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Encroachment of Oriental Bittersweet into Pitcher's Thistle HabitatAuthor(s): Stacey A. Leicht-Young and Noel B. PavlovicSource: Natural Areas Journal, 32(2):171-176. 2012.Published By: Natural Areas AssociationDOI: http://dx.doi.org/10.3375/043.032.0206URL: http://www.bioone.org/doi/full/10.3375/043.032.0206

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Volume 32 (2), 2012 Natural Areas Journal 171

Natural Areas Journal 32:171–176

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Encroachment of Oriental Bittersweet into Pitcher’s Thistle

Habitat

Stacey A. Leicht-Young1

1U.S. Geological SurveyGreat Lakes Science Center

1100 N. Mineral Springs Rd.Porter, IN 46304

Noel B. Pavlovic1,2

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R E S E A R C H N O T E

2 Corresponding author: email: npavlovic@usgs.gov; 1-219-926-8336 x 428

ABSTRACT: Common invasive species and rare endemic species can grow and interact at the ecotone between forested and non-forested dune habitats. To investigate these interactions, a comparison of the proximity and community associates of a sympatric invasive (Celastrus orbiculatus; oriental bittersweet) and native (C. scandens; American bittersweet) liana species to federally threatened Cirsium pitcheri (Pitcher’s thistle) in the dunes habitats of Lake Michigan was conducted. Overall, the density of the invasive liana species was significantly greater in proximity to C. pitcheri than the native species. On the basis of composition, the three focal species occurred in both foredune and blowout habitats. The plant communities associated with the three focal species overlapped in ordination space, but there were significant differences in composition. The ability of C. orbiculatus to rapidly grow and change the eco-logical dynamics of invasion sites adds an additional threat to the successional habitats of C. pitcheri.

Index terms: Celastrus orbiculatus, C. scandens, Cirsium pitcheri, invasive plants, Lake Michigan, threatened plants

INTRODUCTION

Ecotonal habitats occur where there are transitions between different vegetation types. These edge habitats can be formed by both natural processes and human-medi-ated disturbances, and can contain species that benefit from environmental attributes of both sides of the ecotone (Walker et al. 2003). Sand dune ecosystems contain several natural ecotones that are formed as succession from primary to secondary dunes to forest proceeds (Cowles 1899; Ol-son 1958; Maun 2009). A common ecotone occurs between grass-dominated dunes and adjacent forests. There are unique plant species in these different zones as one moves away from the coast including several endemics (Guire and Voss 1963), such as the federally threatened Pitcher’s thistle (Cirsium pitcheri), that occurs in the dunes along the Great Lakes.

Ecotonal habitats can be susceptible to exotic plant invasion because of the edge characteristics often present, such as increased light or amelioration of harsh conditions (Matlack 1994). These exotic species have the potential to negatively impact native species present in these habitats often by outcompeting them for limited resources (Thomson 2005). In the sand dune habitats along Lake Michigan, several exotic species pose a threat to the native species (Mills et al. 1993). One in particular, Celastrus orbiculatus (oriental bittersweet), has been increasing in these plant communities (Leicht-Young et al. 2007a). Although this liana (woody vine) is most often thought of as a forest dweller and can also become dominant in old field succession (Fike and Niering 1999), it

has the ability to move out via vegetative growth into open dune habitats from the forest habitats that form on the lee side of the dunes (Pavlovic and Leicht-Young 2011). From here it can climb small diam-eter woody stems or vines available to it at the edges of the forest and open dunes. This species can also establish by seed under woody vegetation in the open dunes where it is protected from the harsh xeric condi-tions. As a result of its climbing habit, it can blanket and smother native vegetation beneath its expansive growth (Fike and Niering 1999). It is the rampant vegetative growth of this species that could enable it to preempt the habitats commonly preferred by C. pitcheri (Thomson 2005).

The native congeneric liana, Celastrus scandens (American bittersweet), occurs frequently in the open dune and ecotone habitats. This species has declined sub-stantially in the northeastern United States (Steward et al. 2003; Leicht 2005); how-ever, it is still fairly abundant along Lake Michigan from the more open dune habitats into the ecotone between these dunes and forest (Pavlovic and Leicht-Young 2011). It is in these ecotones that the native and invasive bittersweet can compete with each other (Leicht-Young et al. 2007a) and possibly with the federally threatened C. pitcheri that shares this habitat, potentially threatening this species’ successful growth and establishment. Using simple measure-ments of distances between the species and density, coupled with an examination of the plant communities they were as-sociated with, we wanted to determine if the invasive C. orbiculatus poses a threat to C. pitcheri.

172 Natural Areas Journal Volume 32 (2), 2012

METHODS

Survey Methods

In 2005, we assessed the potential im-pact of C. orbiculatus by estimating its density in proximity to C. pitcheri at the property adjacent to the Portage Lakefront and Riverwalk within the Indiana Dunes National Lakeshore, known as West Beach East (hereafter referred to as “WBE”) sec-tion of Indiana Dunes National Lakeshore (41°37’37’’, 87°10’54’’). This location is a successional blowout system with some invasion by Pinus banksiana and Quercus velutina. Within WBE, we surveyed three locations – two blowouts (referred to as “east blowout” and “south blowout”) and one flat area dominated by scattered P. banksiana (“east pines”). At each C. pitcheri plant, we measured the distance in meters to the nearest C. orbiculatus and C. scandens. In addition, in a 1m2 area around 37-focal plants per species (C. orbiculatus, C. scandens, and C. pitcheri), we identi-fied and recorded the presence of all plant species for a total of 111 plots).

Data Analyses

We assessed significant differences in the distance of C. orbiculatus and C. scandens from C. pitcheri using two-way ANOVA, with distance (square root transformed) to C. pitcheri as the dependent variable and species (C. orbiculatus, C. scandens) and location (east blowout, south blowout, east pines) as independent variables. We calculated mean density per hectare by using the formula, density = 10000 / (2 * distance)2, based on the relationships developed by Cottam and Curtis (1956). Densities were log transformed prior to the application of ANOVA. We followed this analysis with nonmetric multidimensional scaling (NMS; Sørenson distance measure) to compare the associate communities of the three species. We also employed clus-ter analysis (Sørenson distance measure and flexible beta (-0.25) linkage method) to determine if the three different loca-tions within our site had different plant communities. We used multiple response permutation procedure (MRPP) to evaluate the significance in plant community com-position among the three species based on

the Sørenson distance measure. We used indicator species analysis to determine what species were characteristic of the fo-cal species habitats. We visually inspected a TWINSPAN-generated ordered table to understand the composition of the two clusters. ANOVA analyses were conducted in SPSS v.12 (SPSS Inc. 2003) and ordina-tion, cluster analysis, MRPP, TWINSPAN, and indicator species analysis in PCORD v.5.31 (McCune and Mefford 2006).

RESULTS

Overall, invasive C. orbiculatus (6.8 ± 0.7 m) did not grow in significantly closer proximity to C. pitcheri than native C. scan-dens (9.7 ± 0.8 m; F1,74 = 0.2, P = 0.22). The location (F2,74 = 0.24, P = 0.81) and the species by location interaction (F2,74 = 2.1, P = 0.13) were not significant. The minimum and maximum distances that a C. orbiculatus was to a C. pitcheri were 0.7 and 18.5 m, respectively, and 0.9 and 27.7 m for C. scandens. However, density of C. orbiculatus was significantly greater than that of C. scandens with respective mean densities of 370 ± 149 and 167 ± 81 per ha (F1,68 = 5.0, P = 0.029). Density did not differ among locations (F2,68 = 0.3, P = 0.74), and there was no significant spe-cies by location interaction (F2,68 = 2.3, P = 0.11).

We observed 39 associated species in the plots adjacent to the three focal species (Table 1). Significant indicators of, respec-tively, C. pitcheri, C. orbiculatus, and C. scandens were: (1) Artemisia campestris ssp. caudata, Corispermum americanum var. rydbergii, and Panicum virgatum; (2) Galium pilosum, Physalis virginiana, Solidago caesia, Quercus velutina, and Tilia americana; and (3) Lithospermum caroliniense var. croceum, Salsola kali, Prunus virginiana and Triplasis purpurea (Table 1).

Community associates of C. pitcheri, C. orbiculatus, and C. scandens overall showed no strong separation in NMS space (variance explained for axes 1, 2, and 3 were 0.19, 0.30, and 0.27; Figure 1); however, cluster analysis showed that there were two main plant communi-ties (chaining = 3.0 %, dendrograph not

shown). The blowouts were characterized by secondary dune vegetation with the most frequent species being Schizachy-rium scoparium, Calomovilfa longifolia, Euphorbia corollata, Panicum virgatum, Pinus banksiana, and Triplasis purpurea (right side of Figure 1). Most frequent in the primary dune vegetation in the east pines were Ammophila breviligulata, and the woody species Juniperus virginiana, Populus deltoides, Prunus pumila, Tilia americana, and Vitis riparia (left side of Figure 1). When we overlaid these two community types on the ordination (Figure 1), the C. pitcheri community in the east pines area (square symbols on left side) is separated from that of the C. orbiculatus and C. scandens communities; while in the east and south blowouts, they spatially appeared more similar (triangle symbols on right side). Nevertheless, composition varied significantly among species overall and by community based on MRPP analysis (Table 2). This separation is less visible in Figure 1 because axis one, the least important axis, is not shown; however, compositional variation along all three axes contributes to MRPP tests.

DISCUSSION

Our results show that C. orbiculatus is invading C. pitcheri habitat and could pose a legitimate threat to C. pitcheri in the dune/forest ecotones near Lake Michigan. Mean densities of Celastrus orbiculatus were nearly twice that of C. scandens in proximity to C. pitcheri. Although C. or-biculatus is commonly viewed as a mesic, forest-dwelling species, it clearly has the ability to survive in these open dune habi-tats. In full sun, we have often observed C. orbiculatus growing under the shade of shrubs, tree saplings, or grapes with ramets extending out into open habitats. These ecotonal habitats are where the threat to C. pitcheri is likely greatest from C. orbicu-latus, because without some type of cover for C. orbiculatus to establish under, this species does not grow as well in wide open dunes where C. pitcheri is also present. We found that wherever these three species are growing, they may overlap in the plant communities they are associated with but seem to occur in slightly different vegeta-tion compositions (Figure 1). In the east

Volume 32 (2), 2012 Natural Areas Journal 173

Table 1. The percent occurrence of the 39 associates of the three focal species. Bolded occurrence values identify indicator species for the focal species. Names follow the nomenclature in the USDA PLANTS database (USDA NRCS 2011). The abbreviations are those that are used in the ordination in Figure 1.

174 Natural Areas Journal Volume 32 (2), 2012

and south blowouts, we saw a fair amount of overlap in the plant communities associ-ated with these species. However, in the vegetation of the east pines area, Celastrus communities were associated with woody species that were lacking where C. pitcheri grew. This may be a result of different environmental conditions created by the nearby P. banksiana trees as compared to the blowout communities.

Both Celastrus species would have the ability to climb the flowering stem of C. pitcheri. However, as C. pitcheri is a monocarpic perennial, it would only be by chance that one of these species would be present at the right time to climb the flowering stalk and potentially impact the flowering/fruiting directly. In fact, of all the C. pitcheri plants we surveyed for this study, only three were flowering adults while the other 34 were juveniles, and none were being physically used as supports. The main threat from C. orbicu-latus is its ability to take on a sprawling, shrub-like habit when growing out in the open unsupported by a host plant, while C. scandens tends to retain its liana growth form and slower growth rate even in open habitats (Gartner 1991; Leicht-Young et al. 2007b; Leicht-Young et al., 2011). These dense monocultures of C. orbiculatus, growing without support across an area, could lead to preemption of C. pitcheri habitat. Cirsium pitcheri requires open sand areas to successfully reproduce and sustain its population (D’Ulisee and Maun 1996). Previous research in old fields has shown that C. orbiculatus can effectively arrest succession and prevent new spe-cies from establishing (Fike and Niering 1999). In addition, in New England, it was

Figure 1. NMS ordination of plant community samples centered on individuals of C. pitcheri (CIPI, black symbols), C. orbiculatus (CEOR, white symbols) and C. scandens (CESC, gray symbols). Squares represent the east pines location while triangles represent the east and south blowout locations (see full descriptions in methods and results). Axes are the two that explained the most variation (see results). Species positions in the ordination space are indicated by the plus signs and their abbreviations can be found in Table 1.

Table 2. Chance-corrected within-group agreement (A) and p values for multiple response permutation procedure test of significant differences in com-munity composition by species.

Volume 32 (2), 2012 Natural Areas Journal 175

observed that rare plant populations were more likely to be lost when one or more invasive species were present than in those areas where these invasive species did not occur (Farnsworth 2004).

Although there are several other important threats to the survival of C. pitcheri in dune ecosystems (i.e., habitat loss, seed predation, deer (Odocoileus virginianus), and rabbit (Sylvilagus floridanus) browse; McEachern et al. 1994; Phillips and Maun 1996), the threat from invasive plants such as C. orbiculatus cannot be underestimated because of their ability to rapidly infiltrate, dominate, and potentially change the ecological characteristics of an area (Fike and Niering 1999). In other regions of the country, C. orbiculatus has been shown to be a top threat to threatened and endan-gered plant species in a variety of habitats (Farnsworth 2004). Control of invasive species in areas with sensitive species is a difficult challenge for land managers, but is necessary for their continued survival (Kirby et al. 2003; Thomson 2005; Meyer and Fourdrigniez 2011). In the case of C. pitcheri, loss of the open dune habitat to natural succession will afford more op-portunities for habitat encroachment by C. orbiculatus.

ACKNOWLEDGMENTS

We thank Emily Palmquist for field assis-tance. We thank the National Park Service at Indiana Dunes National Lakeshore for permission to conduct surveys on their property. Timothy Bell and Julie Stumpf provided helpful reviews of an earlier draft of this article. This article is contribution No. 1653 of the USGS Great Lakes Sci-ence Center. Use of trade, product, or firm names does not imply endorsement by the U.S. Government.

Stacey A. Leicht-Young is a plant ecologist with a PhD in ecology from the University of Connecticut. Her research has examined the biology and ecology of native and invasive bittersweet (Celastrus) as well as impacts on invasive plant species on native plant communities.

Noel B. Pavlovic is a plant ecologist with a PhD in biological sciences from the University of Illinois at Chicago. His re-search encompasses the role of fire in the maintenance of oak savanna biodiversity, long-term demography of Pitcher’s thistle, and management and impacts of invasive plants in Great Lakes ecosystems.

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