Linking In Situ Photochemical Reflectance Index Measurements With Mangrove Carbon Dynamics in a Subtropical Coastal Wetland

Photochemical reflectance index (PRI) has been found to be closely related with vegetation photosynthetic phenology in many ecosystems including boreal evergreen, but its ability to track carbon dynamics in low-latitude evergreen mangrove ecosystems has rarely been assessed. To fill up this gap, this paper explored potential mangrove PRI-carbon links and their environmental controls across temporal scales (diurnal and seasonal), based on 1-year continuous high-resolution (half-hourly) time series measurements of spectral signals, eddy covariance carbon fluxes, and multiple environmental factors at a subtropical mangrove wetland of southeastern China. The diurnal variation of half-hourly mangrove PRI, a U-shaped changing pattern, was consistent with previous PRI studies on boreal evergreen, while the seasonal variation of daily mangrove PRI did not experience a hump-shaped changing pattern as in boreal evergreen. Diurnal and seasonal mangrove PRI variations were significantly correlated with meteorological (radiation and vapor pressure deficit) and carbon-related parameters (gross primary productivity, net ecosystem exchange, and light use efficiency), and the strength of diurnal PRI-carbon correlations varied with meteorological factors with stronger correlations at enhanced radiation and vapor pressure deficit. Our results further indicated that physiology-related PRI performed better than conventional structure-related normalized difference vegetation index in tracking mangrove photosynthetic parameters. To the best of our knowledge, this is the first study to explore the link between high-frequency PRI and carbon dynamics across temporal scales in natural mangrove ecosystems. The PRI-carbon linkage confirmed by this study will improve our understanding of temporal variations in photosynthetic carbon dynamics in mangrove ecosystems.