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Hurricane Disturbance and Recovery of Energy Balance, CO2 Fluxes and Canopy Structure in a Mangrove Forest of the Florida Everglades

1. Introduction

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Mangrove forests are valued ecosystems due to their role in fisheries support, shoreline protection, and as a source of lumber for coastal communities. The role of these forests in global carbon cycling is also being increasingly recognized (Bouillon et al., 2008). Mangrove forests are particularly susceptible to disturbance from tropical storms because of their location along the coast in the tropics and subtropics. During storm events, high winds, surge and sediment deposition and erosion can cause widespread defoliation, tree mortality, and changes in species composition. In extreme cases, the disturbance caused by storms can cause shifts in ecosystem type (Craighead and Gilbert, 1962; Armentano et al., 1995; Ross et al., 2001; Cahoon et al., 2003; Smith et al., 2009).

The recovery of mangrove canopy structure after a tropical storm comes with the production of new leaves, seedling generation, stump sprouting, and development of understory vegetation (Michener et al., 1997; Walker, 1991). The mechanisms governing the recovery of plant-soil carbon cycles, including net ecosystem CO2 exchange (NEE), and the effects of tropical storm disturbance on ecosystem respiration are not well understood. Ellison and Farnsworth (1993) suggested that while structural damage may reduce the ability of mangrove forests to accumulate carbon over the short term, net ecosystem productivity (NEP, expressed in g C m-2 t-1) over longer time periods can also be stimulated through nutrient turnover, lowered sulfide toxicity, and creation of canopy gaps. The impacts of tropical storms on mangrove forest latent (LE) and sensible (H) heat fluxes remain largely unknown.

The research questions we seek to address in this manuscript include, "How are mangrove forest and soil carbon cycles and energy balance impacted by tropical storm disturbance?", "How do the changes in carbon and energy balance reflect the impacts to canopy structure caused by wind damage and storm surge?", and "Over what time scales do forest carbon cycles and energy balance recover to pre-disturbance conditions?". In order to address these questions and to assess the impacts of a tropical storm, it is necessary to account for the variability in forest carbon cycling and energy balance that is expected to occur with inter-annual variability in environmental drivers such as air temperature and salinity. Here we contrast variables such as NEE, LE, and H in a mature mangrove forest in Everglades National Park, Florida, USA before and after Hurricane Wilma, which made landfall in this region in October 2005. Observations of tree mortality, canopy albedo, satellite-based remote sensing data, and changes in sediment elevation provide independent assessments of impact and recovery and also provide the means to relate changes in land surface-atmosphere exchanges to the hurricane impacts on canopy structure. Thus, the principal objectives of the study are to, (1) estimate changes in canopy microclimate, energy partitioning, and seasonal and annual CO2 fluxes caused by Hurricane Wilma, and (2) relate these disturbances to the changes in canopy structure and soil dynamics during recovery from the storm. To address each of these objectives, we differentiate the expected inter-annual variability in ecosystem functioning from the changes that can be attributed to the disturbance.

map showing site locations in Everglades National Park and inset map showing hurricane-impacted zone
Fig. 1. Measurements to determine eddy covariance CO2 and water vapor fluxes have taken place at the Shark River eddy flux Tower site (SRS6) in Everglades National Park since 2004. The site is located 4 km inland from the approximate center of Hurricane Wilma's landfall. Pre- and post-hurricane measurements of sediment surface elevations and forest stand structure have been recorded at Shark River (SH3), Big Sable Creek (BSC) and Lostman's River (LO3) sites. The hurricane-impacted zone is shown in red in the upper right corner (inset) of the figure. [larger image]

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