Redbay (Persea borbonia) abundance and the potential effect of Laurel Wilt Disease in the Bald Head Woods Reserve, North Carolina, USA

Transcription

1 Redbay (Persea borbonia) abundance and the potential effect of Laurel Wilt Disease in the Bald Head Woods Reserve, North Carolina, USA Maureen E. Dewire Department of Environmental Studies, University of North Carolina at Wilmington Submitted in partial fulfillment of Masters of Arts Degree May 2011

2 ! " Table of Contents Abstract 3 Introduction..3 Methods 9 Results...10 Discussion.11 Figures and Tables...14 Map of P. borbonia and LWD Distribution.14 Map of Study Site.15 Graphs of P. borbonia Percent Cover in BHI Woods..16 Tables of Transects Works Cited..19

3 ! # Abstract Laurel Wilt Disease (LWD) has spread rapidly across the southeastern United States since first identified in Georgia in 2004, causing significant mortality of redbay (Persea borbonia) in the coastal plain. Vectored by the invasive redbay ambrosia beetle (Xlyeborus glabratus), LWD is a fungal disease (Raffaelea lauricola) that causes near 100% mortality in infected areas with no effective methods of control identified to date. Aided by anthropogenic actions, LWD is now established in North Carolina, South Carolina, Georgia, Florida and Mississippi. Bald Head Woods Reserve, located on Bald Head Island, North Carolina, is a 191-acre preserve of maritime forest at risk of losing its redbay population should LWD expand to the island. Maritime forests are globally imperiled ecosystems that are generally not well understood. Further, there is even less known about how forest compositions will be altered by the removal of redbay from the system. This study sought to identify the percent cover of redbay in Bald Head Woods in anticipation of the imminent arrival of LWD. Examination of 52 quadrats along five transects within the reserve boundary lines found that approximately 11.4% of the plant cover in the Bald Head Woods is redbay with the majority occurring in the southern portion of the forest. Future monitoring of the current redbay population will be essential to better understanding the impacts of LWD. It is suggested that the current redbay population be categorized by size and closely monitored for LWD infection. If and when LWD impacts Bald Head Woods, it will be important to study the rate of death, the size class affected, changes in forest species composition and signs of successful regeneration. Introduction Maritime forest communities can be found along the eastern seaboard of the United States, stretching from North Carolina south to Florida, restricted to barrier islands and the adjacent mainland (Bellis, 1995). Defined by the North Carolina Coastal Resources Commission as those woodlands that have developed under the influence of salt spray on barrier islands and estuarine shorelines, maritime forest are influenced by several environmental factors: exposure to high levels of salt; strong winds and tidal overwash during storm events; poor nutrient levels in the soil; limited supply of freshwater; unstable soil that is prone to wind or water erosion (Bellis, 1995). There are several distinct but related communities within this coastal zone, defined by

4 ! $ their species composition and physical characteristics (Schafale and Weakley, 1990). Specifically, maritime evergreen forests are located on barrier islands, formed on old sand dunes that are out of the reach of storm surge flooding and away from the most intense salt spray. They differ from other coastal zone communities (maritime deciduous forest, maritime shrub and coastal fringe evergreen) by the presence of hardwoods in the canopy (Q. virginiana and Q. hemispaerica), a tree canopy height of greater than 5m tall, and their limited distribution on barrier islands or the ocean side peninsulas, respectively (Schafale and Weakley, 1990). Although somewhat protected, maritime evergreen forests must cope with a series of stresses including a constant salt spray and disturbance from storm events including hurricanes (Schafale and Weakley, 1990). This community is distinguished from the maritime deciduous forest by the presence of live oak (Quercus virginiana) and laurel oak (Quercus hemisphaerica) as the dominant canopy trees (Bellis, 1995). The Smith Island Complex in southeastern North Carolina is a southern variant of this forest, with Sabal palmetto (Sabal palmetto) an additional member of the forest canopy (Bellis, 1995). Together, these trees provide a network of limbs and foliage that create a canopy, thereby protecting the understory trees, shrubs and vines that include red cedar (Juniperus virginiana), yaupon holly (Ilex vomitoria), Carolina laurel cherry (Prunus caroliniana), redbay (Persea borbonia), flowering dogwood (Cornus florida), wax myrtle (Myrica cerifera), wild olive (Osmanthus americanus), American beautyberry (Callicarpa americana), muscadine grape (Vitis rotundifolia), poison ivy (Toxicodendron radicans) and Smilax spp (Bellis, 1995). Considered globally imperiled ecosystems, maritime forests are a critical component to the overall health and stability of barrier islands providing habitat and foraging material for wildlife, stabilizing the soil and acting as a buffer against severe storms. These features also make

5 ! % maritime forests attractive for development and many have been partially or fully removed to make room for homes and buildings, an action likely to have long-term negative impacts on the health of such forests (Schafale and Weakley, 1990). Habitat fragmentation and the loss of biological and ecological functions as a result of development is one of the greatest challenges facing maritime forests (Bellis, 1995). A more recent threat to the health of maritime forests is the introduction of non-native, invasive species that have the potential to wreak havoc on entire ecosystems. Due in large part to the expansion of global trade, the unintentional introduction of harmful species has increased significantly in recent years (Liebhold et al. 1995). Pimentel et al. estimate an astonishing 50,000 non-native species now occur in the United States with some brought in intentionally, others by accident (2000). While not all introduced species will become established and some are even beneficial, a lack of natural predators outside their normal range allows exotics to rapidly overrun their native counterparts, resulting in the potential for catastrophic losses, both ecologically and economically (Pimental et al. 2000). Invasive species are considered one of the leading causes of biodiversity loss; 49% of imperiled species are in decline in large part due to non-native species, according to federal agencies and The Nature Conservancy (Wilcove et al. 1998). Two of the greatest threats to the health of forests in the United States are the introduction of insects and pathogens. Familiar examples include the chestnut blight fungus (Cryphonectria parasitica) and the hemlock woolly adelgid (Adelges tsugae), both infestations resulting in massive alterations in forests and species composition across the northeastern United States (Spiegel, 2005). In the past decade, a newly introduced species, the redbay ambrosia beetle (Xyleborus glabratus), has established itself across the coastal plain of the southeastern United States. The

6 ! & 2mm-long beetle vectors a non-native fungus (Raffaelea lauricola), resulting in widespread mortality of redbay (Persea borbonia) trees and other members of the Lauraceae family, including sassafras and avocado (Fraedrich et al. 2008). First discovered in 2002 near Georgia s largest shipping port in Savannah, it is assumed that X. glabratus, an ambrosia beetle native to Asia, arrived via cargo ships and wood packing material (Fraedrich et al. 2008). By 2003 and 2004, significant mortality events of redbay trees were being documented in the coastal regions of both Georgia and South Carolina, though the exact cause was not determined until late 2004 by which time eradication was deemed unfeasible (USDA, 2010). Inspection of the dead trees resulted in the discovery of several species of ambrosia beetles, some native, some exotic. One native ambrosia beetle, the black twig borer (Xylosandrus compactus) is known to cause damage to small diameter branches on redbay trees, mimicking the initial damage caused by X. glabratus; the twig borer however does not impact larger limbs or kill the entire tree as X. glabratus does (USDA, 2010). Like most ambrosia beetle species, adult X. glabratus bore into healthy trees, creating tunnels in which they lay their eggs. As part of a symbiotic relationship, the beetle carries a fungus in its mycangia that is released into the tree and serves as food for the developing larvae (Fraedrich et al. 2008). This fungus, R. lauricola, spreads through the vascular tissue of the tree, prohibiting the uptake of nutrients and water, resulting in the wilt appearance and eventual death of the tree, hence the name Laurel Wilt Disease (LWD) (Fraedrich et al. 2008). In an experiment carried out by Fraedrich et al., R. lauricola was found regularly inside the head of X. glabratus confirming their symbiotic relationship (2008). Further in their experiment, they found that when X. glabratus were exposed to redbay trees, 96% of the trees were bored into by the beetles, 70% of the trees died, and R. lauricola was present in 91% (Fraedrich, 2008). Results from these experiments confirmed that the wilting disease was indeed

7 ! ' caused by R. lauricola and vectored by X. glabratus. Once redbay is infected with R. lauricola, mortality occurs within 5 to 12 weeks (Fraedrich, 2008). Due to the relative newness of this disease, many questions remain regarding the infection and transmittal cycle. Ambrosia beetle appear to prefer smaller branches near the crown, though boring holes have been located on the main stems of trees as well (USDA, 2010). Healthy trees do not seem to attract large numbers of beetles initially but rather, as the disease spreads and the tree shows signs of wilt, the number of beetles increases substantially (USDA, 2010). Once leaving the infected tree, ambrosia beetles carry the fungus with them, passing it along to a potentially healthy tree (USDA, 2010). Symptoms of LWD are obvious in the late stages but less so initially, as many native species of ambrosia beetle can leave wilted branches on a number of trees (personal observation). Trees infected with LWD will show drooping foliage and discoloration of the leaves, typically evident first in the crown (USDA, 2010). In later stages, the foliage of the entire tree turns brown or gray, though leaves often remain on the tree upwards of a year or more following death and small tubes, referred to as toothpicks, or piles of sawdust may be seen emerging from the affected tree, an indication of the presence of redbay ambrosia beetle (USDA, 2010). A cross section of the trunk or areas where the bark has been removed will reveal a dark stain if the tree is fighting a fungal infection, another strong indication of the presence of the beetle (USDA, 2010). It is believed at this time that laurel wilt disease is restricted to species in the family Lauraceae (USDA, 2010). Native to the southeastern region of the United States (Figure 1), redbay is the most commonly impacted species, however LWD has been discovered in sassafras (Sassafras albidum), avocado (Persea americana), the federally endangered pondberry (Lindera melissafolia) and the federally threatened pondspice (Litsea aestivalis). Redbay is a common tree

8 ! ( found throughout the coastal plain from Virginia to Texas, especially in maritime forests from North Carolina to Florida (USDA, 2010). Ecologically, the redbay provides food for a variety of birds and mammals as well as serving as one of the only host trees for the Palamedes swallowtail butterfly larva (Papilio palamedes) (Georgia Forestry Commission, nd). Impacts on redbay trees were widespread across coastal Georgia and South Carolina, with all coastal counties in Georgia affected by 2006 and all but one in South Carolina by 2009 (Figure 2). Natural spread of the disease appears to be approximately 20 miles per year but this is believed to be enhanced through anthropogenic activities involving movement of firewood, shipping crates and wood chips (Johnson et al. nd). Currently, the disease has spread through much of Florida and west to Mississippi, confirming humans are likely accelerating the spread of LWD (USDA, 2010). As of March 2011, LWD was confirmed in at least one North Carolina county with indications of its presence in at least four other counties (D. Suiter, personal communication, April 18, 2011). Widespread mortality is likely to continue indefinitely, as there are no management options available at this time, other than educational efforts aimed at reducing the spread by humans (Johnson et al. nd). In anticipation of LWD arriving in North Carolina and specifically on Bald Head Island, a study was carried out to investigate what percent of the Bald Head Woods Reserve was covered by redbay. Bald Head Woods is one of the best remaining examples of maritime forest in the state and a baseline understanding of the redbay population within the reserve will prove critical when attempting to classify the impact of LWD on this community if and when it arrives.

9 ! ) Methods Study Site This project was conducted on Bald Head Island, a small barrier island off the southeastern coast of North Carolina, USA (Figure 3). Bald Head Island is part of the larger natural complex known as the Smith Island Complex, a total of approximately 12,000 acres of marsh and upland. The island is bordered by the Cape Fear River to the west, has extensive east and south-facing beaches and 10,000 acres of salt marsh to the north. The Bald Head Woods (Figure 4) consist of 191 acres of maritime forest, intercepted by two lanes of paved road used primarily by golf carts. The forest is divided into a south and north side, with a single narrow, low-impact trail running the length of each side that is restricted to pedestrian use. The Bald Head Woods canopy is dominated primarily by live oak (Quercus virginiana) and laurel oak (Quercus hemisphaerica) and to a lesser extent, sabal palmetto (Sabal palmetto) and red cedar (Juniperus virginiana) (personal observation). Species of significance in the understory include Carolina laurel cherry (Prunis caroliniana) yaupon holly (Ilex vomitoria), redbay (Persea borbonia), flowering dogwood (Cornus florida), wax myrtle (Myrica cerifera), wild olive (Osmanthus americanus), American hornbeam (Carpinus caroliniana) American beautyberry (Callicarpa americana), muscadine grape (Vitis rotundifolia), poison ivy (Toxicodendron radicans) and Smilax spp (Mayes, 1984). There are a number of mammals present in the woods including white-tailed deer (Odocoileus virginianus), red fox (Vulpes vulpes), grey fox (Urocyon cinereoargenteus), eastern gray squirrel (Sciurus carolinensis), raccoon (Procyon lotor) and the Virginia opossum (Didelphis virginiana) as well as a several rodent species.

10 ! *+! Data Collection Five transects were set up 200 meters apart, running east to west from the intersection of the east and middle trail to just east of the intersection of the loop and middle trail, ensuring that we remained within Reserve boundaries (Figure 4). Along each transect, I measured 10x10m 2 quadrats using a 50m soft measuring tape. I began my first quadrat at the base of the dune ridge on the south side of the woods, and measured every 50 meters along transect until I reached the salt marsh on the north side of the woods. Due to variability of the woods and boundary lines, quadrat numbers varied between 9 and 12 along each transect. Once each quadrat was established, a GPS reading was taken and recorded. Inside each quadrat, I first identified all redbay down to the species level, as both P. borbonia and P. palustris are possible in Bald Head Woods. Once the species were determined, I examined the percent cover of each species present in the quadrat by strict estimation of the total area covered by redbay foliage, and took general note as to the size and health of the trees. Since there was no P. palustris identified, all percent cover by redbay was attributed to P. borbonia. Results Percent cover of P. borbonia varied along each transect, from an average low of 6.4% on Transect 1 to a high of 16.4% on Transect 3 (Figure 5). A total of 52 quadrats were sampled and percent cover was estimated for both the south and north side (Tables 1-5). The south side of the woods averaged 13.8% cover over the course of 32 quadrats and the north side averaged 7.6% cover over 20 quadrats (Figure 6). Overall, the estimated percent cover of P. borbonia in the

11 ! **! Bald Head Woods Reserve is 11.4%. The vast majority of plants seen during this study were either sapling size, new shoots coming from adjacent dead redbay or small sprouts just above the ground. It should be noted however that several impressive specimen trees were found within our study site and these should be monitored closely for indication of LWD in the future. Many of the observed redbay specimens did have branches with dead leaves present, though entirely dead redbay trees were not observed at this time. Discussion Laurel wilt disease has spread rapidly across the southeastern United States, causing extensive mortality of redbay and other Lauraceae species since There are indications of negative economic and ecological impacts as well as changes to forest community structure. Results out of a study done in Georgia have found that LWD has a significant impact on redbay population and size structure, most noticeably on specimens!3cm DBH (Spiegel, 2008). When redbay shrub survival was compared with redbay tree survival in control vs. infected sites, Spiegel found a similar percentage of live shrubs in both the control and infected sites but reported a 37 times higher rate of live redbay trees in the control site, indicating the attack of LWD is dominant on larger class sizes (2008). Confirming this was Spiegel s finding that only 8% of redbay is living at the largest class size of 20-30cm diameter at ground height (DGH), in infested plots (2008). Although the beetles do favor larger specimens, redbay trees as small as 1-1.5cm DGH have been documented with LWD (2008). Redbay regeneration is evident across infected areas but it remains to be seen whether these sprouts will survive or succumb to further beetle infestation once they reach a large enough size (USDA, 2010). Alterations to soil moisture, nutrients, light

12 ! *"! exposure and other parameters in the absence of redbay should all be examined to further understand implications of tree loss. In Georgia, changes were seen in forest community structure within 2-4 years after LWD infestation due to gaps in the canopy as a result of extensive redbay mortality (Spiegel, 2008). Although redbay is only a little over 10% of the forest cover in the Bald Head Woods Reserve, indications from other studies point to potential changes in our community and long term studies should be conducted to determine what species replace redbay. Impacts on other redbay ambrosia beetle host species include the federally listed pondspice (Litsea aestivalis) and pondberry (Lindera melissafolia). To date there have only been a handful of occurrences of LWD on either of these listed species, however those with! inch diameter stems may be more susceptible (D. Suiter, personal communication, April 18, 2011). Other ecological impacts involve species that use redbay has a host plant. The Palamedes swallowtail butterfly (Papilio palamedes), a species found only in the coastal plain of the southeastern United States relies almost entirely on redbay as the host for its larvae and may face survival challenges should the redbay disappear across its range (H. Legrand via D. Suiter, personal communication, April 18, 2011). Redbay trees are also host to psyllid leaf gallers (Trioza magnoliae (Ashmead)) and the loss of redbays may have unknown consequences for this species (Legee, 2006). The fruits and foliage of redbays provide food and shelter for birds and mammals but there is no quantifiable data as of yet on what the impact will be on dependent species ( USDA, 2010). Economic threats are likely to have the greatest impact on the Florida commercial avocado industry, where substantial research is being conducted (USDA, 2010). Other economic impacts could include costs associated with the removal of dead trees, loss of nursery plants and

13 ! *#! subsequent revenue, decreased property values in areas of significant mortality and increase administrative costs necessary for a public education campaign (USDA, 2010). By 2005, eradication was already deemed unfeasible. Slowing down or eliminating LWD does not appear to be a viable option at this point as there are no proven silvicultural or arboricultural treatments (USDA, 2010). An attempt at removing infected trees was made in 2006 on Jekyll Island, Georgia, one of the first areas to see the effect of LWD. All trees with LWD symptoms were removed from private and natural areas, brought to a central location and burned, a total of over 400 trees. Despite the magnitude of this effort, the disease was widespread on the island by the end of 2007 (USDA, 2010). The most realistic option for infested sites may be to allow the disease to run its course and monitor for successful regeneration (USDA, 2010). In order to slow the spread to non-infected areas, options include limiting the movement and transport of infected materials outside of infected counties and ensuring that infected trees are disposed of locally rather than shipped elsewhere (USDA, 2010). The spread of LWD across the entire range of redbay appears inevitable based on the current rate of infestation. Baseline studies of redbay population and forest community structure would be beneficial to natural areas not yet impacted. Future monitoring of these sites could assess the extent of alterations on both forest structure and dependent species like the Palamedes swallowtail butterfly while providing the baseline for the forest to return.

14 ! *$! Figures and Tables Figure 1. Distribution of Persea borbonia in the United States. Figure 2. Range of LWD spread as of March 3, 2011.

15 ! *%! Figure 3. Map of Bald Head Island, North Carolina and the study site, Bald Head Woods. Figure 4. Map of study site, Bald Head Woods Reserve, with transects noted.

16 ! *&!!"#$%&#'(')*"#$'*+',-'.*$.*/0%' *(! *&! *$! *"! *+! (! &! $! "! +! *&,$! *$,#! *+,#! ),"! &,$! -./01234!*! -./01234!"! -./01234!#! -./01234!$! -./01234!%! 1$%/2#342'45$*6&5'7**829':%24'4*'7#24' Figure 5. Percent cover of P. borbonia along each transect. *&!,#$3#/4'3*"#$',-'.*$.*/0%' *$! *"! *+! (! &! $! "! +! 56748! 96.48! ;%<8'=#%8'7**829'>*645'"-'?*$45'>08#' Figure 6. Percent cover of P. borbonia in BHI Woods, south v. north side.

17 ! *'! Transect 1 Trail Side South South South South North North North North North North Quadrat % Cover GPS N W Accuracy 11 m 7 m 7m 7 m 7m 6m 6m 6m 6m 10m Table 1. Percent cover of P. borbonia along Transect 1. Transect 2 Trail Side South South South South South North North North North Quadrat % Cover GPS N W Accuracy 6m 6m 6m 6m 5m 12m 13m 9m 6m Table 2. Percent cover of P. borbonia along Transect 2. Transect 3 Trail Side South South South South South South South North North North North Quadrat % Cover GPS N W Accuracy 7m 6m 6m 6m 6m 6m 6m 6m 6m 6m 6m Table 3. Percent cover of P. borbonia along Transect 3.

18 ! *(! Transect 4 Trail Side South South South South South South South South South North North North Quadrat % Cover GPS N W Accuracy 6m 6m 6m 6m 7m 7m 7m 7m 7m 6m 5m 5m Table 4. Percent cover of P. borbonia along Transect 4. Transect 5 Trail Side South South South South South South South North North North Quadrat % Cover GPS N W Accuracy 5m 7m 5m 9m 5m 5m 6m 5m 7m 6m Table 5. Percent cover of P. borbonia along Transect 5.

19 ! *)! Works Cited Bellis, V.J. (1995). Ecology of maritime forests of the southern atlantic coast: a community profile. United States Department of the Interior. Washington D.C. 94 pp. Fraedrich, S.W., T.C. Harrington, R.J. Rabaglia, M.D. Ulyshen, A.E. Mayfield III, J.L. Hanula, J.M. Eickwort and D.R. Miller. (2008). A fungal symbiont of the redbay ambrosia beetle causes a lethal wilt in redbay and other Lauraceae in the southeastern USA. Plant Disease 92: Georgia Forestry Commission. Nd. Laurel wilt disease associated with redbay ambrosia beetle. Retrieved on April 20, 2011 from Johnson, J., L. Reid, B. Mayfield, D. Duerr, S. Fraedrich. New disease epidemic threatens redbay and other related species. South Carolina Forestry Commission Insects and Diseases. Retrieved April 20, 2011 from! Leege, L.M. (2006). The relationship between psyllid leaf galls and redbay (Persea borbonia) fitness traits in sun and shade. Plant Ecology 184: Liebhold, A.M., W.L. Macdonald, D. Bergdahl and V.C. Mastro. (1995). Invasion by exotic forest pests: a threat to forest ecosystems. Forest Science Monographs 30:1-58. Mayes, C.H. (1984). The flora of Smith Island, Brunswick County, North Carolina (Master s thesis). University of North Carolina at Wilmington, Wilmington, NC. Pimentel, D., L. Lach, R. Zuniga and D. Morrison. (2000). Environmental and economic costs of nonindigenous species in the United States. BioScience 50: Reid, L., J. Eickwort, J. Johnson, and J.J. Riggins. (2010). Distribution of counties with laurel wilt disease symptoms by year of initial detection. Schafale, M. P., and A. S. Weakley. (1990). Classification of the natural communities of North Carolina. North Carolina Natural Heritage Program, Division of Parks and Recreation. North Carolina Department of Environmental, Health, and Natural Resources. Raleigh. 325 pp. Spiegel, K.S. (2005). Impacts of laurel wilt disease on redbay (Persea borbonia) population structure and forest communities in the coastal plain of Georgia, USA (Master s Thesis, Georgia Southern University). Retrieved from s_201005_ms.pdf

20 ! "+! U.S. Department of Agriculture, Forest Service. (2010). Laurel wilt ecological concerns Atlanta, GA: Forest Health Protection, Southern Region. Retrieved from U.S. Department of Agriculture, Forest Service. (2010). Laurel wilt economic impact Atlanta, GA: Forest Health Protection, Southern Region. Retrieved from U.S. Department of Agriculture, Forest Service. (2010). Laurel wilt history Atlanta, GA: Forest Health Protection, Southern Region. Retrieved from U.S. Department of Agriculture, Forest Service. (2010). Laurel wilt management Atlanta, GA: Forest Health Protection, Southern Region. Retrieved from U.S. Department of Agriculture, Forest Service. (2010). Laurel wilt symptoms Atlanta, GA: Forest Health Protection, Southern Region. Retrieved from U.S. Department of Agriculture, Forest Service. (2010). Laurel wilt disease cycle Atlanta, GA: Forest Health Protection, Southern Region. Retrieved from U.S. Department of Agriculture, Forest Service. (2010). Laurel wilt plant susceptibility Atlanta, GA: Forest Health Protection, Southern Region. Retrieved from Wilcove, D.S., D. Rothstein, J. Dubow, A. Phillips and E. Losos. (1998). Quantifying threats to imperiled species in the United States. BioScience 48: