Understanding how the salt marsh vegetation will evolve under future climate conditions is essential for predicting the role of marsh ecosystem services in a warmer climate with higher CO2-concentrations. In a mesocosm experiment in the northern Wadden Sea, the impact of increased temperature (+ 3 °C) and CO2 (800 ppm) on salt marsh vegetation was investigated, assessing biomass production in the pioneer zone and low marsh. The pioneer zone, which was dominated by Spartina anglica and exposed to natural tidal inundations, demonstrated a differentiated response between belowground and aboveground biomass. Aboveground biomass increased in response to enhanced CO2 availability, and belowground biomass increased in response to raised temperatures. Other plant species accounted for less than 18% of the aboveground biomass, and their biomass was suppressed under high CO2 availability. Increased biomass by Spartina anglica may improve resilience toward sea level rise. Hence, the pioneer zone is expected to maintain its coastal protection and blue carbon storage capacity under future climate conditions. The low marsh, which was dominated by Elymus athericus, was exposed to higher than usual tidal inundations and resembled a scenario with increased sea level. The low marsh showed no response in biomass to increased CO2 or temperature, which may be due to the increased flooding. The positive response of Spartina anglica (C4 plant) and the lack of response in Elymus athericus (C3 plant) counter the notion that C3 plants are more productive under future climate conditions and demonstrate that C4 plants can also thrive in future salt marshes.