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Impacts of Beaver Foraging on the Federally Threatened Virginia Spiraea (Spiraea virginiana) along the Cheoah River, NC
C. Reed Rossell, Jr., Kathryn Selm, H. David Clarke, Jonathan L. Horton, Jennifer Rhode Ward, and Steven C. Patch

Southeastern Naturalist, Volume 12, Issue 2 (2013): 439–447

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2013 SOUTHEASTERN NATURALIST 12(2):439–447 Impacts of Beaver Foraging on the Federally Threatened Virginia Spiraea (Spiraea virginiana) along the Cheoah River, NC C. Reed Rossell, Jr.1,*, Kathryn Selm2, H. David Clarke2, Jonathan L. Horton2, Jennifer Rhode Ward2, and Steven C. Patch3 Abstract - Spiraea virginiana (Virginia Spiraea) is a federally threatened shrub endemic to the southern Blue Ridge and Appalachian Plateau physiographic provinces. Observations along the Cheoah River, where the largest population of S. virginiana occurs in North Carolina, indicate that Castor canadensis (Beaver) feed on S. virginiana. However, the effects of Beaver foraging on this imperiled shrub are unknown. To address this issue, we randomly located 50 belt transects (25 m x 2.5 m) along the center of the scour zone of the Cheoah River and assessed all basal stems (less than 2.5 cm in diameter) of woody plants for evidence of browsing by Beaver. We recorded a total of 4963 basal stems, consisting of 59 species. Of those stems, 14.7% were browsed by Beaver. Spiraea virginiana accounted for 3.5% of the total basal stems and ranked fourth in the total number of stems browsed with 8.8%. After adjusting for transect effect, S. virginiana ranked second as a preferred forage species, with 32.0% of its stems browsed. This study suggests that overall browse levels of this community are relatively low compared to other communities inhabited by Beaver because of the high-gradient and turbulent nature of the Cheoah River. Our findings provide strong evidence that S. virginiana is an important food to Beaver, and as a consequence, Beaver foraging has the potential to affect this population. Although more information is needed to determine the long-term impacts, we suspect that in the short-term, S. virginiana may benefit from the levels of browsing found in this study. Field observations suggest that Beaver foraging stimulates asexual reproduction of S. virginiana by inducing rhizomatous growth. Beaver also may help S. virginiana with dispersal, as fresh cuttings are often left at feeding sites and then may be transported downstream for rooting. Introduction Spiraea virginiana Britton (Virginia Spiraea) is a rare shrub endemic to the southern Blue Ridge and Appalachian Plateau physiographic provinces (Ogle 1991a). This species has a limited range and is known to occur in only seven states: Georgia, Kentucky, North Carolina, Ohio, Tennessee, Virginia, and West Virginia (USFWS 1992). It is currently listed as federally threatened and has a global ranking of G2, or globally imperiled (Buchanan and Finnegan 2010). In North Carolina, S. virginiana is listed as endangered (Buchanan and Finnegan 2010), with only nine known populations in the state (C. Wells, USFWS, Asheville, NC, 2010 pers. comm.). Threats to this species include genetic isolation of 1Department of Environmental Studies, University of North Carolina at Asheville, Asheville, NC 28804. 2Department of Biology, University of North Carolina at Asheville, Asheville, NC 28804. 3Department of Mathematics, University of North Carolina at Asheville, Asheville, NC 28804. *Corresponding author - crrossell@aol.com. 440 Southeastern Naturalist Vol. 12, No. 2 small populations, lack of sexual reproduction and dispersal, competition from arboreal and invasive species, and loss of habitat from activities such as damming and road building (Ogle 1991b, USFWS 1992). Spiraea virginiana is a disturbance-dependent species that relies almost exclusively on asexual reproduction through vegetative propagation (Anders and Murrell 2001, Brzyski and Culley 2011, Ogle 1991b). It thrives in scoured areas along high-gradient sections of second- and third-order streams and rivers, where periodic disturbance from flooding maintains its niche by reducing competition from arboreal plants and fast-growing herbs and vines (Anders and Murrell 2001, Ogle 1991b, USFWS 1992). Rooting occurs from decumbent stems, rhizomatous growth, or downstream dispersal of root stocks (Anders and Murrell 2001, Ogle 1991b). Spiraea virginiana is most often found anchored by rhizomes in rock crevices along stream banks, or in areas of loose deposition such as rocky bars and shores (Ogle 1991b, USFWS 1992). As part of a larger study investigating the ecology of S. virginiana along the Cheoah River in North Carolina, where the largest population of S. virginana occurs in the state (C. Wells, 2010 pers. comm.), we observed that Castor canadensis Kuhl (Beaver) commonly feed on S. virginiana. Because this is the first observation of its kind, nothing is known about the effects Beaver have on this rare species. Beaver often exhibit strong preferences for forage species (e.g., Belovsky 1984, Busher 1996, Jenkins 1975, Svendsen 1980), and as a consequence can modify the species composition and structure of plant communities (Raffel et al. 2009, Rosell et al. 2005). Knowledge of forage preferences can help provide insight to the impacts Beaver have on plant communities (Raffel et al. 2009). However, forage preferences can vary widely depending on a variety of factors including geographic location, topography, and associated hydrology of a plant community (Rosell et al. 2005). Numerous studies (e.g., Belovsky 1984, Busher 1996, Brzyski and Schulte 2009, Raffel et al. 2009) have examined the foraging preferences of Beaver throughout North America. However, we are unaware of any studies that have been conducted in alluvial forests of the Appalachian Mountains. The intent of this study was to determine the level of Beaver foraging and the preference rank of S. virginiana in comparison with the most abundant woody species in the riparian shrub community along the Cheoah River. Methods Study area The study was conducted along a 9.2 km reach of the Cheoah River in Graham County, NC (elevation range: 386–524 m; 35°24'N, 83°53'W). This stretch of river is part of the Cheoah River Floodplain Significant Natural Heritage Area, which is designated “nationally significant” by the North Carolina Natural Heritage Program (NCNHP) because of numerous rare plant and animal species (Schwartzman 2012). The first documented occurrence of S. virginana along the Cheoah River was in 1940 by H.R. Totten (NCNHP, Raleigh, NC, unpubl. data). In that same year, this population was considered probably extirpated because of 2013 C.R. Rossell, K. Selm, H.D. Clarke, J.L. Horton, J. Rhode Ward, and S.C. Patch 441 road building (USFWS 1992), and this status was again verified in 1986 (Ogle 1991a). In 2000, S. virginiana was rediscovered along the Cheoah River, and currently, this population is considered the largest in North Carolina (C. Wells, 2010 pers. comm.). The Cheoah River is a dam-controlled, high-gradient system, with large boulder and pool habitats interspersed with Class III and Class IV rapids (NCDoC 2007). From 1928 to 2005, the Alcoa Power Generating Corporation maintained only minimal flow rates for the production of hydroelectric power (Dilts et al. 2005). In 2005, Alcoa established a more natural flow regime to restore the river’s ecological communities (Dilts et al. 2005). Current flows include monthly base flows of 1.1–2.8 m3s-1 (40–100 cfs) and approximately 20 annual high-flow events that peak around 28 m3s-1 (1000 cfs; FERC 2005). The study area receives an average annual precipitation of 167.7 cm, and has an average annual high temperature of 20.3 °C and an average annual low temperature of 6 °C (National Climatic Data Center 2011). The riparian corridor where our study took place is narrow and bordered by US Highway 129. It includes 19 subpopulations of S. virginana, varying in size from one small plant to plants covering more than 20 m2. Subpopulations are defined as a grouping of plants, regardless of the number of genets (Brzyski and Culley 2011). The corridor contains a small alluvial floodplain, with a scour zone extending several meters up steep banks that are dominated by boulders and riprap. The plant community is second-growth, Montane Alluvial Forest (Schafale and Weakley 1990), with a partially open canopy dominated by Liriodendron tulipifera L. (Tulip Poplar), Acer rubrum L. (Red Maple), and Platanus occidentalis L. (American Sycamore). The shrub layer is sparse to moderately dense and dominated by various species including Cornus amomum Mill. (Silky Dogwood), Alnus serrulata (Aiton) Willd. (Tag Alder), and Xanthorhiza simplicissima Marsh. (Yellow Root). The herb layer is sparse and includes Osmunda regalis L. (Royal Fern), Polystichum acrostichoides (Michx.) Schott (Christmas Fern), and Lespedeza cuneata (Dum. Cours.) G. Don (Sericea Lespedeza). Vines are prevalent and include Pueraria lobata Willd. (Kudzu), Toxicodendron radicans (L.) Kuntze (Poison Ivy), and Celastrus orbiculatus Thunb. (Oriental Bittersweet). No data are available on the Beaver population in the Cheoah River system. Beaver were considered extirpated from North Carolina by 1897 and reintroduced to the western portion of the state in 1956 (NCWRC 2011). Observations suggest that the number of Beaver present during our study was relatively small, as only one lodge and one bank den were found. Data collection We randomly placed fifty belt transects along the center of the scour zone to quantify Beaver foraging. A random numbers table was used to generate a downstream location for each transect. Transects were 25 m x 2.5 m and covered approximately 14% of the corridor where S. virginiana occurs. We located transects in areas where the slope did not exceed 15%, because observations indicated that Beaver do not typically forage on the steep, rocky banks of the 442 Southeastern Naturalist Vol. 12, No. 2 Cheoah River. Within each transect, we counted all basal stems of woody plants less than 2.5 cm diameter, identified them to species, and assessed them for evidence of browsing. Stems browsed by Beaver were recognized by sharply angled cuts that usually included incisor marks (Lesica and Miles 2004). Data analysis We summed data across transects for each species, and calculated three descriptive statistics: relative abundance (RA) = number of basal stems of a species in all transects / number of basal stems of all species in all transects x 100, relative use (RU) = number of basal stems of a species browsed in all transects / number of basal stems of all species browsed in all transects x 100, and proportion browsed (PB) = number of basal stems of a species browsed in all transects / number of basal stems of a species available in all transects x 100. We initially used two generalized linear mixed models fit to the data. The response variable for each of the models was the expected proportion of stems browsed for the ten most abundant species. The models used a binomial distribution with a logit link function. Each of the models included transect as a random effect to account for potential autocorrelation of browsing patterns within transects and to avoid pseudo-replication. One model used stem density and species as fixed effects, while the other model used only species as a fixed ef fect. The equation for the models was of the form: log(p / [1 – p]) = Intercept + Transect Effect + Fixed Effects + Random Error. Because the model with the two fixed effects indicated that stem density was not significantly related to browsing (P = 0.12), we used the model with species as the only fixed effect for all subsequent analyses. We estimated the proportion browsed of each species by reverse transforming the Species Effect estimate using the equation: Estimated Proportion Browsed = exp(Species Effect estimate) / [1 + exp(Species Effect estimate)]. We then determined forage preference rank of each species by comparing the estimated proportion browsed of S. virginina to the estimated proportions browsed of the ten most abundant species using least squares means with the Dunnett-Hsu adjustment for multiple comparisons (SAS 2010). We used SAS version 9.2 for all statistical analyses and considered results significant at alpha = 0.05. Results We recorded a total of 4963 basal stems consisting of 59 species. Of those stems, 14.7% were browsed by Beaver. Spiraea virginiana composed 3.5% of the total basal stems, ranking ninth in relative abundance. Spiraea virginiana also accounted for 8.8% of the total stems browsed, ranking fourth in relative use (Table 1). In addition, 35.6% of S. virginiana stems were browsed, ranking twelfth among all species (Table 1). Relative amounts of Beaver foraging differed among the ten most abundant species (P < 0.0001), and varied by transect (estimated variance due to transect effect = 1.05, standard error of estimated variance = 0.28). After adjusting for transect effect using the linear model, S. virginiana ranked second after Robinia pseudoacacia L. (Black Locust) as the most-preferred forage species, although it 2013 C.R. Rossell, K. Selm, H.D. Clarke, J.L. Horton, J. Rhode Ward, and S.C. Patch 443 was not significantly different from Carpinus carolinana Walter (American Hornbeam) (Table 1). Other preferred species included A. serrulata and C. amomum. Least-preferred species included Leucothoe fontanesiana Don (Doghobble), Hydrangea arborescens L. (Wild Hydrangea), and X. simplicissima (Table 1). Discussion The woody plant composition in the study area is typical of other Montane Alluvial Forests in North Carolina, with the exception of S. virginiana (Schwartzman 2012). The finding that S. virginiana was among the ten most abundant species was unexpected, and suggests that a disproportionate number of transects may have been located in areas with S. virginiana. The finding also demonstrates the widespread occurrence of this rare species in the study area and reflects the vigor of the population. The southern Appalachian Mountains are known for their high diversity of woody plants compared to other regions of North America (Whitaker 1956). This diversity is reflected in the high species richness (59 species) in the study area, and is particularly evident compared to other communities inhabited by Beaver. For example, Chabreck (1958) reported 38 woody species around bayous in Louisiana, Bartel et al. (2010) reported 35 species in four wetland communities in the Sandhills of North Carolina, Voelker and Dooley (2008) reported up to 15 species in hardwood forests around Beaver impoundments in southeastern Ohio, and Belovsky (1984) reported 14 species in riparian forests around ponds at Isle Royale National Park in Michigan. Table 1. Beaver forage preferences of the ten most-abundant woody species in a riparian shrub community along the Cheoah River, Graham County, NC, summer 2010. Preference Species RAA RUA PBA APBB PC RankD Cornus amomum 19.2 22.7 16.9 16.9 0.0001 5 Alnus serrulata 13.5 20.5 21.8 14.8 0.002 4 Xanthorhiza simplicissima 9.8 1.3 1.9 1.9 Less than 0.0001 8 Viburnum dentatum 5.6 3.4 8.5 6.5 Less than 0.0001 6 Leucothoe fontanesiana 4.2 0.1 0.5 0.5 0.0002 10 Itea virginica 4.1 1.7 5.9 4.2 Less than 0.0001 7 Carpinus caroliniana 3.7 5.2 20.3 22.4 0.57 3E Robinia pseudoacacia 3.6 19.2 77.0 73.0 less than0.0001 1 Spiraea virginiana 3.5 8.8 35.6 32.0 N/A 2 Hydrangea arborescens 3.4 0.4 1.8 1.2 less than 0.0001 9 ARelative abundance (RA), relative use (RU), proportion browsed (PB); expressed as a percentage of 4963 basal stems from 59 species. BAdjusted proportion browsed (APB) was estimated by reverse transforming the fixed effect (species) of the mixed logistic regression model mixed (see Methods for details) using the equation: APB = exp(Species Effect estimate) / [1 + exp(Species Effect estimate)]. CP-value for the test of the difference between APB of S. virginiana and the other species, using the Dunnett-Hsu adjustment for multiple comparisons. DPreference rank based on APB. EPreference rank of C. caroliniana was not statistically different from S. virginiana (P < 0.05). 444 Southeastern Naturalist Vol. 12, No. 2 In this study, 14.7% of woody plants were browsed by Beaver. In comparison, Northcott (1971) reported 73–76% of woody plants browsed around Beaver colonies in Newfoundland, and Voelker and Dooley (2008) reported 55% and 36% of woody plants browsed around active and abandoned Beaver impoundments, respectively. These differences suggest that the overall extent of browsing at our study site may be less than other communities inhabited by Beaver because of the high-gradient and turbulent nature of the Cheoah River. High-gradient streams reportedly support smaller numbers of Beaver than low-gradient streams (Howard and Larson 1985, Slough and Sadlier 1977). Therefore, it would be expected that overall browse levels should be less along high-gradient streams than lowgradient streams because of fewer Beaver. The finding that the estimated variance of the transect effect was almost four times the standard error suggests that certain attributes differentially impact the suitability of an area for feeding. This finding is supported by others who have reported that variables such as proximity to a den (Fryxell and Doucet 1993, Jenkins 1980, McGinley and Whitham 1985), stream width and gradient (Beier and Barrett 1987, Curtis and Jensen 2004, Howard and Larson 1985), and bank steepness and floodplain width (MacCracken and Lebovitz 2005) can affect local foraging activities. We are uncertain what factors at our study site influenced Beaver foraging, as no habitat data in and around transects were collected. Virtually no information is available on the wildlife value of Spiraea spp. We found only one published account of wildlife feeding on Spiraea spp. Martin et al. (1951) reported that Spiraea spp. composed 2–5% of the diet of Odocoileus virginianus Zimmerman (White-tailed Deer) on Mount Desert Island, ME. In our study, S. virginiana was an important food to Beaver, as indicated by the high relative use and large proportion of stems browsed. Beaver are known to be selective foragers (e.g., Chabreck 1958, Jenkins 1975, Raffel et al. 2009, Shadle et al. 1943, Svendson 1980), as evidenced by the significant species effect in our study. The preference ranks in our study agree with other studies representing the same species, with the exception of S. virginiana (Northcott 1971, Jenkins 1975, Raffel et al. 2009, Shadle et al. 1943, Svendson 1980, Voelker and Dooley 2008). Beaver preference is thought to be related to the digestability and nutritional quality of a species (Doucet and Fryxell 1993, Fryxell 2001) and to the type and amount of secondary metabolites they contain (Bryant et al. 1991). The finding that S. virginiana ranked second with C. carolinana as a preferred forage species suggests that S. virginiana is highly nutritious, although no data are currently available for confirm ation. Conclusions Our study suggests that overall browse levels in this shrub community are relatively low compared to other communities inhabited by Beaver, likely due to the high-gradient and turbulent nature of the site. In addition, the findings provide strong evidence that S. virginiana is a preferred food of Beaver, and as a consequence, Beaver foraging is affecting this shrub population. Although more 2013 C.R. Rossell, K. Selm, H.D. Clarke, J.L. Horton, J. Rhode Ward, and S.C. Patch 445 information is needed to determine the long-term impacts, we suspect that in the short-term, S. virginiana may benefit from the levels of browsing found in this study. Field observations suggest that Beaver foraging stimulates asexual reproduction, by inducing rhizomatous growth, and increases the size and vigor of clones. A similar response to Beaver foraging also has been reported for Populus fremontii Watson (Fremont Cottonwood; McGinley and Whitman 1985). Beaver foraging also may aid S. virginiana dispersal, as Beaver often leave fresh cuttings of S. virginiana at feeding sites, which might then be transported downstream for rooting. However, additional studies are needed to quantify the response of S. virginiana to Beaver browsing and to investigate how habitat variables affect local browsing levels of this rare shrub community. Acknowledgments We thank D. Greene and M. Wallston for field assistance and I. Rossell, E. 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