Mangroves on the Move: Predictions of Storm Surge Effects on Coastal Vegetation

The landward coastal zones of the low-lying habitats are characterized by sharp ecotones between salinity-tolerant (halophytic) vegetation types, such as mangroves, and salinity-intolerant (glycophytic) vegetation types, such as freshwater marsh and hardwood hammocks. Empirical studies show a gradual landward migration of these ecotones in some areas, due to sea level rise (SLR), and evidence in some areas of rapid change from glycophytic to halophytic vegetation, possibly as regime shifts resulting from salinity overwash from storm surges. In this dissertation work, the plausibility of storm surge related regime shifts of glycophytic vegetation was investigated using a coupled hydrological and ecological simulation model, and the resilience of the ecotone was studied using a mathematical model. In view of potential effects of storm surge associate with SLR on Everglades ecosystems, particularly the consequences these pose for the Comprehensive Everglades Restoration Plan, both empirical and modeling studies on coastal vegetation are underway. In this dissertation work, the Spatially Explicit Hammock/Mangrove (SEHM) computer simulation model of the ecotone between those vegetation types was used to show the influence of both abiotic (elevation gradient, groundwater salinity, tidal amplitude, precipitation, freshwater flow) and biotic factors (plant physiology, competitive abilities, dispersal, positive feedbacks between plants and soil salinity) on the mechanisms of ecotone formation. The model simulation results indicate that an environmental gradient of salinity, caused by tidal flux, is the key factor separating vegetation communities, while positive feedback involving the interactions of vegetation types with the vadose zone salinity increases the sharpness of boundaries, and maintains the ecological resilience of mangrove/ hammock ecotones against minor disturbances. The model also shows that the dry season, with its low precipitation, has a strong effect on the position of the mangrove/hammock ecotone. Using a mathematical model of an ecotone vulnerable to possible future changes, I estimated the resilience of the ecotone to disturbances. The specific ecotone is that between two different vegetation types, salinity-tolerant and salinity-intolerant, along a gradient in groundwater salinity. In the case studied, each vegetation type, through soil feedback loops, promoted local soil salinity levels that favor itself in competition with the other type. Alternative stable equilibria, one for salinity-tolerant and one for salinity intolerant vegetation, were shown to exist over a region of the groundwater salinity gradient, bounded by two bifurcation points. This region was shown to depend sensitively on parameters such as the rate of upward infiltration of salinity from groundwater into the soil due to evaporation. I showed also that increasing diffusion rates of vegetation can lead to shrinkage of the range between the two bifurcation points. Sharp ecotones are typical of salt-tolerant vegetation (mangroves) near the coastline and salt-intolerant vegetation inland, even though the underlying elevation and groundwater salinity change very gradually. A disturbance such as an input of salinity to the soil from a storm surge could upset this stable boundary, leading to a regime shift of salinity-tolerant vegetation inland. I showed, however, that, for my model as least, a simple pulse disturbance would not be sufficient; the salinity would have to be held at a high level, as a ‘press,’ for some time. The approach used here should be generalizable to study the resilience of a variety of ecotones to disturbances. The SEHM model has been modified to simulate the mangrove-freshwater marsh ecotone. This model is based on intensive field studies by USGS across a mangrove-marsh ecotone on the Harney River in Everglades National Park. The model indicates that two factors are closely related to storm surge effect on vegetation. One of these is salinity intrusion, which has been proposed as a major disturbance to freshwater wetlands. The other is invasion of mangrove seedlings, which have rarely been reported as drivers for ecotone position changes. The model simulation results indicate that, at least for the cases studied, the regime shift of vegetation from freshwater marsh to mangroves was more sensitive to the density of mangrove seedlings passively transported by the storm surge than to the magnitudes of the salinity intrusion. The observed high salinities after regime shifts in the model are the result of more than simply the salinity overwash from the storm surge. Once mangrove prapagules establish successfully, high salinity can be maintained via evapotranspiration of the invading halophytic vegetation, which leaves salt in the soil. While initial salinity intrusion helps mangrove prapagules compete with dense freshwater marsh, the mangroves, once established, continue to hold the concentration of soil salinity high