Vegetation Types Shift Physiological and Phenological Controls on Carbon Sink Strength in a Coastal Zone

The carbon sink function performed by the different vegetation types along the environmental gradient in coastal zones plays a vital role in mitigating climate change. However, inadequate understanding of its spatiotemporal variations across different vegetation types and associated regulatory mechanisms hampers determining its potential shifts in a changing climate. Here, we present long-term (2011-2022) eddy covariance measurements of the net ecosystem exchange (NEE) of CO2 at three sites with different vegetation types (tidal wetland, nontidal wetland, and cropland) in a coastal zone to examine the role of vegetation type on annual carbon sink strength. We found that the three study sites are stable carbon sinks and are influenced by their distinct physiological and phenological factors. The annual NEE of the tidal wetland, nontidal wetland, and cropland were determined predominantly by the seasonal peaks of net CO2 uptake, release, and duration of CO2 uptake period. Furthermore, the changes in annual NEE were sensitive to climatic variables, as spring mean air temperature reduced the carbon sink strength in the tidal wetland, maximum daily precipitation in summer reduced it in the nontidal wetland, and summer mean global radiation elicited the same effect in the cropland. Finally, a worldwide database of the three vegetation types was compiled, using which we further validated the global consistency of the biological controls. Overall, these results emphasize the importance of considering the underlying mechanisms by which vegetation types influence NEE for the accurate forecasting of carbon sink dynamics across different coastal vegetation types under climate change.