Predicting the distribution, structure, and biomass of mangrove forests is an area of high research interest. Across the Atlantic East Pacific biogeographic region, three species are common and abundant members of local mangrove communities; Rhizophora mangle, Avicennia germinans, and Laguncularia racemosa. Biomass prediction for these species has relied on two approaches: site-specific allometries based on the idea that environmental/climatic differences between sites drive growth differences, or the use of common allometric equations based on the idea that site driven differences are minimal. Meta-analyses of global compilations of interspecific plot level data (e.g., mean canopy height, stand basal area) show trends in size and structure with climatic variables, however this has not been critically evaluated across these species using empirical allometric growth functions. We compared allometric equations derived from 590 individuals within and across nine broadly distributed sites at interspecific and intraspecific levels and explored the influence of climatic variables on allometric slopes and intercepts. Assessing variables that can be used to predict biomass in the field (height, diameter at breast height (DBH), canopy spread), we find interspecific root mean squared errors similar to or smaller than most intraspecific or site-specific equations, particularly when examining sites with sample sizes above recommended values. We also find significant effects of several climatic variables on growth allometries with the strongest effects from minimum temperature followed by precipitation seasonality. Our results suggest that while climate has a clear influence on mangrove allometric growth, common equations may have utility in biomass prediction. Future methodological improvements, particularly larger sample sizes across the entire available size range, combined with data from a broader range of growth conditions will further inform which allometric relationships exhibit the most variability within and across sites and which variables best predict mangrove biomass.

Mangrove ecosystems in the Caribbean are frequently exposed to hurricanes, leading to structural and regenerative change that elicit calls for recovery action. For those mangroves unaffected by human modifications, recovery can occur naturally. Indeed, observable natural recovery after hurricanes is the genesis of the “disturbance adaptation” classification for mangroves; while structural legacies exist, unaltered stands often regenerate and persist. However, among the >7,000 islands, islets, and cays that make up the Caribbean archipelago, coastal alterations to support development affect mechanisms for regeneration, sediment distribution, tidal water conveyance, and intertidal mangrove transgression, imposing sometimes insurmountable barriers to natural post‐hurricane recovery. We use a case study approach to suggest that actions to facilitate recovery of mangroves on Caribbean islands (and similar settings globally) may be more effective when focusing on ameliorating pre‐existing anthropogenic stressors. Actions to clean debris, collect mangrove propagules, and plant seedlings are noble endeavors, but can be costly and fall short of achieving recovery goals in isolation without careful consideration of pre‐hurricane stress. We update a procedural framework that considers six steps to implementing “Ecological Mangrove Restoration” (EMR), and we apply them specifically to hurricane recovery. If followed, EMR may expedite actions by suggesting immediate damage assessment focused on hydrogeomorphic mangrove type, hydrology, and previous anthropogenic (or natural) influence. Application of EMR may help to improve mangrove recovery success following catastrophic storms, and reduce guesswork, delays, and monetary inefficiencies.

Novel combinations of physical oceanographic data and high-definition terrestrial imagery were collected between 2018 and 2021 at three archeological sites located in Everglades National Park (ENP), Southwest Florida, USA. The goal was to quantify geomorphic change along shorelines subjected to a distinct set of environmental conditions to which most archeological sites within ENP are subjected: (1) wave-dominated, open ocean island; (2) coastal island archipelago; and (3) tide-dominated mainland rivulet. The data were then used to assess and prioritize risks to the sites under conditions of a changing climate. The results suggest ENP cultural resources will be subject to increasing risk over time as rising seas and tropical cyclones reduce site elevation relative to mean sea level and local relief, respectively. This will initially result in the submergence of open ocean islands. The loss of these islands, which directly face the Gulf of Mexico and absorb prevailing and event energy, will induce a cascading loss of sites located in the island archipelago and along the mainland shoreline. Archeological investigations conducted within ENP should therefore prioritize wave-dominated, open ocean locations, which in this study are projected to submerge by mid-century. Given the geographic (e.g., susceptibility to tropical cyclones) and geomorphologic (e.g., low-relief coastal zone consisting of unconsolidated sediments) similarity between the study area, the shorelines of the Southeastern United States and in the Wider Caribbean, it is recommended that practitioners responsible for archeological investigations and related resource management planning in these areas should also prioritize open ocean sites to ensure their cultural content is cataloged before it is degraded or submerged under conditions of a rapidly changing climate.

Mangroves can store more sediment organic carbon (SOC) than freshwater and salt marshes. Understanding how mangroves have responded to historical sea-level rise (SLR) is fundamental to assessing their resilience and capacity to store carbon as SLR accelerates. We quantified landscape-level temporal and spatial trends in historical coastal wetland sediment accumulation and associated SOC content (i.e., storage) along coastal-to-inland gradients in Southeast Florida. The observed trends were transgressive and attributed to the historical rise in sea level. Our results indicate an overall significant increase in the SOC content of the historic wetland sediment succession caused by the vertical accumulation and landward migration of carbon-rich mangrove-dominated plant communities (mean = 0.08 g cm−3) into and over carbon-poor wet prairie plant communities (mean = 0.02 g cm−3). The observed historical increase in SOC is predicted to diminish over time as the difference between rates of SLR and vertical sediment accumulation increases and because the landward migration of mangrove-dominated plant communities is now obstructed by a shore-parallel flood-control levee. These results are likely to unfold in other low-latitude coastal wetlands where they are sandwiched between rising seas and an urbanized landscape.

The status and potential degradation of an ecosystem is often difficult to identify, quantify, and characterize. Multiple, concurrent drivers of degradation may interact and have cumulative and confounding effects, making mitigation and rehabilitation actions challenging to achieve. Ecosystem status assessments generally emphasize areal change (gains/losses) as a primary indicator; however, this over-simplifies complex ecosystem dynamics and ignores metrics that would better assess ecosystem quality. Consideration of multiple indicators is necessary to characterize and/or anticipate ecosystem degradation and appropriately identify factors causing changes. We utilize mangrove forests as a model system due to their distribution across a wide range of geographic settings, their position in the inherently dynamic coastal zone, and the multiple natural and anthropogenic pressures they face. We present a conceptual framework to: i) examine drivers of ecosystem degradation and characterize system status, and ii) delineate the roles of biogeographic and geomorphic variability, site history and typology, and references. A complementary workflow is proposed for implementing the conceptual framework. We demonstrate the universal applicability of our conceptual framework through a series of case studies that represent locations with differing drivers of degradation and biogeographic and geomorphic conditions. Our conceptual framework facilitates scientists, conservation practitioners, and other stakeholders in considering multiple aspects of ecosystems to better assess system status and holistically evaluate degradation. This is achieved by critically evaluating suitable comparisons and relevant elements in assessing a site to understand potential actions or the outcome of previously implemented management strategies.

Friess et al. discuss the results of conservation efforts for mangrove forests in recent years.

Hurricanes are a frequent disturbance in the western Atlantic impacting coastal forest structure and persistence. This study addressed the hypothesis that periodic hurricane-associated winds and storm surges can interact with sea-level rise to cause the demise of coastal pine forests, based on the response of the Florida Keys’ last extensive tract of slash pine (Pinus elliottii var. densa) following Hurricane Irma (September 2017). Irma’s winds reached Category 4 strength in the study area on Big Pine Key, and storm surge flooded the island’s low limestone surface with salt water for the third time in 20 years, including Hurricanes Georges in 1998 and Wilma in 2005. Total mortality was 32% and was concentrated in the largest trees. Broken and uprooted trees that died immediately were distributed randomly across the elevation gradient, but trees without major stem damage that succumbed later occupied lower elevations. The extensive salt water flooding at low elevations was likely an agent of the mortality of slash pine, a fresh water–dependent species. Mortality due to Hurricane Irma contributed to a 1998–2018 decline in pine tree density and biomass of 74% and 80%, respectively. These dynamics were characterized by pine recruitment that was minimal in comparison with the three hurricane-driven mortality episodes. The data suggest that the 6 cm sea-level rise over the period exacerbated storm surge mortality by ponding salt water over more of the land surface, and the resulting scarcity of mature trees contributed to recruitment failure. In effect, the population declined at each disturbance, sea-level rise magnified these losses, and the local resilience of the population, i.e., its capacity to recover, was exceeded.

As sea level rises in low-lying coastal islands, salt-tolerant (halophytic) coastal vegetation communities may be able to migrate inland, replacing the freshwater vegetation that is unable to tolerate salt stress. The pace of such shifts may be accelerated by a self-reinforcing feedback between the halophytic vegetation and salinity, as well as by frequent and intensified salinity pulses associated with the increasing impact of storm surges as a consequence of sea-level rise. We used a modification of a previously published spatially explicit individual-based model that simulates impacts on upland freshwater hammock communities from sea-level rise and storm surge to predict the interaction between three coastal communities: mangroves, hammocks, and pinelands. The model simulation predicted two qualitative characteristics regarding the interaction between these three different coastal communities: (1) mangroves and hammock communities tend to have ground water with high salinities, while at the same time pineland ground water salinity is low, and (2) pineland located at lower elevation relative to adjacent hammock will be negatively influenced by higher ground water salinities in hammocks, as it flows toward the lower elevation pineland. We tested these predictions using foliar δ 13 C of Conocarpus erectus collected from Big Pine Key as a proxy for ground water salinity. Measurements of ground water salinity via this proxy confirmed the two predictions of the model. Our approach provides an approximation of the impacts of sea-level rise on terrestrial vegetation communities, including threatened pineland communities, and can be used as a tool for management decisions.

High-resolution satellite imaging represents a potentially effective technique to monitor cyclone-caused environmental damage and recovery over large areas at a high spatial scale. This study utilized a 10-m resolution Sentinel satellite image series to document vegetation changes in a portion of the Florida Keys, USA, over which the core of Category 4 Hurricane Irma passed on 10 September 2017. A previously assembled field survey was used to establish land-cover patterns in the satellite data, and concurrent field measurements verified post-hurricane changes. Normalized difference vegetation index (NDVI) was utilized as a tracer for pre-storm baseline patterns and through 19 post-storm months. NDVI patterns show that the severity of vegetation damage varied appreciably across the area, with the least damage on islands in the western sector of the hurricane’s eye and around its center, and greatest damage on islands just east of the eye. The data reveal that for 2.5 months after the storm, multiple inland vegetation classes showed substantial early regrowth. However, mangrove forests were more negatively affected. The storm caused extensive mortality of black mangrove ( Avicennia germinans ) and red mangrove ( Rhizophora mangle ), corresponding to more than 40% of the total mangrove area on some islands. The full extent of mangrove die-off was not immediately evident, and increased progressively through the first few months after the storm. In addition to demonstrating the utility of high-resolution satellite image series for post-hurricane environmental assessment, this study reveals high-resolution links between vegetation types, their location within the cyclone, and the extent of post-storm recovery.

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Increases in the rate of sea level rise are likely to result in changes to disturbance-adapted coastal forests and associated freshwater resources over the next several decades. In this study, we investigated how the press disturbance of two decades of sea level rise altered island hydrology and interacted with pulse disturbances (frequent hurricane impacts) to alter coastal forest composition and structure. The research was conducted on two low-elevation islands located within the lower Florida Keys: Big Pine and Upper Sugarloaf Keys. Groundwater salinity and vegetation structure and composition were sampled in the early 1990s prior to impact from two hurricanes - Hurricanes Georges (1998) and Wilma (2005) - and again in 2012/2013 in permanent plots inside and outside the boundaries of the islands' freshwater lenses. Using linear mixed effects modeling, we examined whether groundwater salinity varied over time among locations, and investigated whether this variation was reflected in changes in forest structure and composition in three height strata. The results of this analysis revealed that groundwater salinity underlying plots outside the freshwater lens increased over the two decades, while salinity of groundwater at plots inside the freshwater lens remained stable, The greatest shift in composition occurred in the understory strata along a gradient of increasing salinity, and plots located outside the freshwater lens gained species typical of tidally-influenced buttonwood forest and lost glycophytic species. Viewed against the background of recurring hurricane impacts, these findings suggest that sea level rise is currently altering both groundwater resources and the composition of coastal forests in the Florida Keys. Similar dynamics should be observed in low-lying coastal forests within ocean basins subject to increased tropical cyclone activity. Management of these island coastal forests must now consider the continually shifting nature of the resource in light of acceleration in sea level rise.

Animals and plants on low elevation oceanic islands often rely on a thin lens of fresh groundwater and this lens is vulnerable to seawater contamination from storm surge. Documentation of the impact of the storm surge on the freshwater lens and its subsequent recovery is limited. In September 2017, Hurricane Irma made landfall in the Florida Keys as a category 4 storm with storm surge heights in excess of 2 m. This study used Electrical Resistivity Tomography (ERT) to investigate the effect of the storm surge on the freshwater lens of Big Pine Key, FL. The study compared ERT images along three profiles ranging between 220 and 280 m length collected in 2011 with post storm data collected about 3 to 4 months (November 2017/January 2018) and eight (May 2018) months after Irma. The post storm data documented that the storm surge impacted the freshwater lens on all three profiles with low resistivity (i.e., high salinity) zones in the upper 2 m of the groundwater. The increase in salinity was most pronounced in the lower elevations of the profiles. The May 2018 data were collected immediately after 2 weeks of intense precipitation. These data showed 40% recovery of the freshwater lens, most pronounced in the lower elevation of the profiles. This suggests that both the impact of storm surge and the freshwater recovery due to precipitation are most pronounced in low elevation regions where both saline and freshwater can collect at the surface.

Climate change will have long-lasting effects on the availability of fresh water on small, carbonate islands that have isolated fresh groundwater lenses, particularly as sea level rises and rainfall regimes shift. The carbonate islands of the Florida Keys provide an ideal location to study the effect of variable rainfall on the aqueous geochemistry of the islands' groundwater. In a rainfall-driven carbonate system, the expectation is that limestone dissolution will occur within the vadose zone resulting in increased ions in the groundwater. However, geochemical processes are also affected by the salinity of groundwater and the extent of the mixing zone between fresh and salt water. We chose two islands to conduct the study of the shallow groundwater: the largest island in the lower Florida Keys, Big Pine Key (BPK), and a smaller island, Upper Sugarloaf Key (SLK). From May 2011 through April 2012, monthly groundwater samples were collected from 24 shallow (1 m deep) wells located along a fresh to saline gradient on both islands. Groundwater chemistry was compared with rainfall amounts from a weather station on BPK. Saturation indices for aragonite and calcite, generated with geochemical modeling in PHREEQC, were compared to conservative mixing between Gulf of Mexico water and freshwater. Equilibrium to supersaturated conditions with respect to carbonate minerals dominated in all of the groundwater samples. Saturation indices varied with rainfall with the most supersaturated samples observed after a large rain event and samples approaching equilibrium after the longest period without rainfall. Calcium in excess of what would be expected from conservative mixing of fresh water and seawater was observed in all groundwater samples and was elevated at near-shore locations, especially on BPK. Contrary to expectations, dissolution resulting from mixing of freshwater and seawater was not supported in the shallow groundwater. Instead, dissolution within the narrow vadose zone from rain events likely resulted in the excess calcium in the groundwater. Seasonal fluctuations in groundwater composition were primarily observed on the smaller island and were related to the fresh water balance, changing rapidly after a heavy rain event, and suggest that a size threshold has been surpassed for a stable lens. Rising seas will further decrease lens extent and vadose zone depth, reducing the potential for future limestone dissolution.

Some studies published over the past several decades have concluded nourishment of oceanic beaches is a viable strategy to mitigate climate change. However, these were generally too limited in scope to accurately evaluate beach nourishment because each omit one or more of the following: (1) a realistic assessment of potential borrow area sand volume, (2) native beach compatibility, (3) construction costs, (4) all vulnerable geomorphic elements of the coastal zone, and (5) environmental impacts. When all of these parameters are considered, the results are markedly different. To demonstrate our point, we evaluated the recommendations of Houston (2017) using all five parameters. Contrary to Houston, we provide multiple lines of evidence that beach fill projects are not a sustainable strategy to protect or defend oceanic beaches of the Florida panhandle (USA), nor likely most of the world's developed coastlines at risk to the effects of climate change. The nourishment of oceanic beaches as historically constructed will surely continue over the next several decades. But, it must be done as an interim strategy during the formulation and implementation of a robust, long-term adaptive management strategy that incorporates managed withdrawal from the coastline.

Alligator (Alligator mississippiensis) holes are a key feature in the Everglades landscape providing aquatic refuge for alligators and other aquatic organisms. The morphology of the hole as well as its location in the landscape may influence its function as an aquatic refugium. Morphologic and hydrologic characteristics and dynamics of 50 alligator holes in Everglades National Park, Florida were examined based on 1-m resolution digital images and field measurements from 1994 to 2007. Major morphometric parameters of alligator holes calculated for the study area included surface area, diameter, major axis orientation, basin depth, and circularity index. We used basin depth along with surface water modelling from the Everglades Depth Estimation Network to describe alligator hole hydroperiod and examine relationships among morphologic features, habitat, and hydrology. Alligator holes in this study were similar morphologically (pond surface area, basin depth, and sediment depth) among locations and habitats, and for the most part over time with the exception of holes in the Rocky glades. Alligator holes with greater surface area were not necessarily deeper holes. Hole hydroperiod was not correlated with surface area, and larger holes did not dry out less frequently than smaller holes. Although marsh hydroperiods varied by location and habitat, alligator hole hydroperiod did not, illustrating that across the landscape alligator holes provide aquatic refugia under a range of hydrologic conditions. Published in 2012. This article is a US Government work and is in the public domain in the USA.

Use of indicator species as a measure of ecosystem conditions is an established science application in environmental management. Because of its role in shaping wetland systems, the American alligator (Alligator mississippiensis) is one of the ecological indicators for wetland restoration in south Florida, USA. We conducted landscape-level aerial surveys of alligator holes in two different habitats in a wetland where anthropogenic modification of surface hydrology has altered the natural system. Alligator holes were scarcer in an area where modified hydrology caused draining and frequent dry-downs compared to another area that maintains a functional wetland system. Lower abundance of alligator holes indicates lack of alligator activities, lower overall species diversity, and lack of dry-season aquatic refugia for other organisms. The occupancy rate of alligator holes was lower than the current restoration target for the Everglades, and was variable by size class with large size-class alligators predominantly occupying alligator holes. This may indicate unequal size-class distribution, different habitat selection by size classes, or possibly a lack of recruitment. Our study provides pre-restoration baseline information about one indicator species for the Everglades. Success of the restoration can be assessed via effective synthesis of information derived by collective research efforts on the entire suite of selected ecological indicators.

Following 2 y of severe hurricanes in 2004 and 2005, we examined the role of canopy gaps in promoting recruitment and growth of the exotic fern, Old World climbing fern Lygodium microphyllum (hereafter Lygodium), on tree islands of the Arthur R. Marshall Loxahatchee National Wildlife Refuge, Florida. We selected 12 sample tree islands, on which we placed three 1-m(2) plots in a hurricane-caused canopy gap and three plots in an adjacent closed canopy area. Spore traps were placed in canopy gaps and closed canopy areas to quantify the number of spores reaching the forest floor on each island. In addition, in each plot occurrence and growth of Lygodium was measured across four height classes (recruitment class, understory, midstory, and canopy). We predicted that recruitment and growth of Lygodium would be higher in canopy gaps than in closed canopy areas. After 3 y of biannual monitoring, a significantly greater number of spores were found in canopy gaps (4,804 spores.m(2).d) than in closed canopy areas (4,288 spores.m(2).d). Furthermore, we observed significantly greater recruitment and growth in canopy gaps compared with closed canopy areas in the recruitment class only. Presence of recruitment-class Lygodium in canopy gaps increased from four to five treatment areas and decreased from 1 to 0 treatment areas in closed canopy areas. These results suggest differences in recruitment and growth of Lygodium between canopy gaps and closed canopy areas on tree islands after severe hurricanes. However, despite the large number of spores in both canopy gaps and closed canopy areas, recruitment and growth were much lower than expected, with only two treatment areas having an average percent cover greater than 10% in any height class. If conducted within several years after a hurricane, focused monitoring efforts on hurricane-impacted tree islands may allow managers to detect and treat new infestations before they are able to overrun tree islands.

In south Florida, tropical hardwood forests (hammocks) occur in Everglades tree islands and as more extensive forests in coastal settings in the nearby Florida Keys. Keys hammocks have been less disturbed by humans, and many qualify as “old-growth,” while Everglades hammocks have received much heavier use. With improvement of tree island condition an important element in Everglades restoration efforts, we examined stand structure in 23 Keys hammocks and 69 Everglades tree islands. Based on Stand Density Index and tree diameter distributions, many Everglades hammocks were characterized by low stocking and under-representation in the smaller size classes. In contrast, most Keys forests had the dense canopies and open understories usually associated with old-growth hardwood hammocks. Subject to the same caveats that apply to off-site references elsewhere, structural information from mature Keys hammocks can be helpful in planning and implementing forest restoration in Everglades tree islands. In many of these islands, such restoration might involve supplementing tree stocking by planting native trees to produce more complete site utilization and a more open understory.