Long-Term Monitoring of Coupled Vegetation and Elevation Changes in Response to Sea Level Rise in a Microtidal Salt Marsh

Tight interplays between physical and biotic processes in tidal salt marshes lead to self-organization of halophytic vegetation into recurrent zonation patterns developed across elevation gradients. Despite its importance for marsh ecomorphodynamics, however, the response of vegetation zonation to changing environmental forcings remains difficult to predict, mostly because of lacking long-term field observations of vegetation evolution in the face of changing rates of sea level rise and marsh vertical accretion. Here we present novel data of coupled marsh elevation-vegetation distribution collected in the microtidal Venice Lagoon (Italy) over nearly two decades. Our results suggest that: (a) despite increasing absolute marsh elevations (i.e., above a fixed datum), vertical accretion rates across most of the studied marsh were not high enough to compensate for relative sea-level rise (RSLR), thus leading to a progressive marsh drowning; (b) accretion rates ranging 1.7-4.3 mm/year are overall lower than the measured RSLR rate (4.4 mm/year) and strongly site-specific. Accretion rates vary largely at sites within distances of a few tens of meters, being controlled by local elevation and sediment availability from eroding marsh edges; (c) vegetation responds species-specifically to changes in environmental forcings by modifying species-preferential elevation ranges. For the first time, we observe the consistency of a sequential vegetation-species zonation with increasing marsh elevations over 20 years. We suggest this is the signature of vegetation resilience to changes in external forcings. Our results highlight a strong coupling between geomorphological and ecological dynamics and call for spatially distributed marsh monitoring and spatially explicit biomorphodynamic models of marsh evolution.

Automated summary

Tight interplays between physical and biotic processes in tidal salt marshes lead to self-organization of halophytic vegetation into recurrent zonation patterns developed across elevation gradients. Despite its importance for marsh ecomorphodynamics, the response of vegetation zonation to changing environmental forcings remains difficult to predict, mostly because of lacking long-term field observations of vegetation evolution in the face of changing rates of sea level rise and marsh vertical accretion.