Ken W. Krauss, Thomas W. Doyle
U.S. Geological Survey, National Wetlands Research Center, 700 Cajundome Blvd., Lafayette, LA 70506, U.S.A.

Robert R. Twilley²
Center for Ecology and Environmental Technology, Department of Biology, University of Louisiana at Lafayette, P.O. Box 42451, Lafayette, LA 70504-2451, U.S.A.

Thomas J. Smith, III
U.S. Geological Survey, Center for Water & Restoration Studies, c/o Center for Coastal & Watershed Studies, 600 Fourth Street, South, St. Petersburg, FL 33701, U.S.A.

Kevin R.T. Whelan
U.S. Geological Survey, Florida/Caribbean Science Center, c/o Biological Sciences Department, Florida International University, University Park, Miami, FL 33199, U.S.A.

and

Jason K. Sullivan
Johnson Controls World Services, National Wetlands Research Center, 700 Cajundome Blvd., Lafayette, LA 70506, U.S.A.

ABSTRACT

Woody debris is abundant in hurricane-impacted forests. With a major hurricane affecting South Florida mangroves approximately every 20 yr, carbon storage and nutrient retention may be influenced greatly by woody debris dynamics. In addition, woody debris can influence seedling regeneration in mangrove swamps by trapping propagules and enhancing seedling growth potential. Here, we report on line-intercept woody debris surveys conducted in mangrove wetlands of South Florida 9-10 yr after the passage of Hurricane Andrew. The total volume of woody debris for all sites combined was estimated at 67 m³/ha and varied from 13 to 181 m³/ha depending upon differences in forest height, proximity to the storm, and maximum estimated wind velocities. Large volumes of woody debris were found in the eyewall region of the hurricane, with a volume of 132 m³/ha and a projected woody debris biomass of approximately 36 t/ha. Approximately half of the woody debris biomass averaged across all sites was associated as small twigs and branches (fine woody debris), since coarse woody debris >7.5 cm felled during Hurricane Andrew was fairly well decomposed. Much of the small debris is likely to be associated with post-hurricane forest dynamics. Hurricanes are responsible for large amounts of damage to mangrove ecosystems, and components of associated downed wood may provide a relative index of disturbance for mangrove forests. Here, we suggest that a fine:coarse woody debris ratio 0.5 is suggestive of a recent disturbance in mangrove wetlands, although additional research is needed to corroborate such findings.

Key words: Avicennia germinans; disturbance; downed wood; Everglades National Park; Hurricane Andrew; Laguncularia racemosa; necromass; Rhizophora mangle; Rookery Bay National Estuarine Research Reserve; Ten Thousand Islands National Wildlife Refuge.

Woody debris provides an often overlooked, yet potentially important component of mangrove ecosystems (Harmon et al. 1986). The slow decomposition of woody debris following a major disturbance (Spies et al. 1988), for example, has led to speculation that coarse woody debris serves to promote the long-term persistence and supply of nutrients in a forest ecosystem (Harmon & Hua 1991). Similarly, woody debris can persist for many years in tropical mangrove forests (Robertson & Daniel 1989) and can provide erosion control, promote soil pedogenesis, increase site water retention, serve as a potential source of fuel, serve as nursery beds for germinating seeds, and provide habitat for heterotrophic communities (Harmon et al. 1986). Specific to mangroves, woody debris may trap propagules (Pinzón et al. 2003), promote sedimentation (Krauss et al. 2003), and, as mentioned in Allen et al. (2000), potentially increase growth of seedlings proximal to debris stores.

Accordingly, mangrove wetlands in South Florida support diverse food webs, assist with water quality improvement, and protect coastlines against tropical storm surges (Odum et al. 1982, Twilley 1998). Coastal wetland communities are especially important in the Caribbean where tropical storm frequency is high. An estimated 38 storms impacted some part of the Everglades region since 1886 (Doyle and Girod 1997), while shifts in the carbon balance of coastal systems are expected under scenarios of greater storm frequency in the Caribbean (Scavia et al. 2002). The frequency of disturbance in mangrove wetlands is particularly important in comparing ecosystem properties across sites to avoid attributing changes to the wrong environmental gradient.

Disturbances to forests greatly influence the amount of woody debris on the forest floor (Sturtevant et al. 1997, Allen et al. 2000). Determining forest structural changes in response to different environmental impact scenarios (e.g., hurricane, lightning strike), similarly, continues to be an effective way to model ecosystem response (Doyle & Girod 1997, Chen & Twilley 1998). Including woody debris in these assessments may be useful in accounting for site differences in productivity, community dynamics, or carbon biogeochemistry (Harmon & Hua 1991).

Jiménez et al. (1985) report on cases of mangrove tree mortality throughout the Caribbean, but do not include estimates of downed wood. Downed wood as a component of mangrove forest structure has been reported in only two other investigations (Robertson & Daniel 1989, Allen et al. 2000); both studies were from the old world tropics. The purpose of this study, therefore, is to provide an estimate of downed wood for the hurricane-prone mangrove wetlands of South Florida by testing whether the volume of downed wood varied with proximity to the path of a hurricane.