Title: Effects of fire on water quality, plant production, and biogenic accretion in a Juncus roemerianus dominated marsh
MASGC Project Number: R/CEH-27
Principal Investigator: Julia A. Cherry, University of Alabama
Co-PI: Christopher May, The Nature Conservancy of Michigan
Graduate Student: Anna Braswell, University of Alabama Master's Program (Graduated in December 2010)
Background: Coastal wetlands provide a number of goods and services to human communities, including storm abatement, flood mitigation, and biodiversity for recreational activities and coastal economies. The long-term stability of these valuable ecosystems is threatened by habitat loss, habitat degradation, over-exploitation of natural resources, and climate change, especially sea-level rise. Management and restoration of these ecosystems should not only restore critical habitat for ecologically and economically important species, but maintain long-term marsh stability by enhancing accretion as a means of combating relative sea-level rise. Prescribed burning is a common management practice employed in coastal marshes and adjacent pine hammocks of the US to maintain habitat, preserve native biodiversity, promote survival of fire-dependent species, increase food availability for wildlife, and decrease fuel loads that contribute to catastrophic wildfires. However, the effects of fires on nutrient availability, productivity, and accretion of organic matter (biogenic accretion) in Juncus roemerianus marshes of the Gulf coast are poorly understood. Because fires may alter water quality, plant productivity, and ultimately, biogenic accretion within marshes, prescribed burning may be a useful tool for enhancing elevation gain in marshes threatened by submergence due to sea-level rise. In addition, prescribed burning may be a useful tool for removal of hurricane-related debris and restoration of coastal marshes following severe storms. Hurricanes and other tropical storms can also influence marsh accretionary processes via scouring and erosion, sediment and wrack deposition, or salt water intrusion. Because fire and hurricanes have the potential to impact the same Gulf Coast marshes over short periods of time (i.e., in the same year), it is important to understand the ways in which coastal ecosystems respond to multiple disturbances.
Objectives: This aim of this project was to elucidate the effects of large-scale disturbances in a J. roemerianus marsh at Grand Bay National Estuarine Reserve in Mississippi by quantifying changes in water quality and nutrient availability, plant production, biogenic accretion, and hurricane-debris removal before and after hurricanes and a prescribed burn. In fall 2008, Hurricanes Gustav and Ike inundated all study plots with storm surge (Fig. 1), and in January 2009, we subjected half of the transects to a low intensity prescribed burned (200-300°C), thereby permitting us to examine the interactive effects of hurricanes and fire in this coastal marsh.
Study Site & Design: This study was being conducted in marsh along Bayou Cumbest in the Grand Bay NERR located in the Gulf coastal plain in extreme southeastern Mississippi (30°22'58.133"N; 88°26'17.83") . The reserve contains a variety of wetland habitats that are home to many important species of fish and wildlife including brown trout, speckled trout, oysters, brown shrimp, and sea turtles. Salt marshes in the reserve are dominated by the black needlerush, Juncus roemerianus, and represent core marsh habitat bordered by pine savannas on the upland extreme and bayous or bays on the lowland extreme. The marshes of Grand Bay previously received sediment and freshwater input from riverine sources, which no longer flow through the estuary. As a result, the estuary receives little terrestrial sediment input and has been particularly susceptible to coastal erosion. The lack of replenishment by riverine sedimentation makes this estuary particularly useful for investigations of the relative importance of biogenic accretion for long-term marsh stability. The study area consisted of six transects with plots in the low, mid, and high marsh for a total of 18 plots (Fig. 2).
Key Findings: Patterns in vertical accretion were primarily driven by hurricane sediment input, with 15.4 ± 2.4 mm, 5.3 ± 1.8 mm, and 2.4 ± 0.4 mm of hurricane sediment deposition in the low, mid, and high marsh, respectively (Table 1). This initial sediment pulse persisted throughout the remainder of the study, with very little change over time attributable to fire effects. Another consequence of these storms was the movement of wrack, an important fuel source for fires, from the low marsh to the high marsh-pine island boundary. When present, the burn removed this wrack from study plots while also significantly reducing aboveground biomass. Aboveground plant biomass slowly recovered to pre-burn levels over the subsequent year, although the relative speed with which this recovery occurred varied by location within the marsh. Low marsh plots recovered faster than mid or high marsh plots. This was likely due to the fact that high marsh plots experienced less tidal flushing, higher sulfide concentrations, and lower soil redox potentials than low marsh plots. In addition, the slow rate of recovery in burned high marsh locations, in which wrack accumulation from hurricanes was greatest, suggests that certain locations may be more vulnerable to fire than others. Furthermore, the occurrence of a hurricane or intense storm prior to a fire may alter fire behavior and intensity by moving and depositing wrack to new locations within a marsh.
Management Implications: When developing management plans that include prescribed fire, the location and elevation within a marsh and the presence of wrack should be considered to prevent damage to marsh habitat that may be slow to recover and potentially more vulnerable to fire impacts. As climate change contributes to further sea-level rise, understanding the effects of disturbances and management on processes important for vertical adjustment will be imperative for the long-term sustainability of coastal marshes.
Post-Hurricane Characterization of Study Sites :
Variable pH Salinity (ppt) Vertical accretion (mm) Juncus Density (no/m2) Juncus Biomass (g/m2) Juncus community proportion Table 1: Edaphic and biological variables at low, mid, and high marsh plots following Hurricanes Gustav and Ike in 2008 (n=6 for each). Data are means ± 1SE.
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