Primary productivity represents the major input of carbon and biological energy into world’s ecosystems and can be considered as an integrative measure of ecosystem functioning. Mangrove forests dominate tropical and subtropical coastlines and are among the most productive marine ecosystems in the world, ranking second in terms of net primary productivity (NPP) only to coral reefs. The productivity of mangroves represents the outcome and interactions of several factors that operate at distinct global, regional, and local scales. Climate and the relative role of regional geophysical processes (river input, tides, and waves) within a coastal landform are the dominant forcing functions that control the basic patterns of mangrove forest structure and function. At the local scale, variation in topography and hydrology within a coastal landform influence the distribution of soil resources and abiotic regulators, and along with hydroperiod, produce gradients resulting in the development of distinct mangrove ecotypes such as riverine, fringe, basin, scrub, and overwash forests. The magnitude and interaction of these environmental gradients including regulators (i.e., soil salinity, sulfide), resources (e.g., light, nutrients), and hydroperiod (e.g., frequency, duration, and depth of flooding) define a constraint envelope that determines mangrove productivity within a coastal setting.
In the Florida Coastal Everglades (FCE), vegetation patterns of mangroves result from the interaction of environmental gradients and natural disturbances (i.e., hurricanes), creating an array of distinct riverine and scrub mangroves across the coastal landscape. We are investigating how long-term (2000-present) landscape patterns of biomass and total net primary productivity (NPPT), including allocation in above- and below-ground mangrove components, vary inter-annually across gradients in soil properties and hydroperiod in two distinct FCE basins: Shark River Estuary (dominated by riverine mangroves) and Taylor River Slough (dominated by scrub forests). We propose that the allocation of belowground biomass and productivity (NPPB) relative to aboveground allocation is greater in regions with P limitation and permanent flooding.
Our goal is to understand how pulses of fresh and marine water and their associated resources influence mangrove structural and functional attributes and trajectories of ecosystem development in response to accelerated sea-level rise and hurricane disturbances.
Our study area includes mangrove forests of Everglades National Park (ENP), south Florida as part of the Florida Coastal Everglades Long Term Ecological Research (FCE-LTER) program (http://fcelter.fiu.edu/). Mangrove forests of ENP are distributed along the coastal margin and occupy an estimated total area of 144,447 ha, which represents approximately two-thirds of all mangrove cover in south Florida. In 2000, mangrove sites were established each along Shark River (SRS-4, SRS-5, and SRS-6) and Taylor River (TS/Ph-6 and TS/Ph-7) basins. Riverine mangroves along Shark River (southwestern ENP) contain mixed-species of Rhizophora mangle, Avicennia germinans, Laguncularia racemosa, and Conocarpus erectus. SRS-6 is located 4.1 km from the mouth of the estuary, while SRS-5, and SRS-4 are approximately 9.9 and 18.2 km upstream, respectively. All three sites have a distinct tidal hydroperiod, although SRS-4 is also influenced by runoff and groundwater particularly during the wet season. Mangrove sites along Taylor River (southeastern ENP: TS/Ph-7 & TS/Ph-6) are located approximately 1.5 and 4 km inland from Florida Bay. Mangrove zones are dominated by R. mangle scrub forest (tree heights ≤ 1.5 m) with clusters of C. erectus and freshwater Cladium jamaicense-Eleocharis sp. Mangrove waterways along Taylor River are non-tidal systems with flooded conditions, and water flow is determined by the interactions of seasonal precipitation, upland runoff, and wind.
Our long-term data indicate that Taylor River sites showed the highest P limitation (soil N:P > 60), with average NPPT double in higher P environments (Shark River: 17.0 ± 1.1 Mg ha-1 yr-1) compared to lower P regions (Taylor River: 8.3 ± 0.3 Mg ha-1 yr-1). Root biomass to aboveground wood biomass (BGB:AWB) ratio was 17 times higher in P-limited environments demonstrating the allocation strategies of mangroves under resource limitation. Riverine mangroves allocated most of the NPPT to aboveground (69%) while scrub mangroves showed the highest allocation to belowground (58%). Our results suggest that the interaction of lower P availability in Taylor River relative to Shark River basin, along with higher sulfide and permanent flooding account for higher allocation of belowground biomass and production, at expenses of aboveground growth and wood biomass. These distinct patterns of carbon partitioning between riverine and scrub mangroves in response to environmental stress support our hypothesis that belowground allocation is a significant contribution to soil carbon storage in forested wetlands across FCE, particularly in P-limited scrub mangroves. Elucidating these biomass strategies will improve analysis of carbon budgets (storage and production) in neotropical mangroves and understanding what conditions lead to net carbon sinks in the tropical coastal zone.
Our findings also show the high resilience capacity of mangroves in response to hurricane disturbances (Hurricanes Wilma and Irma). Our long-term litterfall data indicate that rates were reduced dramatically across Shark River sites due to canopy defoliation after Wilma’s impact. However, litterfall rates returned to pre-Wilma conditions within less than 5 years. Mangrove foliar residence time returned to pre-Wilma values as leaf NPP recovered over time, suggesting this index could be used as a proxy for canopy recovery and resilience of mangroves. Because hurricane disturbance in south Florida is frequent and increasing, a consideration of the interaction of these effects is critical to evaluate the role of pulsing high-energy disturbances (hurricanes) and pressing changes (sea-level rise) in maintaining forest species composition, productivity, and soil elevation in neotropical mangrove wetlands under climate change.
Victor H. Rivera-Monroy, Robert Twilley, David Lagomasino
National Science Foundation
Contact Edward Castaneda (firstname.lastname@example.org) for more information about the project.
Figure 1. Distribution of mangrove aboveground biomass in the Everglades National Park (ENP), south Florida (Simard et al. 2006). The insert shows the location of Shark River estuary with the highest biomass distribution in near-coast mangroves.
Figure 2. Riverine (left panel) and scrub (right panel) mangrove forests in the Florida Everglades. Contrasting gradients in soil P fertility and hydroperiod control forest structural development and productivity patterns between both basins.
Figure 3. Variation in aboveground wood biomass (AWB) and belowground biomass (BGB) (a) and BGB:AWG ratios (b) in FCE mangrove forests (Castañeda-Moya et al. 2013).
Figure 4. (a) Mean (± SE) foliar residence time (yr) in Shark River mangrove sites from 2001 to 2013. The Hurricane symbol seen below denotes Hurricane Wilma impact in October 2005. (b) Linear regression of foliar residence time and leaf aboveground biomass (AGB) anomaly (% change from 2000 to 2001 leaf AGB mean) across all sites (Rivera-Monroy et al. 2019).