# Response of Mangrove Carbon Fluxes to Drought Stress Detected by Photochemical Reflectance Index
## 4 Discussion
On the annual scale, the PRI0 and sunlit PRI showed a significant decline due to drought stress but seasonal \(\Delta\)PRI didn't show any response. The seasonal \(\Delta\)PRI reflected the daily maximum depression the canopy suffered from, or the maximum of the xanthophyll cycle. In comparison with the facultative component, the response of the constitutive component of PRI was more sensitive to environmental stresses such as drought stress. Therefore, we confirmed that in subtropical mangrove forests, the pigment pool size could be the primary cause of the long-term variations of PRI with additional contribution from the xanthophyll cycle, which agrees with many previous studies [36; 54]. The high correlation coefficients between PRI and carbon fluxes at the diurnal scale verified the ability of PRI to track the carbon dynamics at short time scales. The slightly higher correlation coefficient between PRI and carbon fluxes in 2020 further implied that drought stress could also exert an impact on the daily short-term xanthophyll cycle, but the impact was not as significant as the long-term variations of pigment pool size. In addition, it is shown that the temporal variation patterns of PRI and carbon fluxes were very similar at both seasonal and annual time scales (Figure 3). [PERSON] et al. [42] reported that PRI was a good indicator of photosynthetic capacity at short time scales, showing good correlations between carbon fluxes and PRI. Our results further confirmed that at a longer time scale, PRI was also able to track the response of carbon fluxes to environmental stresses such as drought stress. On the seasonal scale, the relationship between PRI and carbon fluxes was better when mangroves experienced drought stress, which was consistent with previous findings that the PRI-carbon relationship was better under enhanced VPD or water stress [30; 37; 56]. Therefore, we conclude that the relationship between PRI and carbon fluxes performs better under drought stress in mangrove forests. The RF analyses indicated that PRI was mainly correlated with lagged GPP and advanced VPD. Overall, the decreasing PRI values following higher VPD and reduced rainfall did serve as an early indicator of the declines of carbon uptake. PRI variations were mainly caused by pigment pool size and the response to changing environmental factors were relatively faster than the response of carbon dynamics. The relationship between PRI0 and carbon fluxes almost mimicked the PRI-carbon relationship, indicating that constitutive pigment pool size played an important role in driving the seasonal variations of carbon fluxes, especially under drought stress. The poor relationship between seasonal \(\Delta\)PRI and carbon fluxes suggested that the seasonal variations of carbon fluxes were barely relevant to the changes of the xanthophyll cycle. This contrast between PRI0 and seasonal \(\Delta\)PRI indicated that the PRI0 was more representative in tracking the seasonal variations of carbon fluxes. Our analyses of PRI and mangrove carbon dynamics suffered from several limitations and uncertainties. First, due to the lack of continuous meteorological observations over a decadal time scale, we were not able to explicitly quantify the level of the drought stress investigated in this study. Further studies might be needed to better assess the extent of this drought stress. Second, the difference in the footprint size of spectral and carbon flux measurements might affect the PRI-carbon relationship [42], but we're not able to explicitly assess this effect with current measurements. Third, our PRI sensors were installed at a fixed position, while PRI signals could be affected by other factors such as sun-target-view geometry [56]. Fourth, there were uncertainties in NEE and GPP, which were estimated from raw EC measurements based on several hypotheses [57]. Fifth, in addition to drought stress, the decline of carbon-related variables in 2020 could also be affected by other environmental stresses [58; 59].