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Beech Bark Disease Reduces Sus scrofa (Boar) Rooting Intensity in Great Smoky Mountains National Park
Kileigh B. Welshofer and David B. Vandermast

Southeastern Naturalist, Volume 15, Issue 4 (2016): 669–680

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Southeastern Naturalist 669 K.B. Welshofer and D.B. Vandermast 22001166 SOUTHEASTERN NATURALIST 1V5o(4l.) :1656,9 N–6o8. 04 Beech Bark Disease Reduces Sus scrofa (Boar) Rooting Intensity in Great Smoky Mountains National Park Kileigh B. Welshofer1 and David B. Vandermast2,* Abstract - This study examines the effect of beech bark disease (BBD) on Sus scrofa (European Wild Boar) rooting in high-elevation Beech gaps of Great Smoky Mountains National Park. In 2011, we sampled vegetative cover by stratum (canopy, shrub, herb) and European Wild Boar rooting extent in pre-existing fenced boar-exclosure plots and corresponding unfenced plots. We also used data from previous studies to compare frequencies of individual herbaceous species collected pre-BBD to those collected post-BBD. Our results indicate that mortality of Fagus grandifolia (American Beech) trees due to BBD and the consequent growth of a dense shrub-layer significantly reduced boar rooting in gaps within the Beech stands. We found that herbs were affected by both European Wild Boar and the dense shrub-cover following American Beech mortality; however, some plant species remained abundant, possibly because they were protected from detection within the shrubs. Introduction Exotic organisms are species that have been introduced to an environment in which they do not naturally occur. Upon introduction to a new ecosystem, exotic species can become established and disrupt the ecological niches of the existing native organisms in the region (Atkinson 1996, Donlan and Wilcox 2008). Although the subject of invasive species has been widely studied throughout the last century, little is known about the interactions among 2 or more invasive species within the ecosystem they have invaded. Since its migration into Great Smoky Mountains National Park (GRSM) from a nearby hunting preserve during the 1940s, Sus scrofa L. (European Wild Boar, hereafter Boar) has been causing reductions in species richness and diversity by rooting through the forest understory (Bratton 1975, Jones 1959). Fagus grandifolia (American Beech, hereafter Beech) forests in high-elevation (>1524 m) forest stands are the preferred summer habitat for the omnivorous Boars, supplying them with an abundance of diverse herbaceous plants and other organisms for consumption. Foraging by ungulates, such as that conducted by Boars, is known to drastically change forest ecosystems (Russell et al. 2001). Rooting is known to cause damage to tree roots, and destroy seedlings and wildflowers, which reduces understory cover and diversity in GRSM (Bratton 1974) both through direct consumption and by uprooting of non-target plants in search of food. Beech bark disease (BBD) was first documented at GRSM in 1993, where this invasive insect–fungal-pathogen complex was identified as killing mature Beech 1Department of Environmental Studies, Elon University, Elon, NC 27244. 2Department of Biology, Elon University, Elon, NC 27244. Corresponding author - dvandermast@elon.edu. Manuscript Editor: John Riggins Southeastern Naturalist K.B. Welshofer and D.B. Vandermast 2016 Vol. 15, No.4 670 trees near Clingman’s Dome (Houston 1994). Following its discovery in 1993, BBD spread throughout eastern GRSM Beech forests, but had not, as of the time of data collection for this study, extended to the far-western Beech gaps. The combination of the Cryptococcus fagisuga Lindinger (Beech Scale Insect) and 1 of 2 fungal species, commonly Nectria coccinea var. faginata Desm. and less-commonly Nectria galligena (Bres.) Rossman & Samuels, has led to the death of as many as 90% of American Beech trees in some high-elevation forests. Mature Beech (in excess of 8 cm DBH) experience the greatest mortality—smaller trees are often not infested by the scale insect (Vandermast 2005). The disease is initiated with the establishment of the insect, and alteration of the Beech bark. The Nectria fungus is then able to infect the lateral meristematic tissue, eventually girdling the tree and dissociating the bark from the sapwood, a characteristic unique to BBD (Ehrlich 1934, Mahoney et al. 1999) that allows for accurate forensic determination of the cause of tree death. Beech bark disease has been shown to have complex effects on the vegetative community. It causes significant reduction in above-ground biomass and alters the dominance of Beech in the forest canopy (Forrester et al. 2003). In GRSM, Beech forests suffering a reduction in canopy coverage because of BBD are rapidly colonized by dense thickets—sometimes in excess of 30 stems/m2—of Beech sprouts and Rubus canadensis L. (Blackberry) (Vandermast 2005). Beech trees may also be affected by Boar rooting. Beech sprouts have been found to be significantly taller and more abundant when exposed to loosened soils and other rooting conditions (Howe et al. 1981). This change in composition and density of the understory may alter the availability of food resources and accessibility of Beech forests to Boars. Further, the lack of mature Beech canopy due to BBD has changed the vegetation of the forest floor (Morin and Liebhold 2015). In the Beech gaps of GRSM, these changes have led to a greater density of sapling-sized Beech trees and dense thickets of Blackberry canes (Vandermast 2005). In this study, we examined the (1) extent to which BBD-induced changes in forest structure affected Boar rooting, and (2) extent to which these changes impacted the frequency of specific plant species in the high-elevation Beech forests of GRSM. We hypothesized that death of the Beech canopy due to BBD reduced the extent of Boar rooting in high-elevation Beech forests in GRSM. Furthermore, we hypothesized that herbaceous species capable of existing in the dense thickets of Beech sprouts and Blackberry undergo a reprieve from Boar rooting and occur at higher frequencies than they would in forests with Boar rooting. Field-site Description High-elevation Beech gaps are a unique forest type dominated by Beech trees (relative abundance is >50%). They are typically located on southward-facing slopes in the gaps between mountain peaks (Whittaker 1956). Beech trees within the gaps are nearly genetically identical due to their clonal reproduction through root sprouts of canopy trees (Morris et al. 2004). Beech gaps are deciduous islands, often entirely surrounded by spruce–fir forests throughout the high elevations of the Southeastern Naturalist 671 K.B. Welshofer and D.B. Vandermast 2016 Vol. 15, No. 4 park (Russell 1953). Two dominant types of understory occur within the Beechgap ecosystem: an open and Carex spp.-dominated habitat with some shrubs, and one with a diverse, forb-dominated herbaceous layer that is enhanced by the dense shade provided by the canopy (Whittaker 1956). Boars appear to avoid rooting in Beech gaps where sedges are the dominant understory plants; therefore, in this study we only utilized plots in Beech forests where the understory was forb-dominated (Vandermast 2005). We sampled 20 pre-existing plots, including 10 Boar “exclosures” and 10 corresponding “control” (non-exclosure) plots (see Supplemental File 1, available online at http://www.eaglehill.us/SENAonline/suppl-files/s15-4-S2277-Vandermast-s1, and, for BioOne subscribers, at http://dx.doi.org/10.1656/S2277.s1). Exclosures consist of a chain-link fence up to 1.8 m (6 ft) high that protected part of the Beech gap from Boar rooting. The Boar exclosures and control plots were established in the 1980s by GRSM personnel specifically for the purpose of studying the effects of Boar rooting on Beech-gap flora. The exclosures vary in size from just over 100 m2 to >1 ha. In all cases, the areas sampled in the exclosures and the control plots were equal. Methods During the summer of 2011, we followed the Carolina Vegetation Survey (CVS; Peet et al. 1998) protocol to sample tree, shrub, and herb cover in 10 exclosure and 10 corresponding unfenced control plots. Trees tend to grow slowly and are often stunted in high-elevation Beech gaps. The tree canopy typically consists of Beech with a few co-occurring species and is usually 18–22 m tall. Shrub cover ranges from 2–2.5-m high and consists of Blackberry and tree saplings (mostly Beech). Herb cover is less than 1 m tall. The standard unit of observation in the CVS protocol is the 100-m2 module. We estimated the area affected by Boar rooting within each module. For purposes of consistency, we estimated Boar-rooting area in the same way as the vegetation cover. Boar rooting is identifiable from other forms of animal rooting in Beech forests because Boars overturn much larger patches of forest floor (often in excess of 100 m2), compared to Ursus americanus (Pallas) (American Black Bear) and Odocoileus virginiana Zimmermann (White-tailed Deer), which only scrape soil in distinct locations (usually less than 10 m2). It is not an exaggeration to say that Boar rooting resembles rototilling of the forest soil. We analyzed the relationship between BBD-caused Beech mortality and Blackberry cover in both exclosure and control plots because BBD killed Beech trees in both areas. We examined the relationship between Blackberry cover and the Boar rooting in control plots only because these were the only areas accessible to Boars. In both cases, we used correlation analysis to examine these relationships. Furthermore, we separated Beech forests into those with high levels of Beech mortality (n = 12) and those with low levels of Beech-mortality (n = 8). High-mortality plots contained many dead Beech trees with the characteristic mortality pattern caused by BBD. Low-mortality sites were those where BBD had not yet begun to kill Southeastern Naturalist K.B. Welshofer and D.B. Vandermast 2016 Vol. 15, No.4 672 Beech trees, and mortality in these plots appeared to be representative of normal background tree-mortality rates. We compared Blackberry cover and Boar rooting between high- and low-mortality plots using a two-sample t-test (a = 0.05) in SigmaPlot v.9.0 (Systat Software, Inc., San Jose, CA). We did not collect data on individual herbaceous species. Thus, in order to examine changes in individual species over time, we used data collected by Russell in 1953 (prior to the introduction of both BBD and Boars to the Beech gaps he studied) to compare with data collected by Kay and Vandermast 2009, which was the most recently collected usable data. Also, because Russell’s paper only reported frequency data for herbs, we converted the 2008 species cover data to frequency values for comparison purposes. The 2008 data were collected using the CVS protocol in the same exclosure and control plots used in our 2011 study. Thus, we were able to compare frequencies of plant species in Beech gaps from pre-exposure (Russell 1953 data) to those exposed only to BBD (2008 exclosure data), and those exposed to both rooting and BBD (2008 unfenced-control data). Bratton’s (1974) paper predicted that boar rooting would harm certain herbaceous species in GRSM Beech gaps. To use Bratton’s species list to compare Russell data with our 2008 data, we categorized susceptible species as follows: Aster spp. (asters), Carex spp. (sedges), Lilium superbum L. (Turks-cap Lily), Monarda didyma L. (Bee Balm), and Poa alsodes A. Gray (Grove Bluegrass), Rudbeckia laciniata L. (Cutleaf Coneflower), Trillium erectum L. (Wake Robin), and Viola spp. (violets). In addition, there were plant species documented by Russell that were not predicted by Bratton to be threatened by Boar rooting. Those species include Angelica triquinata Michx. (Filmy Angelica), Cuscuta sp. (dodder), Houstonia serpyllifolia Michx. (Thymeleaf Bluet), Oxalis montana Raf. (Mountain Woodsorrel), and Blackberry. Results Across the Beech gaps in this study, we found a negative correlation (r = -0.522, r2 = 0.272) between canopy cover and shrub cover, confirming previous observations that shrub cover increases with the death of the Beech canopy (Fig. 1). There is much variation in our data, but it appears that when canopy cover is less than 60%, Blackberry cover increases dramatically in most plots. Beech forests with >60% canopy cover had low Blackberry cover. Furthermore, we found that there was a strong, negative correlation between the area affected by Boar rooting and shrub-cover (r = -0.925, and r2 = 0.856; Fig. 2). In forests where Blackberry cover was >70%, the area affected by Boar rooting declined to almost nothing. We noted that game trails were evident in the dense Blackberry thickets, and Boars likely used these trails. The area of Beech forests affected by Boar rooting was significantly higher at sites with low levels of Beech mortality (32.7 ± 5.23%) than in those with high levels of Beech mortality (1.6 ± 0.5%) (2-sample t-test: P < 0.01, df = 9; Fig. 3). Shrub-cover was high (83.3 ± 11.5%) in plots with high mortality (tree cover = 16.3 ± 14.2%) but low (16.3 ± 12.9%) in plots experiencing low mortality (tree cover = Southeastern Naturalist 673 K.B. Welshofer and D.B. Vandermast 2016 Vol. 15, No. 4 64.7 ± 10.6%). We found no statistical evidence for a difference in average percentage of herb-cover in high-mortality vs. low-mortality plots (2-sample t-test: P = 0.69, df = 19). However, it is likely that different species of herbs were present in plots with low vs. high levels of Beech mortality. Figure 2. Correlation between shrub cover and Boar rooting in unfenced control plots in high-elevation Beech gaps in Great Smoky Mountains National Park. Figure 1. Correlation between shrub cover and American Beech canopy cover in highelevation Beech gaps in Great Smoky Mountains National Park. Southeastern Naturalist K.B. Welshofer and D.B. Vandermast 2016 Vol. 15, No.4 674 All of the species Bratton (1974) predicted to be susceptible to Boar rooting occurred at lower frequencies in the 2008 data (post-BBD) than when observed pre-BBD by Russell. Of particular interest, Turk’s-cap lily, which occurred at a frequency of nearly 40% in the Russell data, was almost non-existent in the 2008 data (Fig. 4). Also, for all species except asters and sedges, the frequencies were higher in the unfenced control plots where these plants were subject to the effects of both Boar rooting and BBD than in the plots where Boars were excluded. Of the additional herbaceous species, Blackberry and dodder occurred more frequently in 2008 than in the pre-BBD data (Fig. 5). Mountain Woodsorrel, which Figure 4. Frequencies of herbaceous species identified by Bratton (1974) as being susceptible to Boar rooting. The pre-BBD frequencies are from Russell (1953), the Rooting/BBD data are from 2008 unfenced controls where plants were subject to both rooting and BBD, and BBD only data are from Boar exclosures where plants were subject only to BBD. Figure 3. Average percent of plot area affected by Boar rooting in Beech forests with high levels of Beech mortality due to BBD (32.7%) vs. low levels of Beech mortality (1.6%) (P < 0.01). Plots with high mortality had numerous trees exhibiting the characteristic mortality pattern caused by BBD. Plots with low mortality were in areas that BBD had not yet reached. The low mortality represents a background mortality rate. Southeastern Naturalist 675 K.B. Welshofer and D.B. Vandermast 2016 Vol. 15, No. 4 occurred at a frequency of almost 40% in the 1953 data, was not observed in 2008 and Thymeleaf Bluet was observed far more frequently in unfenced plots than in exclosures. Our analysis of tree species identified both by Russell (1953) and in 2008 indicates that all tree species occurred at reduced frequencies in the later data set (Table 1). This finding could be due to a combination of BBD and Boar rooting, but high-elevation forests in GRSM are subject to many modern stressors including excess nitrogen input and ozone damage. Four species commonly found in Beech forests in 2008 were not listed by Russell in his study: Acer saccharum (Sugar Table 1. Frequencies (% of modules in which a tree occurred) of tree species in 1953 and 2008. In 2008, trees in control plots were subject to both BBD and Boar rooting and trees in exclosures were subject to BBD only. 1953 Frequency 2008 Frequency Tree Species Russell Total Control Exclosure Acer pensylvanicum L. (Striped Maple) 29 8.3 11.8 3.8 Abies fraseri (Pursh) Poir. (Frasier Fir) 36 13.3 11.8 15.4 Acer rubrum L. (Red Maple) 21 3.3 0.0 7.7 Amelanchier laevis Wiegand (Smooth Shadbush) 43 6.7 0.0 15.4 Betula alleghaniensis Britton (Yellow Birch) 50 38.3 38.2 38.5 Fagus grandifolia Ehrh. (American Beech) 100 70.0 79.4 57.7 Picea rubens Sarg. (Red Spruce) 86 23.3 35.3 7.7 Prunus serotina Ehrh. (Black Cherry) 29 13.3 8.8 19.2 Acer saccharum Marsh. (Sugar Maple) 38.3 38.2 38.5 Acer spicatum Lam. (Striped Maple) 23.3 5.9 46.2 Aesculus flava Sol. (Yellow Buckeye) 26.7 29.4 23.1 Quercus rubra L. (Red Oak) 16.7 8.8 26.9 Figure 5. Frequencies of herbaceous species that Bratton (1974) did not predict as being susceptible to Boar rooting. These are species that displayed a difference in the frequency observed between pre-BBD and Boar introduction in 1953 and post-BBD and Boar introduction in 2008. Southeastern Naturalist K.B. Welshofer and D.B. Vandermast 2016 Vol. 15, No.4 676 Maple), A. spicatum (Mountain Maple), Aesculus flava (Buckeye), and Quercus rubra (Red Oak). Of these 4 species, 2 occurred at higher frequencies in exclosures than in control plots; Mountain Maple occurred in 46.2% of exclosures and 5.9% of controls, and Red Oak occurred in 26.9% of exclosures and 8.8% of controls. This observation suggests these species are susceptible to Boar rooting or other disturbances prevented by the exclosure fencing. Discussion In this study, we examined how 1 invasive species, BBD, could alter the use of high-elevation Beech forests in GRSM by another invasive species, the European Wild Boar. Our results indicate that the increase in Blackberry cover following canopy Beech death from BBD reduced the amount of Boar rooting in these forests. Boar rooting was most likely reduced because the dense blackberry growth inhibited access and reduced resources for the Boars. Dense Blackberry growth following deforestation or death of canopy trees is not unique to the high-elevation Beech forests of GRSM. Rapid Blackberry growth in southern Appalachian clear-cuts has been associated with slow growth of shadeintolerant species during secondary succession (Wilson and Shure 1993). Blackberry has long been an important colonizer of high-elevation forest gaps in GRSM (Crandell 1958), but its rate of colonization, density, persistence, and consequent capacity to inhibit forest reorganization on these sites may be enhanced by atmospheric-N deposition (Tilman 1987). Atmospheric-N deposition is highest at high elevations in GRSM (Fowler et al. 1988), and N-mineralization is highest in high-elevation hardwood forests when rates are compared among different plant communities along an elevational gradient (Knoepp and Swank 1998). At densities of 30 stems/m2 (Vandermast 2005), Blackberry inhibits the access Boars would normally have to these forests and likely decreases food availability for them. As a corollary, our data also showed that Boar rooting was greatly reduced in Beech forests with high mortality versus those free of BBD with low mortality. While our data show that the dense shrub layer is likely to exclude Boars from accessing these areas, it is also possible that the lack of beech nuts once produced by mature beech trees prior to BBD could also decrease the likelihood of Boars foraging in these areas. BBD has complex effects on the vegetative communities where it occurs. The disease causes a significant reduction in above-ground biomass and decreases the dominance of Beech in the forest canopy (Forrester et al. 2003). Results from a previous study suggested that, although tree species such as Picea rubens (Red Spruce) and Sugar Maple become more abundant in diseased Beech forests, Beech sprouts remain the most abundant regenerating species. In a healthy Beech gap ecosystem, the shade provided by the canopy suppresses the germination and growth of Blackberry, leaving room for the regrowth of Beech trees (Vandermast 2005). When BBD kills the mature Beech trees, the canopy no longer provides shade for the forest understory. The additional sunlight reaching the forest floor changes the microclimate of the area and leads to changes in vegetative composition (Gehlhausen et al. 2000), including rapid Blackberry growth. Southeastern Naturalist 677 K.B. Welshofer and D.B. Vandermast 2016 Vol. 15, No. 4 Shrubs can consume much of the forest’s space and nutrients; thus, understory plants normally found in a healthy Beech forest may not be able to acquire essential resources for survival (i.e., sunlight, soil nutrients). A study in northern California found that the development of a dense shrub layer affected herbaceous species abundance of native grasslands. The study showed a decline in many herbaceous species after the invasion of Baccharis pilularis DC. (Coyote Brush). However, some shade-tolerant species may benefit from the protection from predators provided by the shrub layer (Hobbs and Mooney 1986). In our study, some herbaceous species (trilliums and violets, in particular) in Beech gaps had higher frequencies in our control plots, where Boars had access. These herbs may have survived Boar rooting, or become reestablished after it, and can grow in and among the Blackberry canes where they are protected from rooting. Bratton’s predictions that certain herbaceous species would be susceptible to Boar rooting were borne out by the results of our analysis. Our data indicated that, while all herbaceous species survived in Beech gaps, all of Bratton’s susceptible groups occurred at lower frequencies in the 2008 data than they did in the 1953 data. Interestingly, within the 2008 data, some of these species occurred at higher frequencies in areas where they were susceptible to Boar rooting than they did in the exclosures. We propose that rooting in the control plots may have increased the frequency of these species compared to the exclosures for several reasons. First, competition from other plant species because of the lack of rooting within the exclosures could exclude these species from growing within the plots. Second, the upturned soils resulting from rooting could provide better habitat for seedling germination than a forest floor with thick leaf litter. Rooting has been shown to enhance plant species richness in wetland habitats (Arrington et al. 1999). Seed dispersal may also increase with Boar rooting. A study in Germany found that Boar rooting increased the dispersal distance of species across forested and open ecosystems (Heinken et al. 2005). However, the difference in effects of BBD and Boar rooting on the herbaceous community of Beech gaps is difficult to distinguish because of the ubiquitous nature of the disease. Additionally, the exclusion of all foraging species within the exclosure plots may also affect the presence of herbaceous species. Some species recorded by Russell in 1953 that were not considered by Bratton as susceptible to rooting declined in frequency while others increased. Two species found more commonly in 2009 than in 1953 were Blackberry and dodder. The increase in Blackberry frequency is due to the mortality of canopy Beech trees. Dodder regularly parasitizes Blackberry in the southern Appalachians (Radford 1968) and would be expected to increase in frequency proportional to the increase in Blackberry frequency (Musselman 1986). It is possible that some of the differences in frequencies noted in this study compared to present studies could be due to the differences in sampling seasons (Bratton and Russell in early-mid summer and 2008 data collected in early August). Some herbs that emerge in the spring may have completed the above-ground part of their lifecycle by early August. If this is the case, the 2008 data may have underestimated Southeastern Naturalist K.B. Welshofer and D.B. Vandermast 2016 Vol. 15, No.4 678 frequency. However, given that all species except Mountain Woodsorrel were observable in early August, it is unlikely that this difference is meaningful. Our results suggest that the effects BBD on the structure of Beech gaps could limit Boar rooting in Beech gaps and possibly provide a temporary hiatus from rooting for species capable of surviving in the Beech sprout and Blackberry thickets that develop following the death of the canopy Beech trees. Restoration of the Beech gaps to their pre-BBD structure and composition seems unlikely. The reduced rooting of Boars because of the dense shrub layer is not a permanent solution to protecting existing plant species in Beech gaps. As noted, many species continue to decline with the presence of both the Boar and BBD in Beech forests. While it is certainly possible that excess nitrogen-deposition, warming temperatures, and ozone alter plant populations in these forests, we found that Boar rooting and the dense growth of Blackberry change the frequencies of some species. Beech saplings and other trees will eventually grow tall enough to shade the Blackberry thickets, which will become less dense and even disappear from the forest. This transition will return the forest to an approximation of its pre-BBD structure, but it also will provide enhanced conditions for Boar rooting. The control of either invasive species (BBD or Boar) is extremely difficult. 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