Low-lying coastlines are among the first ecosystems directly impacted by hurricane disturbance and chronic sea-level rise. Studies that explore responses of coastal biota to climate change are needed to develop adaptation strategies. Coastal species may cope with environmental changes over the short term, but eventually they will be eliminated unless inland migration can occur. The mechanisms underlying successful upslope establishment are poorly understood. The main hypothesis tested in this study was that inland migration of downslope species is impeded by upslope vegetation composed of species largely intolerant of salinity and deposition pulses. These biological barriers can be disassembled by hurricane-generated storm surge, followed by a reshuffling of community assemblages, and opening of opportunities for species from seaward positions to establish further inland.
In early Summer 2009, five transects were established at points along East Bay River, a tidal river ecosystem on Eglin Air Force Base in northwestern Florida that encompasses saline to fresh water conditions. Each transect was surveyed for elevation and extended perpendicular to the river into upland habitat to include 1-meter, 2-meter, and 3-meter changes in relative elevation from river’s edge. In August 2009 nine vegetation plots were located haphazardly in each of the three elevation classes alongside each transect (n = 135). Three plots were designated controls, three were assigned to a single surge treatment, and three plots were assigned to a double surge treatment with a two-year interval separating the surge applications (Experiment 1). In 2010 a second set of plots (n = 135), similarly organized with random assignments to the three surge treatments, was established along each of the transects. Plots established and initially surged in 2010 also received propagule additions of dominant native species from across the estuarine gradient. In addition, the volume of saline water applied for the second surge application in 2012 was doubled (Experiment 2). All vascular plant species were identified and their cover estimated in each plot prior to experimental storm surge treatments and annually thereafter through the 2013 growing season. In 2010, a third set of plots was established (n = 75) to test the effects of storm surge that included sediment deposition (Experiment 3). Composition was surveyed on an annual basis in control and treatment plots following the surge treatments (2010 to 2013). In 2011, a reciprocal transplant experiment (Experiment 4) was initiated to investigate the feasibility of upslope, assisted migration of dominant species representing different parts of the gradient, with and without removal of standing vegetation (n = 135 plots).
Compositional trajectories differed significantly among years and elevation zones but communities were largely resilient to experimental storm surge disturbances and showed no significant differences between surge treatment levels, except along one of the inland transects (Experiment 1). In contrast, plant composition exhibited significant change with surge treatment effects, which were part of higher order interactions that included time and elevation in Experiment 2. Propagule additions had no discernible effect on post-disturbance recovery. In general, community disassembly was less pronounced near the bay where halophytes adapted to regular tidal inundation dominate; disassembly was greatest in upslope and upstream communities. Trajectories of the high and intermediate elevation communities, located along the two most inland transects, were also influenced by two prescribed fires in Experiments 1-3. Disassembly was greatest and vegetation recovery slowest in plots that received a second, double-volume storm surge (Experiment 2) and where sediment was added to the surge waters (Experiment 3). Compositional shifts were largely driven by in situ mortality and reduced abundance of species intolerant of the surges, rather than upslope migration of species. Several species emerged as key indicators of elevation-specific responders (e.g., winners and losers) to storm surge treatments. Two of the transplanted species, the brackish marsh dominant Juncus roemerianus and fresh marsh dominant Cladium mariscus, exhibited multi-year survival in plots spanning the estuarine and elevation gradients (Experiment 4), including habitats that received prescribed burns.
Collectively, Experiments 1-3 indicate that inland communities are more vulnerable to the effects of storm surge, particularly when the surges are more sustained and lead to sediment deposition. Taken together with climate change models that project increasingly common intense tropical storms, these results suggest that “ecologically naïve” vegetation will be impacted most by these events, leading to substantial compositional changes. The initial winners will be species that are resilient to these events, and the losers will be ones that are sensitive. Where mortality is high and recovery is limited or slow, regeneration windows may be longer lived such that some downslope species are capable establishing in upslope habitat. Distance from source population and dispersal distance will be factors that influence the rate of inland migration.
Upslope and inland establishment of some species can be expedited through assisted migration. Vegetation cover may be beneficial for newly transplanted individuals, but removal of standing vegetation appears to have positive effects on later survival of transplants when competition may pose a bigger challenge. Species that are transplanted must be tolerant of disturbances, such as fire, that are typical in their new, upslope habitats.
This research can inform management regarding ecological scenarios consistent with climate change forecasts and provide a potential tool for intervention. Chronic and acute disturbances (i.e., sea level rise, storm surge, fire) are drivers that have complex effects on movement of coastal species. As disturbance regimes are altered by climate change, they are likely to cause shifts in species’ distributions in unpredictable ways. Movement of species is also influenced by barriers in the landscape; methods to overcome these obstacles will be increasingly important as the climate envelopes of species change spatially. Deliberate, assisted migration can be a prescriptive and effective approach if management efforts are matched with climate change. This type of intervention may be especially necessary when dispersal is limited by natural and anthropogenic barriers.