# Response of Mangrove Carbon Fluxes to Drought Stress Detected by Photochemical Reflectance Index
## 4 Discussion  
The measurements of less rainfall and higher VPD than the multi-year average confirmed that the mangrove wetland experienced drought stress from late 2019 to late 2020 (Figure 2c). Consequently, the deficit of rainfall spanning several seasons during this period lowered the carbon sink and PRI (Figure 3). Although this effect didn't show as soon as the rainfall decreased, it was likely that the response of mangroves to drought stress was not as rapid as other environmental factors. Moreover, the rainfall of spring and summer in 2020 only summed up to 662.9 mm, which was much less than that of previous years (Table 1). Reduction of rainfall during this period could suppress mangrove carbon uptake since mangrove forests in subtropical monsoon climates often have stronger carbon sequestration capacity in spring [8]. The RF analyses also indicated that rainfall was a main factor driving NEE and PRI variations (Figure 7). [PERSON] and [PERSON] [52] proved that PRI could serve as an optical indicator of spring photosynthetic activation in evergreen conifers, which experienced a significant temperature increase from winter to summer. However, the seasonal variations of temperature in mangroves are not as significant as those in boreal evergreens. Thus, spring rainfall is more likely to be a limiting factor of carbon sink potential in subtropical mangroves. The sustained deficit of rainfall also resulted in higher VPD (Figure 1b), which further inhibited the growth of the plants [53]. The combined influence of rainfall and VPD accounted for the decline of carbon assimilation. According to the ranking of RF-based variable importance, mangrove carbon fluxes were heavily affected by VPD with obvious one- or two-month lag effects, which was similar to previous studies [20; 21; 22]. Another important factor affecting mangrove carbon sink is tidal salinity, which has been often reported to cause physiological drought when reaching relatively higher values [41]. The salinity started to increase in late 2019, and the annual average salinity of 2020 was relatively higher (Figure 2c), probably due to the drought stress. Therefore, we concluded that the sustained deficit of rainfall, higher VPD and salinity inhibited the carbon sequestration, consequently leading to a significant decline of mangrove carbon uptake in 2020. The seasonal patterns of PRI were similar to previous studies showing the highest (lowest) values in summer (winter) [54]. Over the four years, the significant decline of PRI in 2020 (Figure 3) implied that drought stress could affect the canopy pigment composition. The RF analyses also indicated that rainfall and VPD were two major factors in driving PRI variations, where PRI increased with rainfall and decreased with VPD. [PERSON] et al. [55] observed an obvious decline in PRI during the extreme drought with small diurnal change relative to seasonal changes. [PERSON] et al. [37] also found that the PRI signal was more sensitive under water-limited conditions. To further explore the response of pigment composition to drought stress, the seasonal variations of PRI0 and sunlit PRI also provide some indications. Temporal variations of PRI are mainly controlled by two physiological processes, one of which is xanthophyll cycle activity controlling the diurnal changes of PRI, and the other is the pigment pool size dominating the seasonal variations of PRI PRI0 and sunlit PRI in our results shared quite similar temporal variation patterns across time scales. The seasonal variation patterns of PRI0 and sunlit PRI were consistent across years (Figure 6), which implied that the size of the pigment pool was affected by seasonal variations of environmental factors [38], in particular for PAR. This is consistent with a previous study showing that PRI was radiation-dominated with additional effects from high VPD [42]. The typical seasonal variations of subtropical mangrove forests have higher and lower PAR in summer and winter, respectively [42], leading to similar seasonal variations of two components of PRI.