yield and LUE on the seasonal scale is mainly due to changes in canopy structure.After using MTVI2 to correct the effect of the structural effects on SIF_yield,the correlation between SIF_yield and LUE was significantly reduced.These experimental findings have deepened the understanding of the mechanisms between canopy SIF and photosynthesis.?3?Influencing characteristics of direct and scattering radiation changes on SIF and its relationship with GPP.The study analyzed the relationship between clearness index?CI?and SIF quantitatively,then evaluated the impact of direct and scattering radiation changes?cloudy and sunny?on the changes of SIF at different time scales.Results show that there is a positive correlation between SIF and CI,indicating that SIF has a higher correlation with direct radiation.The correlation between GPP and CI is low,which is mainly caused by the higher photosynthesis intensity under cloudy conditions than under sunny conditions.Further,the study shows that the SIF-GPP relationship under different atmospheric conditions demonstrates a significant difference.Generally,the slope and R² of the SIF-GPP linear regression model on cloudy days is larger than that of the sunny model even in different ecosystems.In summary,this study focuses on the influence of canopy structure and environmental conditions on the relationship between SIF and GPP.Firstly,we analyze the coupling rule of SIF-GPP at multiple time scales and the difference of SIF-GPP correlation at different ecosystems based on multi-scale remote sensing data.Then,we further investigate the effects of canopy structure and atmospheric conditions on the SIF-GPP relationship at the seasonal scale.The result provides a theoretical and experimental basis for a better understanding of the SIF-GPP association mechanism and influencing factors,which is of great importance to the global carbon cycle research.