Mangrove transgression into Everglades marshes: a process to be accepted or resisted?

A post provided by Mike Ross, John Meeder, and Susana Stoffella

Mike Ross and Jack Meeder emerge from a helicopter as the Southeast Saline Everglades research project was initiated in 1996. The mangrove scrub in the foreground had recently been killed back during a freeze event that had little effect in the adjacent tree island. Photo credit: Mike Ross

This post refers to the article Transient vegetation dynamics in a tropical coastal wetland: sea-level rise, glycophyte retreat, and incipient loss in plant diversity published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.13267)

This article, the latest contribution from our research group on coastal processes in the Southeast Saline Everglades (SESE), began more than thirty years ago. In 1994, John Meeder and Mike Ross initiated a study of ecosystem dynamics in the southeastern panhandle of Everglades National Park (ENP), at a time when the role of 20th-century sea-level rise on south Florida vegetation patterns had already been recognized. Taylor Alexander and a University of Miami ecology class had identified the remains of freshwater-dependent slash pines on lower slope positions in Key Largo, signifying their replacement by more salt-tolerant trees (Alexander 1974). Alexander’s work alerted Ross’ research group at National Audubon Society’s Florida Keys Science Center to employ similar physical evidence and aerial photos to chronicle the time course of a dramatic pine forest recession in the lower Keys caused by sea-level rise-driven saltwater encroachment (Ross et al. 1994). After coming across a paper by the pioneer vegetation ecologist Frank Egler, who had noted the transgression of a sparsely vegetated dwarf mangrove-dominated “white zone” into freshwater marshes (Egler 1952), we turned our attention to the southeastern Florida mainland, where we used the easily recognizable interior boundary of the white zone as a marker of coastal vegetation transgression (Ross et al. 2000). The study demonstrated that the white zone had advanced furthest in areas cut off from upstream water sources, thereby identifying freshwater delivery along with sea-level rise (3.0 mm.yr-1 during 1940-1994) as important elements in recent coastal dynamics. The implications were not lost on planners of the Comprehensive Everglades Restoration Program (CERP), whose broad objective was to restore the pre-development Everglades landscape by reallocating freshwater, or “making the water right”. In the coastal zone, this purpose could be sought by increasing freshwater delivery, with the re-establishment of glycophytic plant species and reversal of mangrove advance serving as measures of success.  

With the support of Everglades National Park, we returned to the SESE in 2016. During the 22-year period between studies, the rate of sea-level rise in south Florida had increased by about half, i.e., to 4.4 mm.yr-1. As our current paper describes, this rise has been accompanied by a tangible wave of mangrove advance and saltwater encroachment through the landscape, documented as well by a change in invertebrate fauna from a freshwater assemblage to one characteristic of brackish water environments (Meeder et al. 2022). At the trailing edge of the wave, the dwarf mangrove canopy within the white zone has closed perceptibly, eliminating herbs that formerly occupied the interstices between Rhizophora mangle shrubs. At the wave’s leading edge, Rhizophora mangle has increased in abundance everywhere, becoming the dominant species in 4 of 11 marsh sites formerly dominated by sawgrass (Cladium jamaicense), the iconic freshwater macrophyte of the Everglades interior. Notably, the invasion of Rhizophora mangle and a few other halophytes have resulted in an increase in mean α-diversity among plant assemblages in leading-edge communities, but β-diversity across all 27 study area sites declined slightly. Our research shows that the transgression of mangrove ecosystems in the SESE has continued unabated since first observed by Egler in the 1940’s, appearing to have accelerated in the last few decades. Mangrove invasion has yielded a transgressive stratigraphic sequence in which marine peats are deposited over freshwater marl sediments (Meeder & Parkinson 2018). Faced with a further acceleration in the rate of sea-level rise to 9.6 mm yr-1 during 2016-2023, the question for CERP scientists and ENP natural resource managers is what, if anything, to do about it?

Given that the coastward portions of all SESE wetlands lie within the jurisdiction of Everglades or Biscayne National Parks, responses to the sea-level rise-driven changes discussed above may be usefully considered through the Resist-Accept-Direct (RAD) framework advanced by the National Park Service and several other federal agencies (Lynch et al. 2021). Using the RAD approach, CERP planners, local resource managers, and stakeholders would consider the ecological, societal, and financial feasibility (costs and benefits) of resisting, accepting, or directing mangrove encroachment into Everglades freshwater marshes. As we interpret it, the RAD framework focuses on setting coherent objectives; hybrid solutions that combine two or three RAD alternatives or apply them in different site-specific contexts within a single management unit are possible.

Till now, based on the notion that mangrove transgression is a direct result of saltwater encroachment in coastal wetlands, the management strategy has generally been to Resist. Freshwater is released from canals to flow through the coastal wetlands, an approach that can benefit the receiving estuaries by restoring the historical distribution and volume of freshwater that supply nearshore environments. The operation also reduces the release of waters high in inorganic nutrients from point sources at canal mouths, instead transforming these nutrients into more beneficial organic forms before they reach the estuary. Given the rapidity of sea-level rise, however, the Resist strategy has not been successful in reversing or halting mangrove advance into freshwater marsh, thereby reestablishing the habitat variability (beta diversity) that formerly characterized the SESE coastal zone. Moreover, it carries the considerable risk of raising waters to levels that strain the capacity of mature mangroves to produce new recruits, or even to survive, leading to open water conditions. Thus, as practiced till now, the Resist strategy of “just add water” falls short in ecological feasibility.

If ecosystem services with real value to humans are considered, management designed to prevent mangrove transgression also fails the financial feasibility test on several counts, including inhibiting storm surges, building soils, and reducing greenhouse gas emissions. It is well recognized that mangrove forests in the Greater Everglades maintain and build soil elevation more effectively than adjacent marshes, thereby having a greater capacity to forestall coastal flooding (Krauss et al. 2011). Moreover, from a global perspective, recent research indicates that mangroves in the transitioning SESE landscape are sinks for carbon, while marshes are sources of global carbon (Yannick et al. 2024).

The important role of the Resist strategy in Everglades restoration is not unjustified, however. It is based on strong and widespread public support, which in RAD results in high scores on societal feasibility. The Everglades is recognized worldwide as a special ecosystem due to its size and unique physical and biological characteristics. It is also one that has been diminished and degraded in order to advance human purposes, and there is great support for re-establishing the pre-development biota and landscape pattern to the extent possible. In the SESE this includes extensive coastal marshes fringed by mangroves in a narrow band along the shoreline. It also includes healthy estuaries that receive higher volumes of freshwater within a seasonal cycle. Unfortunately, the rapidly accelerating rate of sea-level rise makes restoring the coastal Everglades of 1900 an unimaginably difficult task. Our experience in these wetlands suggests that a strategy based exclusively on historical landmarks and resistance to rising seas should be replaced by a longer-term vision that balances all three RAD elements. In developing this vision, management should continue to Resist mangrove encroachment in productive marshes distant from the coast, Accept that mangroves will play an expanded role in the lower Everglades of the future, and Direct change such that healthy mangrove forests will serve as hydrological and biological connectors between major Everglades flow-ways and downstream estuaries that will be adequately supplied with freshwater.

References:

  • Alexander, T. R. (1974) Evidence of recent sea level rise derived from ecological studies on Key Largo, Florida. Environments of South Florida: present and past. Miami Geological Society, Coral Gables, FL.
  • Lynch, A. J., Thompson, L. M., Beever, E. A., Cole, D. N., Engman, A. C., Hawkins Hoffman, C. et al. (2021) Managing for RADical ecosystem change: applying the Resist-Accept-Direct (RAD) framework. Frontiers in Ecology and Environment. 19(8): 461-469. https://doi.org/10.1002/fee.2377
  • Meeder, J.F., Adelgren, N., Stoffella, S. L., Ross, M.S. and Kadko, D.C. (2022) The paleo-ecological application of mollusks in the calculation of saltwater encroachment and resultant changes in depositional patterns driven by the Anthropocene Marine Transgression. Frontiers in Ecology and Evolution, 10. https://doi.org/10.3389/fevo.2022.908557
  • Ross, M. S., J. J. O’Brien, and Sternberg, L. da S.L. (1994) Sea-level rise and the reduction in pine forests in the Florida Keys. Ecological Applications 4(1): 144-156. https://doi.org/10.2307/1942124
  • Yannick, D., S. Oberbauer, C. Staudhammer, J. Cherry, and Starr, G. (2024) Carbon dynamics of a coastal wetland transitioning to mangrove forest. JGR Biogeosciences 129. https://doi.org/10.1029/2023JG007991

Brief personal summary: Mike Ross is a Professor of Ecology in the Department of Earth & Environment at Florida International University (FIU). His research experience extends beyond South Florida and the Caribbean to the southern Appalachians and the southern boreal forests of Alberta. John Meeder is Research Professor Emeritus in FIU’s Institute of Environment. He applies a geological perspective to his wetland research in the Big Cypress Swamp, the Everglades, southwestern Louisiana brackish marshes, and the eastern Caribbean. Susana Stoffella is a Research Analyst at FIU. Her research interest is in the application of analytical techniques to understand vegetation-environment relationships and the effects of disturbance on plant communities. All three are affiliated with the South Florida Terrestrial Ecosystems Lab (softel.fiu.edu), and longtime associates of the FIU’s Institute of Environment (formerly the Southeast Environmental Research Center).