Hydrologic restoration in a dynamic subtropical mangrove‐to‐marsh ecotone

Abstract

Extensive hydrologic modifications in coastal regions across the world have occurred to support infrastructure development, altering the function of many coastal wetlands. Wetland restoration success is dependent on the existence of hydrologic regimes that support development of appropriate soils and the growth and persistence of wetland vegetation. In Florida, United States, the Comprehensive Everglades Restoration Program (CERP) seeks to restore, protect, and preserve water resources of the greater Everglades region. Herein we describe vegetation dynamics in a mangrove‐to‐marsh ecotone within the impact area of a CERP hydrologic restoration project currently under development. Vegetation communities are also described for a similar area outside the project area. We found that vegetation shifts within the impact area occurred over a 7‐year period; cover of herbaceous species varied by location, and an 88% increase in the total number of mangrove seedlings was documented. We attribute these shifts to the existing modified hydrologic regime, which is characterized by a low volume of freshwater sheet flow compared with historical conditions (i.e. before modification), as well as increased tidal influence. We also identified a significant trend of decreasing soil surface elevation at the impact area. The CERP restoration project is designed to increase freshwater sheet flow to the impact area. Information from our study characterizing existing vegetation dynamics prior to implementation of the restoration project is required to allow documentation of long‐term project effects on plant community composition and structure within a framework of background variation, thereby allowing assessment of the project's success in restoring critical ecosystem functions.

Coastal wetland adaptation to sea level rise: Quantifying potential for landward migration and coastal squeeze

“…These are estuaries where ecological impacts and transformations due to sea level rise are expected to be very large. Within each of these six estuaries, large areas of land will be affected by sea level rise as tidal saline wetlands migrate landward and replace upslope and upriver ecosystems (Doyle et al., ; Flower, Rains, & Fitz, ; Howard et al., ; Krauss, From, Doyle, Doyle, & Barry, ; Langston et al., ; Williams, Pinzon, et al., ). Hence, these are estuaries where there is much value in future‐focused and climate‐smart conservation planning efforts that promote landward migration and also manage habitats at risk of conversion to tidal saline wetlands.…”

“…Our regional-scale comparison identifies certain estuaries where the potential for landward migration and coastal squeeze are high (see colour intensity of estuaries in Figure 2, see isolated estuaries in Figures 3-6 and see low estuary ranks in Table 1). Those analyses indicate that the potential for landward migration of wetlands is very high in the following six estuaries: (1) (Doyle et al, 2010;Flower, Rains, & Fitz, 2017;Howard et al, 2017;Krauss, From, Doyle, Doyle, & Barry, 2011;Langston et al, 2017;Williams, Pinzon, et al, 1999). Hence, these are estuaries where there is much value in future-focused and climate-smart conservation planning efforts that promote landward migration and also manage habitats at risk of conversion to tidal saline wetlands.…”

Review of the ecosystem service implications of mangrove encroachment into salt marshes

“…Interestingly, experimental removal of mangrove cover at an encroachment site in Port Aransas showed an opposite trend to the above, whereby accretion above feldspar marker horizons decreased with increasing mangrove cover (Guo et al., ). Following restoration of freshwater sheet flow across wetlands at Ten Thousand Islands National Wildlife Refuge, surface elevation change was found to decrease in mangrove and transition marshes, but remained stable in landward brackish marshes over the same period, despite relatively consistent rates of vertical accretion between each of the vegetation types (Howard et al., ).…”

“…Biotic interactions between mangrove and marsh species, plus positive biological feedbacks (such as increased resilience to frost damage with mangrove age), may also influence the distribution and rate of mangrove expansion and salt marsh loss, beyond what would be predicted from temperature changes alone (Guo, Zhang, Lan, & Pennings, ; Howard et al., ; Osland et al., ; Peterson & Bell, ). The drivers of within‐range mangrove expansion are less clear and may include rises in relative sea level (whereby mangroves migrate upslope into salt marshes) (Kelleway et al., ; Raabe, Roy, & McIvor, ; Rogers, Battley, et al., , Rogers, Wilton, & Saintilan, ); changing rainfall patterns (Eslami‐Andargoli, Dale, Sipe, & Chaseling, ); increasing atmospheric CO 2 concentration and soil nutrients (McKee, Rogers, & Saintilan, ; McKee & Rooth, ); sedimentation and geomorphic change associated with catchment land use (Swales et al., ); changes in tidal influence and freshwater inflow (Howard et al., ), among other factors (Saintilan & Williams, ).…”

Warming accelerates mangrove expansion and surface elevation gain in a subtropical wetland

“…In some sites in Australia and New Zealand, increases in mangrove abundance are driven by nonclimatic processes such as sediment delivery and subsidence (Lovelock et al, ; Rogers, Saintilan, & Heijnis, ; Swales, Bentley, & Lovelock, ) as well as changes in rainfall patterns (Eslami‐Andargoli, Dale, Sipe, & Chaseling, ). In some sites in Florida, USA, and Mexico, this expansion is thought to be due to sea‐level rise (Krauss, From, Doyle, Doyle, & Barry, ; López‐Medellín et al, ) and changes in freshwater inputs (Howard et al, ; Krauss et al, ). However, in other regions, like in China, the cause of expansion remains unknown (Durango et al, ).…”