# Response of Mangrove Carbon Fluxes to Drought Stress Detected by Photochemical Reflectance Index
## 5 Conclusions  
In this study, the responses of PRI and carbon fluxes to climate fluctuations and drought stress were examined in a subtropical mangrove forest with continuous high-frequency time-series measurements from 2017 to 2020. The ability of PRI to track temporal dynamics of mangrove carbon fluxes was confirmed at both short-term (diurnal) and long-term (seasonal and inter-annual) scales. The time series of daily PRI and carbon uptakes both showed decreasing trends when experiencing drought stress. Time series of monthly PRI were most correlated with advanced time series of VPD with one month ahead and lagged time series of GPP with one month later, which confirmed the ability of PRI as an early indicator of drought-induced down-regulation of carbon uptake. Compared to annual mean values in the first three years (13.22, 12.53, and 12.98 umol m\({}^{-2}\) s\({}^{-1}\)), the annual mean value of NEE decreased by 10.7% in 2020. Correspondingly, the annual mean value of PRI decreased by 29.0% in 2020. PRI and carbon fluxes were significantly correlated at diurnal, seasonal, and annual scales, where the relationship performed better under drought stress. Dark-state PRI, i.e., the constitutive PRI component, showed similar temporal variation as PRI in response to drought stress, while delta PRI, i.e., the facultative PRI component, did not show any response to drought stress. It implied that PRI variations in this subtropical mangrove were mainly caused by the variations in pigment pool size rather than the xanthophyll cycle. This study highlights the potential of PRI to serve as an early and readily detectable indicator to track the response of mangrove carbon cycle to climatic anomalies such as drought stress.  
Conceptualization, X.Z.; methodology, X.Z. and Y.L.; formal analysis, Y.L. and X.Z.; investigation, Y.L. and X.Z.; writing--original draft preparation, Y.L.; writing--review and editing, Y.L. and X.Z.; visualization, Y.L. and X.Z.; supervision, X.Z.; project administration, X.Z.; funding acquisition, X.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Special Project on National Science and Technology Basic Resources Investigation of China (2021 FY100704), the National Natural Science Foundation of China (31600368), the National Key Research and Development Program of China (2017 YC0506102), the Natural Science Foundation of Fujian Province, China (2020J01112079), the Youth Innovation Foundation of Xiamen, China (3502Z20206038), and the Fundamental Research Funds for the Central Universities of China (20720210075).  
The data presented in this study are available on request from the corresponding author.  
We are grateful to members from Coastal Ecology & Remote Sensing lab led by [PERSON] at Xiamen University and staff at Zhangjiang Estuary Mangrove National Nature Reserve for their help in the fieldwork.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.