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Climate drivers of Zizaniopsis miliacea biomass in a Georgia, U.S.A. tidal fresh marsh

Tidal fresh marshes are at least as productive as nearby salt marshes, but much less is known about controls on primary production in tidal fresh vs. salt marshes. We studied a tidal fresh marsh in Georgia, U.S.A., dominated by the C3 grass Zizaniopsis miliacea. We documented seasonal variation in Z. miliacea above‐ground biomass and below‐ground macro‐organic matter over 1 yr, and annual variation in end‐of‐season aboveground biomass over 15 yr in creekbank and midmarsh zones. Aboveground biomass showed a distinct peak in July and October. Belowground macro‐organic matter was much greater than aboveground biomass and peaked in October. Overall productivity was similar to that of salt marshes downstream. Z. miliacea end‐of‐season above‐ground biomass showed a classic hump‐shaped “subsidy‐stress” relationship with plot elevation, but on average the creekbank supported about twofold more above‐ground biomass than the midmarsh, and both zones varied in biomass about 1.7‐fold among years. Annual variation in above‐ground biomass was negatively correlated with maximum and mean temperature in both zones, and positively with river discharge in the creekbank zone. Sea level, precipitation and water column salinity showed biologically plausible trends with respect to biomass. The responses of Z. miliacea to abiotic drivers were muted compared with the responses of nearby salt marshes dominated by Spartina alterniflora. Temperature was more important for Z. miliacea, whereas drivers of porewater salinity were more important in the salt marsh. Likely future changes in temperature, precipitation, and river discharge may pose a threat to the high productivity of tidal fresh marshes.