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Ken W. Krauss1, Thomas W. Doyle1, Robert R. Twilley2, Kevin R.T.K. Whelan3, and Thomas J. Smith III4 1 US Geological Survey, National Wetlands Research Center, Lafayette, LA, 70506 (USA) 2 University of Louisiana at Lafayette, Center for Ecology and Environmental Technology, Lafayette, LA, 70504 (USA) 3 U.S. Geological Survey, Florida/Caribbean Science Center, Miami, FL, 33199 (USA) 4 U.S. Geological Survey, Center for Water & Restoration Studies, St. Petersburg, FL, 33701 (USA)

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Abstract

Taylor River Slough [larger image]

Volume of woody debris in forests provides an often over-looked, yet important ecosystem service. Here, we report on line-intercept woody debris surveys conducted in mangrove wetlands of south Florida between 9 and 10 years after the passage of Hurricane Andrew (1992). The volume of woody debris for all sites combined was estimated at 67 m³ha^-1, and varied from 13-181 m³ha^-1 depending upon differences in forest height, storm circulation quadrant, and maximum estimated sustained wind velocities. Greater volumes of woody debris were found within the storm’s eyewall and immediate right quadrants. Likewise, smaller percentages of fine woody debris volume relative to coarse woody debris volume were associated with greater storm force, also reflecting differential rates of decomposition. Hurricanes are responsible for large amounts of damage to mangrove ecosystems and components of downed wood may provide a relative index of disturbance among similar mangrove forests.

Study crew near Lostmans Ri Everglades National Park [larger image]

Disturbance to forests greatly influences 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., hurricanes, lightning strikes, hydroperiod fluctuations) continues to be an effective way to predict ecosystem response and persistence (Doyle & Girod 1997; Chen & Twilley 1998). Including downed woody debris in these assessments may be important in accounting for site differences in productivity, community dynamics, and carbon biogeochemistry among ecosystems (Harmon & Hua 1991).

Downed wood as a component of mangrove forest structure has been explored in two other investigations (Robertson & Daniel 1989; Allen et al. 2002); both studies were from the old world tropics. The purpose of this study is to provide an estimate of downed wood for the hurricane-prone mangrove wetlands of south Florida. We tested whether the volume of woody debris and relative distribution of fine versus coarse woody debris varies with proximity to the path and windspeed of Hurricane Andrew (1992) as a landscape level gradient of disturbance, and use this gradient to suggest a relative disturbance index for mangrove forests in south Florida.

Survey sites

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Line intercept - coarse woody debris [larger image]		Go-no-go gauge - fine woody debris [larger image]

• Differences in combined woody debris (i.e., coarse + fine debris) by sample region were significant (F_4,18 = 13.74; P0.001) – Table 1
• Peak woody debris was associated with eyewall and immediate right quadrant impact zones of Hurricane Andrew (ENP-UP and ENP-MID) – Table 1; Fig 1
• When analyzed separately, fine woody debris and coarse woody debris differed by region, with statistical groupings varying only slightly from combined measurements – Fig 1; cf. Table 1
• Coarse woody debris comprised the largest percentage of downed wood on most surveyed mangrove stands, with a ratio of fine:coarse debris ranging from 0.37 to 0.82 on RB-UP, ENP-UP, ENP-MID, and ENP-LOW sites – Fig 1
• Relationships between forest height and woody debris volume were weak – Fig 2

Table 1. Mean total volume of woody debris in south Florida mangrove forests.
	Mean volume (m3ha-1)
Diameter/Decay Class	RB-UP	ENP-UP	ENP-MID	ENP-LOW	ENP-East	Mean
Fine (7.5 cm)
0.0-1.0 cm	1.41	2.53	1.16	1.03	0.19	1.20
1.0-2.5 cm	3.78	5.15	4.49	4.31	3.95	4.35
2.5-7.5 cm	12.53	22.36	29.88	17.79	8.06	18.48
Coarse (>7.5 cm)
Sound	0.00	0.26	0.00	0.15	0.00	0.09
Intermediate	9.47	11.22	11.54	11.29	1.36	9.28
Rotten	14.12	5.12	85.34	16.83	2.74	34.00
Total	41.31 b	97.63 ab	132.41 a	51.40 ab	16.31 c	67.40

Fig 1 Distribution of fine and coarse woody debris by region (± 1 s.e.), and means for combined woody debris (center line) from south Florida mangrove wetlands. Values followed by the same letter among regions for either fine woody debris (capital letters) or coarse woody debris (lower-case letters) are not significantly different at = 0.05. [larger image]

Fig 2 Relationship between woody debris volume and forest height for two ranges of model-projected Hurricane Andrew windspeeds from south Florida mangrove wetlands. [larger image]

Ten Thousand Islands NWR [larger image]

Shifts in the relative percentage of coarse woody debris versus fine woody debris by region and the decomposition state of larger wood further indicates that the majority of the coarse woody debris in ENP-UP, ENP-MID, and ENP-LOW sites may have been from Hurricane Andrew. In fact, a fine:coarse woody debris ratio of 0.44 and 0.37 for ENP-UP and ENP-MID, respectively, indicates that lower ratios may indicate relatively higher disturbance regimes when compared to sites of similar forest stature. Fine to coarse woody debris ratios exceeded 0.75 for RB-UP and ENP-LOW; ENP-East sites (3.0) were not comparable on the basis of a much reduced relative forest stature. A low fine to coarse woody debris ratio of 0.11 was found on mangrove sites on Kosrae, Federated States of Micronesia, under an accelerated individual tree harvest regime relative to mangrove forests of Pohnpei (0.56) and Yap (0.61: Allen et al. 2000). Such analysis of downed wood components may provide a relative index of disturbance among similar mangrove forests in a geographical region.

It was hypothesized that, perhaps, pre-hurricane surveys of forest structure can indicate potential woody debris fall during a hurricane. But, only as much as 28% of the variation in the volume of woody debris can be explained by the linear relationship of woody debris with forest height.

Total stand volume has been shown to be an equally poor indicator of downed woody (Allen et al. 2000), yet the relationship with forest basal area can be strong (Santiago 2000). Determining a more appropriate metric to predict the volume of downed wood in Florida mangrove forests may involve intensive studies on plot-level variation within forests, and should be a focus of future woody debris research efforts on long-term ecological research plots.

Allen, J. A., K. C. Ewel, B. D. Keeland, T. Tara, and T. J. Smith III. 2000. Downed wood in Micronesian mangrove forests. Wetlands 20: 169–176.

Chen, R., and R. R. Twilley. 1998. A gap dynamic model of mangrove forest development along gradients of soil salinity and nutrient resources. J. Ecol. 86: 37–52.

Doyle, T.W., and G. F. Girod. 1997. The frequency and intensity of Atlantic hurricanes and their influence on the structure of south Florida mangrove communities. In H. F. Diaz and R. S. Pulwarty (Eds.), Hurricanes: Climate and Socioeconomic Impacts. pp. 109–120. Springer-Verlag, Heidelberg, Germany.

Harmon, M. E., and C. Hua. 1991. Coarse woody debris dynamics in two old-growth ecosystems. BioScience 41: 604–610.

Harmon, M. E., J. F. Franklin, F. J. Swanson, P. Sollins, S. V. Gregory, J. D. Lattin, N. H. Anderson, S. P. Cline, N. G. Aumen, J. R. Sedell, G. W. Lienkaemper, K. Cromack Jr., and K. W. Cummins. 1986. Ecology of coarse woody debris in temperate ecosystems. Adv. Ecol. Res. 15: 133–302.

Robertson, A. I., and P. A. Daniel. 1989. Decomposition and the annual flux of detritus from fallen timber in tropical mangrove forests. Limnol. Oceanogr. 34: 640–646.

Santiago, L. S. 2000. Use of coarse woody debris by the plant community of a Hawaiian montane cloud forest. Biotropica 32: 633–641.

Sturtevant, B. R., J. A. Bissonette, J. N. Long, and D. W. Roberts. 1997. Coarse woody debris as a function of age, stand structure, and disturbance in boreal Newfoundland. Ecol. Appl. 7: 702–712.

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Related information:

SOFIA Project: Vegetation Dynamics in Land-Margin Ecosystems: The Mangroves of South Florida

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