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Analysis and Simulation of Propagule Dispersal and Salinity Intrusion from Storm Surge on the Movement of a Marsh-Mangrove Ecotone in South Florida

Discussion

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Earlier studies have reviewed the effects of large storms on coastal ecosystems (e.g., Craighead and Gilbert 1962; Craighead 1971; Ellison and Stoddart 1991; Roth 1992; Ellison 1993; Smith III et al. 1994; Field 1995; Michener et al. 1997; Paerl et al. 2001; Doyle et al. 2003; McLeod and Salm 2006; Gilman et al. 2006, 2007; Smith III et al.. 2009) or modeled possible effects of storms on mangrove ecosystems (e.g., Doyle et al. 2003). Our modeling approach differs from previous models by focusing on the dynamics of the ecotone between coastal halophytic and glycophytic vegetation (mangrove and freshwater marsh) and by modeling the ecotone as being maintained by self-reinforcing feedbacks between the biotic and abiotic environment. The hypothesis that halophytic and glycophytic vegetation represent alternative stable states over areas of coastal landscape implies that the ecotone is susceptible to regime shifts. Our model can be used to estimate the resilience of the ecotone to realistic effects of storm surge events in southern Florida. We explicitly investigated different levels of two significant factors, salinity intrusion and the transport of mangrove propagules, associated with the storm surge. These two factors are prerequisites for triggering vegetation changes, as both an increase in salinity and sufficient mangrove propagules are needed to create the positive feedback between plants and soil porewater salinity necessary to cause the regime shift. We observed that the spatial extent of the regime shift responded sensitively to the duration of salinity intrusion, it is just that increased salinity concentrations did not persist long enough after Hurricane Wilma to drive this change. It is uncertain whether other types of storms might have a different influence on ecotonal movement, as depicted theoretically with only a 2-year post-storm retention of salinity (Fig. 5). The ecotone was resilient to passive mangrove propagule transport if the increase in salinity was washed away by precipitation quickly after the storm surge.

Hurricane Wilma was not predicted to cause significant hydrology changes at the marsh-mangrove ecotone study site SH5 for several reasons. First, SH5 is 9.5 km upstream from the Gulf of Mexico on the Harney River, so the storm surge was greatly dampened at that point. Intrusion of water from Harney River by Hurricane Wilma was much fresher (~5) than marine (~35) water and even less saline than groundwater at SH4. In addition, saline water intrusion was reduced prior to Wilma from heavy rainfall during wet season. Furthermore, the slightly lower elevation of SH5 helped in the accumulation of freshwater from upstream during wet season. Other studies of hurricane effects have found similar brief durations of salinization and thus no long-term effects on vegetation (e.g., Chabreck and Palmisano 1973; Conner et al. 1989; Flynn et al. 1995) but have noted that longer durations may cause changes in plant communities (McKee and Mendelssohn 1989; Howard and Mendelssohn 2000). Longer durations of salinity can occur when a storm surge is followed by a prolonged dry period, especially in areas where saline water is trapped in swales (Hook et al. 1991) or behind berms (Pearlstine et al. 2010). For such reasons, hurricanes might cause more severe changes in hydrology at other locations, e.g., the Mississippi Delta from Hurricanes Katrina and Rita (Day Jr. et al. 2007; Schriever et al. 2009). The "lower Saline Everglades" is an area where regime shifts might be expected. Egler (1952) noted that a succession of storm tides could lead to a buildup of salinity on the flat, poorly drained terrain, as well as "bring in vast numbers of Rhizophora fruits, which root and grow rapidly." The transition of that area from a mixture of C. jamaicense and R. mangle to pure R. mangle noted by Ross et al. (2000) 40 years later could have occurred as such a shift.

Although sea level rise and storm surges have been considered to be primary drivers of coastal vegetation changes, the mechanisms by which these two factors trigger vegetation changes are different. On the one hand, sea level rise is a gradual change that can eventually push environmental conditions (e.g., groundwater salinity) past a threshold, beyond which an affected vegetation zone could shift from glycophytic to halophytic vegetation. On the other hand, a regime shift caused by storm surge is a large disturbance that pushes the system beyond the threshold all at once, such that it cannot return to the original vegetation state (glycophytic) but moves to the alternative state (halophytic). If the disturbance is not so large that it pushes the system outside its domain of ecological resilience, the ecosystem can return to its original state following a disturbance. Evapotranspiration is the process causing the largest water output from the Everglades ecosystem (Saha et al. 2012), and it tends to increase soil salinity, as salt is left in the soil. Plants can adjust their transpiration in response to changing soil salinity conditions and, therefore, modify local salinity conditions. If an external salinity disturbance lasts for a long time, mangroves can invade freshwater marsh habitats and then maintain high salinity via evapotranspiration. The passive transports of mangrove propagules then overcome the mangrove dispersal limitation on the upper inland. Although there are rare reports of mangrove seedling dispersal after a storm surge (Rathcke and Landry 2003), multiple experiments on mangrove dispersal indicate that pulse dispersal and retention after storm surge is possible (Rabinowitz 1978a, b; Sousa et al. 2007; Peterson and Bell 2012; Van der Stocken et al. 2013). Perhaps this research may encourage empirical studies to investigate mangrove dispersal and establishment after a storm surge.

Acknowledgments

We appreciate the thorough and helpful reviews of two reviewers for the journal and one for USGS peer review. JJ was supported as Postdoctoral Fellow at the National Institute for Mathematical and Biological Synthesis, an Institute sponsored by the National Science Foundation, the U.S. Department of Homeland Security, and the U.S. Department of Agriculture through NSF Award #EF-0832858, with additional support from The University of Tennessee, Knoxville. This research was partially supported the USGS project "A land of flowers on a latitude of deserts" funded by the National Climate Change and Wildlife Science Center and supported by the FISCHS Project (Future Impacts of Sea Level Rise on Coastal Habitats and Species) at the USGS Southeast Ecological Science Center, funded by USGS Ecosystems Mapping and the USGS Greater Everglades Priority Ecosystems Science. This material was developed in collaboration with the Florida Coastal Everglades Long-Term Ecological Research program under National Science Foundation Grant No. DBI-0620409.

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