Coordinated Measurements Of Solar-induced Chlorophyll Fluorescence And Eddy Covariance Over A Mangrove

Because coastal wetland ecosystem provides many important ecosystem services and has high productivity,more and more ecologists pay attention to understand the carbon budget of coastal wetland ecosystem.Mangrove is an important blue carbon ecosystem,so accurate characterization of gross primary productivity(GPP)is critically important in assessing mangrove carbon budgets,but current knowledge of the seasonality of GPP in evergreen mangroves is very limited.Remote sensing of Solar-induced chlorophyll fluorescence(SIF)has emerged as a promising approach to approximating GPP across ecosystems including crop,grassland,deciduous forests and evergreen forests,but its capability for tacking GPP in mangroves has not been assessed.Therefore,the objectives of this study are to analyze the sensitivity of GPP-SIF relationship to plant photosynthetic physiology over a mangrove.This study took a subtropical mangrove in Zhangjiang estuary of Fujian province in southeast China as the research area.We explored dynamic changes of SIF and GPP across different time scales based on one-year time series measurements of hyperspectral and eddy covariance system,and then discussed the relationship between SIF-GPP of subtropical mangrove and its response to environmental stress.The main work of this paper is as follows:(1)In this paper,the daily and seasonal time scales of SIF and GPP were studied in the subtropical mangroves.SIF and GPP showed the similar rule in the time scale of day and season,they rised first and then fall in the time scale of day,and they changed in the same peak and trough in the time scale of season.Meanwhile,the SIFy(SIFy:the ratio of SIF over absorbed photosynthetically active radiation(APAR))and LUE(Light Use Efficiency:the ratio of GPP over APAR)was analyzed.The relationship between SIFy and LUE were more dynamic than that between SIF-GPP,and SIFy and LUE tend to be stable at noon,but separate from each other in the morning and afternoon.These findings confirm that SIF can be used as a potential remote sensing index of mangrove canopy photosynthesis.(2)Through linear and non-linear analysis,this paper explored the fitting relationship between SIF and GPP,which have the general similar change rule on the time scales of day and season,but their fitting function relationship showed the nonlinear and linear time scale dependence on the time scales of day and season.The nonlinear regression function GPP=(18.7 × SIF-2.93)/(SIF+0.24)explained 14%of the variation on the 30 min time scale.On a long seasonal time scale,SIF-GPP relationship was best described by linear regression function(GPP=14.29+4.01 ×SIF;R2=0.17).(3)This paper analyzed the effects of environmental stress such as high temperature,high light,high water level on the relationship between SIF and GPP.When environmental stress occurs around noon(excessive light and high VPD),the relationship between SIF and GPP will be reduced.While in a certain range,the rise of tidal water level will increase the relationship between SIF and GPP in mangrove.This study provides the first,high temporal-resolution,continuous SIF measurements in mangroves and highlights the importance of characterizing the impacts of environmental conditions on the SIF-GPP relationship.Future studies should take into account these issues in order to reduce the uncertainty in estimating GPP from remotely sensed SIF measurements.This has important practical significance for a comprehensive understanding of the carbon cycle process of the mangrove wetland ecosystem,understanding its driving mechanism,and better accomplishing the mangrove conservation and planning work in the protected area.