Effects of invasive Amur honeysuckle (Lonicera maackii) and white-tailed deer (Odocoileus virginianus) on survival of sugar maple seedlings in a southwestern Ohio forest.
Invasive plants can affect native plants in a variety of ways ranging from lowering fitness to altering soil nutrient pools (Vila et al., 2011). These effects occur when invasive plants compete with and/or displace native species which can alter community structure (Mack et al., 2000; D'Antonio and Meyerson, 2002; Stohlgren et al., 2003). Native herbivores may lessen these impacts by grazing on introduced plants (Creed and Sheldon, 1995; Vila and D'Antonio, 1998; Case and Crawley, 2000) or they may increase the spread and impact of invasive plants by foraging on their native competitors (Edwards et al., 2000; Kellogg and Bridgham, 2004; Rooney, 2009).
Nonnative shrubs that invade native forests can create a dense vegetation layer that negatively affects growth and survival of seedlings of native trees and, thereby, can affect recruitment and relative abundance of tree species into forest canopies (Merriam and Feil, 2002; Fagan and Peart, 2004; Webster et al., 2006; Richardson and Rejmanek, 2011). The shrub Amur honeysuckle (Lonicera maackii; hereafter honeysuckle) was introduced to the U.S.A. from Asia in 1896 and now is prevalent in many eastern, midwestern, and southern states, as well as provinces in eastern Canada (USDA, NRCS 2013). Honeysuckle negatively affects abundance and richness of native tree seedlings (Hutchinson and Vankat, 1997; Collier et al, 2002; Hartman and McCarthy, 2008), and also reduces abundance of seedlings of sugar maple (Acer saccharum; Hutchinson and Vankat, 1997; Gorchov and Trisel, 2003).
Abundance of white-tailed deer (Odocoileus virginianus, hereafter deer) in Ohio has increased seven-fold since 1980 (data courtesy of T. Rooney, Wright State University, Dayton, OH). Selective browsing by overabundant deer reduces species richness and overall abundance of tree seedlings (Rooney and Waller, 2003; Collard et al, 2010; Tanentzap et al, 2011), as well as abundance and growth of individual species such as sugar maple and northern red oak (Quercus rubra; Tilghman, 1989; Waller and Maas, 2013). Selective grazing by deer on browse-sensitive seedlings (such as white ash (Fraxinus americana) and sugar maple; Tighman, 1989) also may alter species composition in regenerated forests by favoring survival of seedlings that are less sensitive to browsing such as American beech (Fagus grandifolia), black cherry (Prunus serotina), and honeylocust (Gleditsia tricanthos; Tilghman, 1989; Rooney and Waller, 2003; Long et al., 2007). Conversely, Powers and Nagel (2009) found no impact of deer on density or diversity of tree seedlings, including sugar maple, and Hartman and McCarthy (2004) determined that the use of solid tube tree protectors to limit deer browsing did not improve survival of tree seedlings.
Interactions between deer and invasive plants reflect intensive deer herbivory that enhances or reduces the impact of exotic plants in a non-additive way (Vavra et al., 2007; Christopher et al, 2014). Identifying these interactions is important because both deer and invasive plants may have to be controlled to avoid continued or increased negative impacts on native vegetation. For example Webster et al. (2008) found growth of tree seedlings increased only when abundance of both deer and exotic Japanese stiltgrass (Microstegium vimineum) was reduced. Similarly, Waller and Maas (2013) found growth of red oak seedlings was least with garlic mustard (Alliaria petiolata) present and deer excluded but was greatest when both deer and garlic mustard were absent. Gorchov and Trisel (2003) identified a possible deer x honeysuckle interaction by finding that removal of honeysuckle increased growth of sugar maple seedlings protected by wire cages, whereas growth of unprotected sugar maple and northern red oak seedlings was reduced by removal of honeysuckle. Apparently, honeysuckle inhibited deer browsing, although the wire cages also protected seedlings from herbivores other than deer. Similarly, Cipollini et al. (2009) concluded when deer density was high, management methods that leave dead honeysuckle stems standing may protect understory plants from deer grazing.
Despite studies documenting the negative impact of honeysuckle and deer on the abundance of sugar maple seedlings, studies of the effect of these species on the survival of sugar maple seedlings are lacking. Ascertaining the rate at which sugar maple seedlings decline as a result of the impacts of an invasive shrub and an overabundant herbivore would provide information to assess whether these forces could alter species composition of deciduous forests. Therefore, our overall objective was to determine the independent and interactive effects of honeysuckle and deer on survival of sugar maple seedlings. We hypothesized the survival of sugar maple seedlings would be reduced by both honeysuckle and deer and that a deer x honeysuckle interaction would further reduce the survival of sugar maple seedlings because the density of deer increases in areas invaded by honeysuckle (Allan et al., 2010) and because deer forage on honeysuckle berries and disperse their seeds (Castellano and Gorchov, 2013).
STUDY SITE AND EXPERIMENTAL DESIGN
Our study was conducted at the Cincinnati Nature Center, a 405 ha preserve located near Milford, Ohio (39.12061 N, 84.24786 W). Our study site was dominated by sugar maple, American beech, Q. muehlenbergii (chinkapin oak), northern red oak, Carya ovata (shagbark hickory), C. cordiformis (bitternut hickory), and A. rubrum (red maple). Saplings of Asimina triloba (pawpaw) and sugar maple were common in the understory along with the native shrub Lindera benzoin (spicebush).
We used experimental plots established in Apr. 2005 and located on a west-facing slope, a portion of which had been invaded by honeysuckle (Christopher et al., 2014). Richness of herb species, total number of herbs, and environmental factors (i.e., wind velocity, slope, soil compaction, and litter depth) did not differ between the area with and without honeysuckle prior to the start of the study (Christopher et al., 2014). Twelve 10 x 10 m plots were located in the invaded northern portion of the study site in a 2 x 2 arrangement of honeysuckle present/removed and deer present/excluded (n = 3 replicate plots of each treatment assigned randomly). Each invaded plot contained 8-10 honeysuckle shrubs comprising 7590% cover. Six additional plots were in the uninvaded southern portion of the study site, approximately 75 m away from the portion of the site containing honeysuckle, in a 1 x 2 arrangement, with treatments of honeysuckle absent and deer present/excluded (n = 3 replicates of each treatment assigned randomly). Deer were excluded with 2.4 m high deer fencing (Benner's Gardens, Conshohocken, Pennsylvania). Density of deer at our study site was 30-35/[km.sup.2] (aerial survey 2006, Cincinnati Nature Center) and observations of deer foraging in deer-present plots confirmed their presence during the study period. Honeysuckle shrubs were removed immediately after plot establishment by cutting them off at ground level, removing the clippings, and painting the stumps with 2% glyphosate (Roundup[R], Monsanto Company, St. Louis, Missouri). We occasionally found and removed honeysuckle seedlings in the uninvaded area, confirming that this area was suitable to honeysuckle invasion and represented an uninvaded control.
CENSUS OF SUGAR MAPLE SEEDLINGS
Sugar maple seedlings were selected for study because they were the most abundant tree species at our study site and were present on all plots (Table 1). In late May (spring) 2006, we individually marked 30 sugar maple seedlings in 12 of the experimental plots and all sugar maple seedlings in the remaining six plots because abundance of sugar maple seedlings was <30 in those plots (i.e., we marked 18 and 12 seedlings in honeysuckle present-deer excluded plots, respectively; 25 seedlings in honeysuckle removed-deer excluded plots; 16, 25, and 11 seedlings in honeysuckle present-deer present plots). We censused marked seedlings in early Oct. (autumn) 2006, late May (spring) 2007, and early Oct. (autumn) 2007, recording the number that remained.
To determine the independent and interactive effects of deer and honeysuckle on the survival of sugar maple seedlings, we conducted a factorial survival analysis with the survival package in R (Therneau and Grambsch, 2000; version 3.0.2, R Development Core Team, 2013) to analyze survivorship of sugar maple seedlings. We used a parametric analysis with censoring because individuals were still alive during the last census. In the data matrix, we included a vector called the censoring indicator that distinguished between those seedlings that died during a particular census period from those that remained alive during that census period. Those seedlings that were alive during the final census period were so indicated. We constructed two models: one determined the independent effect of deer and honeysuckle on survivorship of seedlings over the 2 y census period and the other determined the effect of the deer x honeysuckle interaction on survivorship of tree seedlings over the 2 y census period. For each model we determined whether survivorship of seedlings differed between deer-present and deer-excluded plots and among honeysuckle absent, honeysuckle-present, and honeysuckle-removed plots. The significance of these pairwise contrasts was evaluated by a z-test. We compared the model including independent deer and honeysuckle effects to the model containing deer x honeysuckle using a likelihood ratio test to determine if the deer x honeysuckle interaction affected seedling survivorship.
Survival of sugar maple seedlings was higher on honeysuckle-absent plots (64% remained after 2 y) than on honeysuckle-present plots (33% remained after 2 y) and was higher on deer-excluded plots (48% remained after 2 y) compared to deer-present plots (39% remained after 2 y; Fig. 1; direction of differences was the same on all replicate plots). Both deer and honeysuckle significantly affected survival of sugar maple seedlings ([chi square] = 66.83, df = 3; P < 0.01; Fig. 1). Survival was lower on deer present than on deer excluded plots (z = -2.88, P < 0.01). Survival also was lower on honeysuckle-present plots (z = -5.99, P < 0.01) and honeysuckle-removed plots (z = -6.38, P < 0.01) than on honeysuckle-absent plots. There was no difference in survival on honeysuckle-present plots compared to honeysuckle-removed plots (z = 0.124, P = 0.90). The deer x honeysuckle interaction was not significantly different from the model containing independent effects of deer and honeysuckle ([chi square] = 3.63, df = 2; P = 0.16), although because of low replication, power to detect an interaction was limited.
We found survival of sugar maple seedlings was negatively affected by the presence of honeysuckle and deer but these two factors did not interact. Several factors may explain why survival of sugar maple seedlings was higher on honeysuckle-absent plots than on honeysuckle-present plots. While woody vegetation (i.e., spicebush and pawpaw) formed a modest native understory, the shrub layer created by honeysuckle with its extensive stem structure was denser and more extensive than that attributed to native woody vegetation. This shrub layer, in combination with the prolonged leaf retention of honeysuckle (i.e., from Mar.-Nov.; McEwan et al., 2009; Smith, 2013) created conditions that differed from those in forest tracts without honeysuckle. Dense layers of honeysuckle decrease illumination at the forest floor (i.e., 1% of full sun; Luken et al., 1997; Woods, 1993) and may have altered ground-level microclimate, soil moisture, or soil nutrients, contributing to decreased survival of sugar maple seedlings (Hutchinson and Vankat, 1997). There was no difference in wind velocity, soil compaction, or litter depth in plots containing honeysuckle (data from Online Resources in Christopher et al., 2014).
Sugar maple seedlings also may be negatively affected by shoot or root competition with honeysuckle (Gorchov and Trisel, 2003) or by allelopathy from honeysuckle roots (Doming and Cipollini, 2006; Cipollini et al., 2008). Christopher et al. (2014) found abundance of herbs on honeysuckle-removed plots did not differ from honeysuckle-absent plots 2-3 y after removal of honeysuckle. We did not see such a rapid rebound in survival of sugar maple seedlings after removal of honeysuckle; survival did not differ between honeysuckle-present (33% remained after 2 y) and honeysuckle-removed plots (31% remained after 2 y). Cipollini and Doming (2008) found that soil taken from fields containing honeysuckle inhibited growth of an annual plant in greenhouse trials. Such residual allelopathic effects also may have affected survival of sugar maple seedlings, thereby accounting for the lower survivorship of sugar maple seedlings in honeysuckle-removed plots compared to honeysuckle-absent plots.
[FIGURE 1 OMITTED]
Decreased survival of sugar maple seedlings on deer-present plots was most likely because these seedlings are a preferred browse species (Tilghman, 1989; Horsley et al,. 2003). Deer browsing also reduces abundance of herbaceous vegetation (Christopher et al., 2014), which may affect tree seedlings indirectly by altering microclimate near the ground (Rooney and Waller, 2003) or may result from deer preferentially browsing on some herbs. Christopher et al. (2014) determined abundance and number of leaves of false Solomon's seal (Maianthemum racemosa), a species heavily grazed by deer (Frankland and Nelson, 2003), was significantly reduced in deer-present plots. Lower density of false Solomon's seal in deer-present plots could have negatively affected survival of sugar maple seedling by making them more accessible as browse for deer.
Boerner and Brinkman (1996) studied establishment and mortality of tree seedlings at the Tucker Nature Preserve in central Ohio from 1984 to 1993. Mean longevity for most tree seedlings was 5-7 mo, with > 95% dying during their first year. Fewer than 2% of tree seedlings persisted > 2 y. The median life span for sugar maple seedlings was approximately 1 y with only 14% alive at the end of the first year. This survival was lower than our results perhaps because density of deer differed between the studies. Although they did not have direct evidence, Boerner and Brinkman (1996) argued seedling mortality was largely attributable to deer browsing rather than low light stress from canopy trees or competition among seedlings. Their conclusion agrees with ours, that deer negatively impact sugar maple seedling survival.
Contrary to our prediction, we did not find the deer x honeysuckle interaction affected survival of sugar maple seedlings. Similarly, Aronson and Handel (2011) reported no significant effect of an interaction between deer and stiltgrass on tree seedlings. However, other studies have reported significant interactions between deer and invasive plants (e.g., deer x stiltgrass negatively affected herbs and woody vegetation, Knight et al, 2009; Duguay and Farfaras, 2011; deer x garlic mustard negatively affected herbs and tree seedlings, Knight et al., 2009; Waller and Maas, 2013; deer x honeysuckle negatively affected herbs, Christopher et al, 2014). Comparing studies that only detected effects on woody vegetation or tree seedlings, we found no significant interaction but Duguay and Farfaras (2011) and Waller and Maas (2013) reported significant interactions suggesting that a deer x invasive species interaction effect on tree seedlings varied with the particular invasive plant species and whether that species was woody or herbaceous.
Lack of a honeysuckle x deer interaction suggests that land managers could implement control measures specifically for either honeysuckle or deer, without experiencing unintended effects on survival of sugar maple seedlings by the nontarget species. In cases where deer x invasive plant interactions were identified, less than desired effects on target vegetation would result if control measures were not implemented on both deer and the invasive plant (Baiser et al., 2008; Knight et al., 2009; Dornbush and Hahn, 2013; Weller and Maas, 2013; Christopher et al,. 2014). However, land managers face a difficult decision when deciding whether to manage deer and honeysuckle separately or together because there is an interactive effect on abundance of native forest herbs and growth of individual herbs (Christopher et al" 2014) but not on tree seedlings (this study).
Acknowledgments.--We thank C. Christopher for introduction to the field site and for technical support, G. Klein, S. Lang, B. Loomis, O. Mirza, and K. Scherff for advice and assistance with field work, T. Culley for comments on early versions of the manuscript, T. Rooney for sharing data on population size of deer in Ohio, the Cincinnati Nature Center for permission to conduct research on their property, and the Department of Biological Sciences, University of Cincinnati, for financial support.
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SUBMITTED 7 AUGUST 2014
ACCEPTED 18 MARCH 2015
JESSICA D. LOOMIS, STEPHEN F. MATTER and GUYN. CAMERON (1)
Department of Biological Sciences, University of Cincinnati, Cincinnati, Ohio 45221
(1) Corresponding author: Telephone (513) 556-9740; Fax: (513) 556-5299 e-mail: firstname.lastname@example.org
Table 1.--Total number of seedlings of each tree species counted in all experimental plots and mean ([+ or -] SE) abundance/plot for each species; n = 18 10 x 10 m plots Number of Abundance Tree species Common name seedlings of seedlings Acer negundo Box elder 12 0.67 [+ or -] 0.12 Acer saccharum Sugar maple 530 29.4 [+ or -] 8.41 Asimina triloba Pawpaw 16 0.89 [+ or -] 0.43 Carya cordiformis Butternut hickory 8 0.44 [+ or -] 0.33 Fagus grandifolia American beech 3 0.17 [+ or -] 0.17 Fraxinus americana White ash 130 7.22 [+ or -] 2.23 Prunus serotina Black cherry 90 5.00 [+ or -] 1.67 Quercus muehlenbergii Chinkapin oak 4 0.22 [+ or -] 0.14 Ulmus americana American elm 68 3.78 [+ or -] 0.71
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|Author:||Loomis, Jessica D.; Matter, Stephen F.; Cameron, Guy N.|
|Publication:||The American Midland Naturalist|
|Date:||Jul 1, 2015|
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