Tree mortality provides early warning sign of ecosystem functional transition in coastal freshwater forested wetlands

Globally, coastal forested wetlands are increasingly affected by relative sea level rise. However, the mechanisms underlying coastal wetland degradation remain unclear. Using the eddy covariance approach, we evaluated the long-term (i.e., 2009–2019) net ecosystem exchange (NEE), associated hydrology, and ecosystem structural changes in a forested wetland in North Carolina, USA. We quantified tree mortality in response to changes in hydrology. The ecosystem shifted from a net C sink (NEE = −3.68 Mg C ha−1 yr−1) pre-2010 to a net C source (NEE = 0.87 Mg C ha−1yr−1 to 7.59 Mg C ha−1yr−1) afterward. We ascribe the ecosystem C loss to increasing tree mortality (i.e., from 1.6% in 2009 to 45.8% in 2019), partly due to relative sea level rise (R2 = 0.62). Tree mortality, and consequent shift of NEE, provide early warning signs of a transition in this ecosystem more than 20 km inland from the coast.