Temperature optimum for marsh resilience and carbon accumulation revealed in a whole‐ecosystem warming experiment

Coastal marshes are globally important, carbon dense ecosystems simultaneously maintained and threatened by sea‐level rise. Warming temperatures may increase wetland plant productivity and organic matter accumulation, but temperature‐modulated feedbacks between productivity and decomposition make it difficult to assess how wetlands and their thick, organic‐rich soils will respond to climate warming. Here, we actively increased aboveground plant‐surface and belowground soil temperatures in two marsh plant communities, and found that a moderate amount of warming (1.7°C above ambient temperatures) consistently maximized root growth, marsh elevation gain, and belowground carbon accumulation. Marsh elevation loss observed at higher temperatures was associated with increased carbon mineralization and increased microtopographic heterogeneity, a potential early warning signal of marsh drowning. Maximized elevation and belowground carbon accumulation for moderate warming scenarios uniquely suggest linkages between metabolic theory of individuals and landscape‐scale ecosystem resilience and function, but our work indicates nonpermanent benefits as global temperatures continue to rise. Coastal marshes are globally important, carbon dense ecosystems simultaneously maintained and threatened by sea‐level rise. Here, we actively increased aboveground plant‐surface and belowground soil temperatures and found that a moderate amount of warming consistently maximized root growth, marsh elevation gain, and belowground carbon accumulation at our mid‐Atlantic site. At higher temperatures, marsh loss was associated with increased carbon mineralization and increased marsh breakup. Extrapolated regionally, we expect marsh resilience to decrease with warming at low latitudes, increase with warming at high latitudes, and have a temporarily positive effect in mid‐latitude marshes uniquely linking individual metabolic theory and landscape‐scale ecosystem resilience.