Large slow-growing hydrophytes increase wetland carbon storage.

Wetlands, among Earth's most carbon-dense ecosystems, are vital for climate change mitigation. While plant diversity has been widely shown to increase soil carbon storage in terrestrial ecosystems, its influence in natural wetlands remains unclear. Here, using data from 1,268 natural wetlands surveyed in the US National Wetland Condition Assessment (NWCA), we examined how trait-based plant diversity (functional diversity) and composition (functional identity) affect soil carbon storage. We show that functional diversity had a minimal effect on carbon stocks, and its influence was weakened by elevated soil nutrient availability and non-native plant stress. In contrast, soil carbon storage was generally greater in wetlands dominated by larger, slow-growing and highly hydrophytic plants. Moreover, the benefits of functional identity were contingent on higher water levels and lower human disturbance. These findings suggest that the conservation and restoration of wetlands dominated by large, conservative and hydrophytic species under hydric conditions could help achieve climate change mitigation goals.