Temperature and salinity are important regulators of mangrove range limits and productivity, but the physiological responses of mangroves to the interactive effects of temperature and salinity remain uncertain. We tested the hypothesis that salinity alters photosynthetic responses to seasonal changes in temperature and vapor pressure deficit (D), as well as thermal acclimation of leaf respiration, in black mangrove (Avicennia germinans). To test this hypothesis, we grew seedlings of A. germinans in an outdoor experiment for ~12 months under four treatments spanning 0 to 55 ppt porewater salinity. We repeatedly measured seedling growth and rates of leaf net photosynthesis (Asat), stomatal conductance to water vapor (gs), Rubisco carboxylation (Vcmax), and electron transport for RuBP regeneration (Jmax) at prevailing leaf temperatures, as well as rates of leaf respiration at a common temperature (Rarea25). We developed empirical models describing the seasonal response of leaf-gas exchange and photosynthetic capacity to temperature and D, and the response of Rarea25 to changes in mean daily air temperature. We tested the effect of salinity on model parameters.
Results/Conclusions
Average rates of Asat and gs were slightly higher under intermediate salinities (18 to 35 ppt) compared to high salinity (55 ppt). However, salinity had no effect on the biochemical parameters of photosynthesis (Vcmax, Jmax) and individual measurements of Asat, gs, Vcmax, and Jmax showed a similar response to seasonal changes in temperature and D across all salinity treatments. Rates of Rarea25 increased modestly as salinity increased, but individual measurements of Rarea25 showed a negative relationship with mean daily air temperature that was consistent across all salinity treatments. We conclude that photosynthetic responses to seasonal changes in temperature and D, as well as seasonal temperature acclimation of leaf R, are largely consistent across a broad-range of salinities in A. germinans. These results might simplify predictions of CO2 fluxes in mangrove habitats.