MANTRA: A coupled vegetation competition and groundwater simulation model to study effects of sea level rise and storm surges on coastal vegetation

Background/Question/Methods

Highlighted by the global climate change phenomenon, the potential future effects of sea level rise (SLR) and storm surge overwash events on coastal vegetation have been of great interest. Low lying coastal areas are especially vulnerable to SLR and storm surge overwash events, which can have both acute and long-term effects on vegetation and on soil and groundwater salinities, resulting in loss or shift in coastal vegetation as one of the ecological impacts. This loss or shift of vegetation may be irreversible, hence affecting the biodiversity and posing risks of habitat loss critical to native species. To help predict, understand, and prepare for the consequences of these climate-related impacts on both the short-term dynamics of salinity in the soil and groundwater and the long-term effects on vegetation, the U.S. Geological Survey’s spatially explicit model of vegetation community dynamics along coastal salinity gradients (MANHAM) is integrated into the USGS groundwater model (SUTRA) to create a coupled hydrology-salinity-vegetation model, MANTRA.  MANTRA simulates the possible long-term effects of both gradual sea level rise and storm surge events on the ecotone between salinity tolerant (halophytic) and salinity intolerant vegetation, by simulating competition under changing groundwater salinity and other environmental conditions.

Results/Conclusions

MANTRA simulations were applied to a coastal site in the Florida Everglades where a freshwater hardwood hammock may be vulnerable to transition to halophytic mangrove vegetation. Two storm surge scenarios were simulated.  In one it was assumed that salinity overwash was accompanied by heavy damage to the hammock trees.  In the other scenario only moderate damage to tree was assumed, but a relative drought of four years was imposed following the storm, thus limiting washing out of the salinity. In both cases a regime shift to mangroves followed the storm surge within a couple of decades.  Storm surges with moderate or less damage and without extended droughts did not produce a regime shift. Another scenario without storm surges, but with steady sea level rise caused the hardwood hammock to be restricted to a narrow band after 150 years.  It is envisaged that MANTRA would help in projecting the effects of overwash and climate change events on groundwater salinity as well as potential changes in vegetation composition. Further, potential countermeasures and strategies for ecosystem preservation can be further explored via MANTRA simulations.