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Hurricane Impacts to Tree Islands in Arthur R. Marshall Loxahatchee National Wildlife Refuge, Florida
Cristina A. Ugarte, Laura A. Brandt, Stefani Melvin, Frank J. Mazzotti, and Kenneth G. Rice

Southeastern Naturalist, Volume 5, Number 4 (2006): 737–746

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2006 SOUTHEASTERN NATURALIST 5(4):737–746 Hurricane Impacts to Tree Islands in Arthur R. Marshall Loxahatchee National Wildlife Refuge, Florida Cristina A. Ugarte1,*, Laura A. Brandt2, Stefani Melvin2, Frank J. Mazzotti3, and Kenneth G. Rice4 Abstract - Tree islands—small, wetland forest communities imbedded in a matrix of freshwater marsh—characterize Arthur R. Marshall Loxahatchee National Wildlife Refuge in South Florida. The establishment and spread of invasive exotic plant species were hypothesized to alter tree-island communities and prolong recovery times from hurricane disturbances. During the fall of 2004, two hurricanes, Frances and Jeanne, caused damage to these tree islands. We examined the spatial extent of damage to tree islands and tree species across the Refuge by sampling 74 islands. Each tree island was assigned an overall damage rating based on both the openness of the canopy and the type and quantity of damage received. Distance from the eye-wall of the hurricanes, tree-island size, average tree height on the island, and relative abundance of invasive exotic plants were examined as predictors of damage. Over 85% of the sampled tree islands had damage, with most of the damage occurring in the center of the Refuge. Most tree islands were found to have moderate damage, (i.e., snapped large branches and less than 50% canopy cover removed). Persea palustris (swamp bays) had more snapped trunks than Ilex cassine (dahoon holly) and Myrica cerifera (wax myrtle). Islands with larger trees had heavier damage than islands with shrubs or smaller trees. Fifty-eight percent of the tree islands sampled had either Lygodium microphyllum (lygodium) and/or Melaleuca quinquenervia (melaleuca). The only island with severe damage had abundant lygodium that appeared to have caused the entire canopy to collapse. These hurricanes present a unique opportunity to investigate recovery patterns of tree islands in an ecosystem impacted by invasive exotics. They also provide an opportunity to examine patterns of spread and recruitment of lygodium and melaleuca. Introduction Hurricanes are a natural process in the Everglades landscape. Under natural conditions, most communities recover from disturbances; however, fragmentation and other human induced stresses such as changes in water management and introduction of exotic species may change how a community responds to disturbances such as hurricanes. A first step in documenting 1School of Natural Resources and the Environment, University of Florida, Everglades National Park Field Station, 40001 State Road 9336, Homestead, FL, 33034. 2US Fish and Wildlife Service, Arthur R. Marshall Loxahatchee National Wildlife Refuge 10216 Lee Road, Boynton Beach, FL, 33437. 3Department of Wildlife Ecology and Conservation, FLREC, University of Florida, 3205 College Avenue, Fort Lauderdale, FL 33314-7799. 4United States Geological Survey, Florida Integrated Science Center, Center for Water and Restoration Studies, University of Florida Field Station, 3205 College Avenue, Fort Lauderdale, FL. 33314-7799. *Corresponding author: caugarte@ifas.ufl.edu. 738 Southeastern Naturalist Vol. 5, No. 4 responses to disturbances is evaluating immediate impacts to the vegetation. In fall 2004, we had an opportunity to conduct an evaluation of hurricane damage to tree islands in the Arthur R. Marshall Loxahatchee National Wildlife Refuge, FL. On September 5 and 26, 2004, the eye-walls of hurricane Frances, a Category-2 storm, and hurricane Jeanne, a Category-3 storm, passed through northern Palm Beach County, FL within 12 km of each other. Hurricane Frances produced hurricane-force winds that extended 85 miles from the center of the eye, affecting the Arthur R. Marshall Loxahatchee National Wildlife Refuge (Refuge), whose center was approximately 77 miles from the center of the storm. Tropical-force and hurricane-force winds from Jeanne affected mostly the northern portion of the refuge. Hurricane Jeanne was the first Category-3 storm to make landfall on the east coast of Florida, north of Palm Beach since 1899. Of particular concern to Refuge staff were immediate and long-term impacts to tree islands, areas of wetland forest communities embedded within the marsh. Although these forests have been exposed to repeated hurricanes over the years (Noel et al. 1995, Roman et al. 1994), the impact of hurricane disturbance combined with current stressors that include hydrologic modification and colonization of invasive exotics could exacerbate wetland forest health and prolong recovery times (Loope et al. 1994, Roman et al. 1994). Because hurricanes can widely scatter the seeds of winddispersed plants (Oberbauer et al. 1996), large-scale disturbance from hurricanes could provide an opportunity for wind dispersing exotics to colonize new areas (Snitzer et al. 2005). The presence of exotics may ultimately change a community’s “natural” successional trajectory (Loope et al. 1994, Ogden 1992, Roman et al. 1994). Control of exotic plants, particularly Lygodium microphyllum (Cav.) R. Br. (lygodium) and/or Melaleuca quinquenervia (Cav.) S.T. Blake (melaleuca) is a management priority at the Refuge. Recent systematic reconnaissance flights (SRFs), which assess percent of exotics within 1-km x 1-km grid cells, estimated that over 60% of the cells within the Refuge contained lygodium or melaleuca (Woodmansee et al. 2005). How the spread of these exotics will be influenced by the hurricanes is not clear. However, there is concern that physical damage to tree islands (i.e., removal of the canopy and increasing light levels) will make them prime colonization sites for lygodium and melaleuca within these habitats. The goals of this work were to 1) determine the spatial patterns of hurricane effects to tree islands throughout the Refuge, 2) classify hurricane damage to tree-island vegetation, 3) assess species-specific damage, and 4) assess how certain characteristics (i.e., presence and abundance of exotics on tree islands, tree height, and distance from the center of the eye of the two storms) correlate to tree-island vegetation damage. 2006 C.A. Ugarte, L.A. Brandt, S. Melvin, F.J. Mazzotti, and K.G. Rice 739 Methods Site description The Arthur R. Marshall Loxahatchee National Wildlife Refuge (Refuge) includes approximately 57,000 ha of northern Everglades ridge and slough habitat which is a mosaic of open sloughs, wet prairies, sawgrass strands, and tree islands. Tree islands are a signature landscape feature of the Refuge and provide habitat for a variety of plants and animals (Brandt et al. 2003). The majority of tree islands are small (< 0.13 ha) islands that formed from floating peat mats that settled on the peat surface and became colonized with bayhead vegetation. Tree canopies are composed primarily of Persea palustris (Raf.) Sarg. (swamp bay), Ilex cassine L. (dahoon holly), and Myrica cerifera L. (wax myrtle). Ferns (Acrosticum spp., Osmunda spp.), Chrysobalanus icaco L. (cocoplum), and Cephalanthalus occidentalis L. (button bush) occur in shrub and ground cover (Brandt et al. 2003). In late October and early November 2004, tree islands were systematically sampled to assess hurricane impacts. We used ArcGis 9.0 to place a 2-km x 2-km grid over the Refuge area, and used the vertices of 100 of the 2 x 2 cells within the area of the Refuge accessible by airboat as sample points. Upon visiting a point, the closest tree island was sampled. If no tree island occurred at the point, or the tree island was less than 10 m in width, we sampled the nearest tree island greater than 10 m in width. We were able to sample tree islands at 74 selected points. Of the other 26 sampling points, 11 had no tree islands within at least 2 km, 4 were continuous dense shrubs and inaccessible, and 11 were dominated by melaleuca, with no typical tree island flora. Tree islands were circled by airboat to assess damage and to collect information on tree-island area using a global positioning system (Garmin MAP 76™). Two scales of damage were assessed at each tree island: the entire island and individual trees by species. Each tree island was assigned an overall damage rating based on both the openness of the canopy and the type and extent of damage received (Lawrence 1995): Severe damage = uprooted trees/leaning over 90 degrees, with 􀂕 75% open canopy. Heavy damage = major trunk snap off, leaving 􀂕 50 and < 75% open canopy. Moderate damage = large side branches snapped and leaning trees, leaving 􀂕 25 and < 50% open canopy. Light damage = small branches down, leaves stripped, and leaning trees, leaving 􀂕 10 < 25% open canopy. No damage = no visible damage and less than 10% open canopy. Since tree height is often related to damage severity in other South Florida wetland forests (Noel et al. 1995), each tree island was assigned a canopy height category based on the majority of trees present: trees 740 Southeastern Naturalist Vol. 5, No. 4 (larger than 3 m), small trees (less than 3 m) and shrubs (primarily willow heads with an open canopy). In addition, we recorded the exotic plant species present and estimated their cover as: abundant (greater than 50% cover of the tree island), moderate (between 10–50%), or few (less than 10%). Individually damaged trees were counted and categorized into one of three damage classes by species: uprooted/leaning; main trunk snapped off; and side branch off. Surface maps of damage and exotics were created using the spatial analyst module in ArcGIS 9.0 with an inverse distance-weighted function. Because there were so few islands in either the no damage or severe damage category, we collapsed damage types into two categories for statistical analyses: damage—which included the severe, heavy, and moderate categories; and no damage—which included the no damage and light categories. The hurricane tracks were obtained from NOAA, and a distance measurement from each island to the eye-wall was calculated using ArcGIS. We used a forward stepwise logistic regression analysis (SPSS ver. 11.5), to assess if damage or no damage was a function of distance to the eye-wall of hurricanes Jeanne or Frances, tree height, island size, and abundance of melaleuca and lygodium separately. Results We sampled 74 tree islands throughout the refuge, ranging in size from 0.03 ha to 1.4 ha (Fig. 1) and tallied 364 damaged trees. A little over one third of the islands had moderate damage (38.5%), while 24% had heavy damage, 23% had light damage, 14% had no damage, and only one had severe damage (Fig. 2). Of the 364 trees we tallied, the most common damage type across these islands was major trunk snapping (39%), followed by uprooted/leaning trees (34%; Table 1). A greater proportion (61%) of swamp bay, which was the tallest of the tree species sampled, had major trunk snaps compared to dahoon holly which was usually leaning/uprooted (Fig. 3). Wax myrtle was most often leaning/uprooted. Melaleuca had very little damage relative to other species. Of the 74 islands sampled, 58% had either melaleuca or lygodium present, and 19% had both species. Melaleuca was present on 41% of the islands while lygodium was present on 38% (Table 2). The most severely Table 1. A.R.M. Loxahatchee National Wildlife Refuge 2004 hurricane damage counts to common tree island tree species. S = swamp bay, D = dahoon holly, W = wax myrtle, M = melaleuca, C = cypress, W = willow, B = button bush, and Co = cocoplum. Damage type S D W M C W B Co Total % Uprooted/leaning 25 47 54 0 2 3 1 1 34.0 Major trunk snap 73 40 33 3 1 3 0 2 39.0 Side branch snap 21 28 43 1 0 4 0 0 24.0 Total % stems damaged 32 29 33 0.01 0.8 3 0.3 0.8 2006 C.A. Ugarte, L.A. Brandt, S. Melvin, F.J. Mazzotti, and K.G. Rice 741 damaged island was heavily infested with lygodium. There was no damage gradient throughout the Refuge, and distance from the hurricane center was not a significant predictor in our model. It was apparent, however, that the most damage occurred in the central part of the Refuge (Fig. 2). The only significant explanatory variable in the forward stepwise logistic regression model was tree height, which explained 32% of the variation in the model. Islands with larger trees were five times more likely to obtain heavy damage than islands with smaller trees or shrubs (Table 3). Figure 1. Distribution of sizes (ha) of tree islands in A.R.M. Loxahatchee National Wildlife Refuge sampled for hurricane impacts in October/November 2004. Mean size of 74 tree islands = 0.18 with st. dev.= 0.21 ha. Table 2. A.R.M. Loxahatchee National Wildlife Refuge 2004 counts and relative abundance of most common exotics, melaleuca and lygodium, on 74 sampled tree islands. Abundant Moderate Few Species Presence (< 50% canopy) (50–10%) (< 10%) Lygodium 38% (n = 28) 12.0% (n = 9) 9.0% (n = 7) 16% (n = 12) Melaleuca 41% (n = 30) 7.0% (n = 5) 7.0% (n = 5) 27% (n = 20) Presence of either exotic 58% (n = 43) No exotics present on island 31% (n = 31) 742 Southeastern Naturalist Vol. 5, No. 4 Figure 2. Landscape-level hurricane damage to tree islands in A.R.M. Loxahatchee National Wildlife Refuge 2004 in reference to hurricane tracks. Black = severe damage, dark grey = heavy damage, moderate grey = moderate damage, light grey = light damage, and white = no damage. Each open circle represents a sampled island. Discussion Over 85% of the tree islands sampled after hurricanes Frances and Jeanne had damage. Most islands sustained moderate impacts to their canopy with the snapping of large branches and between 25–50% of the canopy removed. We found that most of the heavier damage occurred in the center of the Refuge; 2006 C.A. Ugarte, L.A. Brandt, S. Melvin, F.J. Mazzotti, and K.G. Rice 743 however, there were patchy areas of heavy damage throughout the Refuge. This patchiness often occurs in other forms of natural disturbances, and may be due to a combination of microhabitat variation and wind-field microbursts, a phenomenon reported to occur with hurricanes (Powell et al. 1996). Islands with taller trees were five times more likely to suffer damage than tree islands with shorter trees or shrubs. Shrubbier islands, dominated by wax myrtle, were found more often in the northern and southern parts of the Refuge, and may partially explain why we saw more damage to islands in the central part of the Refuge. The types of damage most often sustained, major-trunk snaps and uprooted or leaning trees, create openings in the canopy and can result in tree mortality. The extent of damage varied by species. The greater proportion of main-trunk snaps for swamp bay compared to dahoon holly was not surprising as other researchers have noted similar species-specific damage Table 3. Forward stepwise logistic regression of 2004 hurricane damage in A.R.M. Loxahatchee National Wildlife Refuge as a function of tree height on sampled tree islands (r2 = 0.324, G = -47.000). Exp(B) Variable B S.E. Wald df Sig. odds ratio Step 1(a) Tree height 1.637 0.466 12.341 1 0.0001 5.138 Constant -3.787 1.310 8.363 1 0.0040 0.023 Figure 3. Percent damage to tree islands in A.R.M. Loxahatchee NWR from 2004 hurricanes based upon tree height; shrub, small-tree, and large-tree dominated islands. Seventy-four tree islands were sampled. 744 Southeastern Naturalist Vol. 5, No. 4 (Duever 2005, Gresham et al. 1991, Putz et al. 1983, Slater et al. 1995, Walker et al. 1992). Suggested reasons for species-specific damage include habit, wood strength, and specific density. Putz et al. (1989) found that growth rates and probability of snapping is inversely correlated to wood strength, and that larger trees tended to resprout less than smaller trees. The results of this study suggest that the larger swamp bays, which form a significant component of the canopy on Refuge tree islands, have weaker wood strength than other treeisland species, and may not regenerate as quickly as the smaller swamp bays. The level of damage to a tree species does not necessarily mean that trees will die or that the structure of communities will be altered in the long term. For example, short-term mortality was low, 12%, for trees in hardwood hammocks of Everglades National Park after Hurricane Andrew, despite up to 85% severe damage to upland trees (Slater et al. 1995). We found very little damage to melaleuca relative to native species. Although we did not detect relationships between the relative abundance of exotics and hurricane damage, we do feel that they may cause additional stresses to these islands. Lygodium, in particular, may severely impact these islands because its large, heavy, rhizomatous mats may cause larger trees to become “top heavy” and increase the probability of snapping or uprooting. We found one case of severe damage in the Refuge where the entire tree island canopy had collapsed. It was probable that under heavy winds, the weight of the lygodium mat in addition to weak stems of trees toppled the trees, leaving almost no structure whatsoever. In a general review of hurricanes in the Everglades, Duever et al. (1994) suggested that there are few long-term effects on community characteristics and that canopies fill in after a few years. Walker et al. (1992) also noted that trees rapidly regained pre-hurricane foliage and suggested that a species’ ability to re-sprout may be more important in structuring post-hurricane communities than its ability to survive hurricane-force winds. Long-term changes in community structure may occur in unaltered systems from natural disturbances (Snitzer et al. 2005). However, additional human-caused stress may exacerbate recovery patterns that influence community structure. At the Refuge, hydrology and invasive exotic species are two human-caused stressors that may affect recovery of tree islands. Altered hydrology may limit regeneration of swamp bay and dahoon holly if water levels are such that tree islands are flooded for extended periods. Particularly in the south-central part of the Refuge, tree islands may be flooded above their median height for 350 days, which may not allow for germination and survival of seedlings (Brandt et al. 2006). A greater threat to short-term tree-island recovery is the threat of colonization by lygodium and melaleuca. Fifty-eight percent of the sampled islands had either lygodium or melaleuca or both. Both of these exotic species have high germination rates and high growth rates and are likely to colonize openings on islands before native species. Snitzer et al. (2005) found greater recruitment of exotics in gaps of high light after hurricane disturbance. We are currently monitoring 2006 C.A. Ugarte, L.A. Brandt, S. Melvin, F.J. Mazzotti, and K.G. Rice 745 recruitment of lygodium on islands that were lightly, moderately, and heavily affected by hurricanes. Understanding how exotics colonize hurricane- disturbed tree islands will provide information that can be used to target priority areas for exotic control efforts. Hurricanes present a unique opportunity to investigate recovery patterns of tree islands in a human-altered ecosystem. In particular, hurricanes in south Florida provide an opportunity to examine patterns of spread and recruitment of lygodium and melaleuca. If damage sustained to tree islands and location of islands within the Refuge are important predictors of invasiveness, then managers can use this information to help prioritize efforts to remove exotics. Acknowledgments This study was supported by the Critical Ecosystem Studies Initiative and the Priority Ecosystems Studies program, US Department of Interior, and the School for Natural Resources and the Environment, University of Florida. The Fish and Wildlife Service and O.L. Bass provided logistic support. We thank C. Bugbee for assistance in the field, and K.R.T. Whelan for a review of the manuscript. Literature Cited Brandt, L.A., D. Ecker, I. Gomez Rivera, A. Traut, and F.J. Mazzotti. 2003. Wildlife and vegetation of bayhead islands in Arthur R. Marshall Loxahatchee National Wildlife Refuge. Southeastern Naturalist 2:179–194. Brandt, L.A., G.L. Martin, and F.J. Mazzotti. 2006. Topography of pop-up bayhead tree islands in Arthur R. Marshall Loxahatchee National Wildlife Refuge Florida Scientist 69(1):19–35. Duever, M. 2005. Hurricane Hugo: Impacts on natural ecosystems and their subsequent recovery. Presentation at the 2005 annual meeting of the Society of Wetland Scientists, Charleston, SC. Duever M.J., J.F. Meeder, L.C.Meeder, and J.M. McCollom. 1994. The climate of south Florida and its role in shaping the Everglades ecosystem. Age, origin, and landscape evolution of the everglades peatland. Pp. 225–248, In S. Davis and J. Ogden (Eds.). Everglades, The Ecosystem, and Its Restoration. St. Lucie Press, Delray Beach, FL. Gresham, C.A., T.M. 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