# Response of Mangrove Carbon Fluxes to Drought Stress Detected by Photochemical Reflectance Index
###### Abstract  
The photochemical reflectance index (PRI) has been often used as a physiology-based remote sensing indicator of ecosystem carbon fluxes. However, the assessments of PRI in tracking long-term carbon fluxes with climatic anomalies in mangroves are very limited. In this study, four-year (2017-2020) continuous time series measurements from tower-based eddy covariance and spectral systems in a subtropical mangrove were used to explore the ability of PRI in tracking the response of mangrove carbon fluxes to climate fluctuations and drought stress. The results showed that the temporal dynamics of daily PRI and carbon fluxes shared similar variation patterns over the study period, experiencing simultaneously decreasing trends under drought stress. Compared with the first three years, annual mean values of NEE in 2020 decreased by 10.7% and PRI decreased by 29.0%, correspondingly. PRI and carbon fluxes were significantly correlated across diurnal, seasonal, and annual time scales with better fitness under drought stress. Dark-state PRI (PRI0), the constitutive component of PRI variation due to seasonally changing pigment pool size, showed similar temporal variation as PRI in response to drought stress, while delta PRI (APRI), the facultative component of PRI variation due to diurnal xanthophyll cycle, showed no response to drought stress. This study confirms the ability of PRI to track temporal dynamics of mangrove carbon fluxes on both short-term and long-term scales, with the temporal variation of PRI largely affected by the long-term constitutive pigment pool size. This study highlights the potential of PRI to serve as an early and readily detectable indicator to track the response of the mangrove carbon cycle to climatic anomalies such as drought stress. 1  
[PERSON], X.  
tidal inundation [4], and drought stress [10]. Among these, drought stress is one of the major chronic environmental stresses that adversely affect mangrove growth [11]. Drought stress due to reductions in rainfall can result in a decline in mangrove productivity or even an exacerbation of mangrove loss [10; 12; 13; 14]. The decline of freshwater inputs and increasing evapotranspiration with drought stress can pose unfavorable salinity conditions for mangroves [15; 16], and thus mangrove carbon assimilation rates might decrease dramatically during dry seasons [17]. Compared with mangroves experiencing adequate rainfall, mangroves in drier areas tend to be shorter [18]. Annual rainfall has been identified as a good predictor of mangrove ecosystem carbon stocks [19]. In addition, many studies have emphasized the necessity for considering the lag or cumulative effects of environmental factors that might be more important than their direct effects on mangrove growth [20; 21; 22]. To track the response of mangrove forests to drought stress in the context of climate change, there is a pressing need for continuous monitoring of mangrove carbon dynamics, which are highly heterogeneous with strong temporal and spatial variations [23; 24]. The Eddy covariance (EC) technique [25] is typically applied to measure ecosystem gas exchange across various ecosystems at a high temporal resolution, but spatially extensive applications of the EC technique are often limited by its high instrument and infrastructure costs. To overcome this limitation, tower-based optical remote sensing measurements within the flux tower footprint are being conducted aiming to seek the linkage between optical remote sensing and ecosystem carbon dynamics. The light use efficiency (LUE) model is one of the widely used frameworks for integrating optical sampling and flux measurements. One of the established optical sampling methods within this framework is the photochemical reflectance index (PRI), which has been proved to be able to reflect the physiological regulations of the plants [26; 27].