COS 110-4 - Simulating responses of low marsh vegetation to fluctuating salinity in Earth system models

Background/Question/Methods

Coastal and estuarine wetlands sequester carbon at rates higher than most terrestrial ecosystems, yet Earth system models that simulate global carbon and other biogeochemical cycles are limited in their ability to simulate ecosystem processes. Tidal fluctuation of salinity is a unique and key characteristic of these ecosystems that we are working to implement in the Energy Exascale Earth System model (E3SM) land model. Although vegetation in salt marshes are well-adapted to elevated salinity, salinity can still control water use, stomatal conductance, and carbon uptake. We developed a salinity response function in the E3SM Land Model (ELM) that can be set for each plant functional type, allowing for representation of varying plant tolerances to salinity. We used this approach to simulate vegetation dynamics in response to fluctuating salinity at the Plum Island Ecosystems Long Term Ecological Research site, Massachusetts. The site is a low marsh dominated by Spartina alterniflora. We drove the model using site measurements of water level, salinity, and meteorology.

Results/Conclusions

Salinity at the Plum Island low marsh site ranged from about 25-40 ppt throughout the growing season. Without a plant response to salinity, the model overestimates gross primary production (GPP) by 2x. We used a Gaussian function to define an optimal salinity for maximum productivity and a salinity tolerance that controls how sensitive productivity is to increases in salinity. Simulations using the salinity function lowered the modeled GPP and improved the accuracy. Some additional modifications were required to improve phenology, as the model still consistently overestimated GPP during leaf onset and leaf offset. Because sea level rise is expected to increase saltwater intrusion in many estuaries and coastal ecosystems, being able to represent wetland vegetation responses to changing salinity will be critical for understanding changes in carbon uptake and storage in coastal wetlands.