Controls on Evapotranspiration Rates in a Mangrove Forest Following Hurricane Wilma in the Everglades, FL

Mangrove forests are found along the world’s subtropical and tropical coastlines, causing them to frequently bear the brunt of tropical storms and hurricanes. Many mangrove ecosystem changes resulting from such storm disturbances have been widely studied, but there has been little work looking at how these natural disturbances might affect mangrove evapotranspiration (ET) and the effects it might have on water balances in surrounding areas. Our study focuses on the controls on ET during the wet seasons between 2004 and 2011 in a mangrove-dominated portion of the Everglades, FL. The hypothesis explored is that defoliation and canopy structural changes following hurricane Wilma (October 2005) altered the temperature profile, the energy balance closure, and the capacity of the canopy to control ET rates. To test these hypotheses, we used models incorporating the Bowen ratio, decoupling coefficient, canopy conductance, and Enhanced Vegetation Index (EVI), with data inputs from the Shark River AmeriFlux tower (SRS6) and from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS). Hurricane Wilma destroyed approximately 30% of the mangrove forest canopy, which took approximately 5 years to recover. In most forests, such a loss in canopy would result in a decrease in ET, but our analysis shows an increase in ET. Unlike terrestrial trees or even freshwater wetland trees, mangrove species are very responsive to the salinity of their soil and their stomatal conductance, and so transpiration is largely a function of that salinity. Our analysis suggests that the hurricane did not cause much change in the stomatal conductance due to the availability of water during the wet season, however, it instead caused a shift in the energy balance towards higher turbulent heat fluxes, and therefore increased evaporation, through the increased radiation exposure and resultant heating of the water underneath the canopy. Such a shift towards evaporation resulted in more atmospheric control of ET (as opposed to canopy control), which can be described by a high value of the decoupling coefficient. The demonstrated increase in ET after the hurricane and the canopy recovery time are large enough that such storm effects should be included in future hydrology models. Understanding the effects of disturbances, such as hurricane Wilma, on the ET and water balance of the south Florida mangrove forests is crucial to water management strategies for the future of the Everglades.