Salt Marsh Light Use Efficiency is Driven by Environmental Gradients and Species-Specific Physiology and Morphology

Light use efficiency (LUE) of salt marshes has not been well studied but is central to production efficiency models (PEMs) used for estimating gross primary production (GPP). Salt marshes are typically dominated by a species monoculture, resulting in large areas with distinct morphology and physiology. We measured eddy covariance atmospheric CO2 fluxes for two marshes dominated by a different species: Juncus roemerianus in Mississippi and Spartina alterniflora in Georgia. LUE for the Juncus marsh (mean = 0.160 +/- 0.004 g C mol(-1) photon), reported here for the first time, was on average similar to the Spartina marsh (mean = 0.164 +/- 0.003 g C mol(-1) photon). However, Juncus LUE had a greater range (0.073-0.49 g C mol(-1) photon) and higher variability (15.2%) than the Spartina marsh (range: 0.035-0.36 g C mol(-1) photon; variability: 12.7%). We compared the responses of LUE across six environmental gradients. Juncus LUE was predominantly driven by cloudiness, photosynthetically active radiation (PAR), soil temperature, water table, and vapor pressure deficit. Spartina LUE was driven by water table, air temperature, and cloudiness. We also tested how the definition of LUE (incident PAR vs. absorbed PAR) affected the magnitude of LUE and its response. We found LUE estimations using incident PAR underestimated LUE and masked day-to-day variability. Our findings suggest that salt marsh LUE parametrization should be species-specific due to plant morphology and physiology and their geographic context. These findings can be used to improve PEMs for modeling blue carbon productivity.