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Effect of removal of Hesperis matronalis (dame's rocket) on species cover of forest understory vegetation in NW Indiana.

INTRODUCTION

A major issue in prescribing management for control of exotic invasive plant species in disturbed or degraded natural areas is the potential response of the existing plant community to invasive species removal. Invasive plant species can have a variety of effects on the communities and ecosystems they invade, including changing soil attributes (Ehrenfeld et al., 2001; Ehrenfeld, 2003), light availability (Webb et al., 2000; Martin and Marks, 2006), physical structure (Woods, 1997) and fire frequency (D'Antonio and Vitousek, 1992; Brooks et al., 2004) of a site. In addition, one of the most frequently observed changes associated with establishment of an invasive species is a decrease in native plant abundance and diversity (Parker et al., 1999; Alvarez and Cushman, 2002). However, it is often assumed, usually without quantification, that invasive species cause diversity and abundance of native species within the plant community to decrease (McCarthy, 1997; Alvarez and Cushman, 2002; Stinson et al., 2007), and with eradication of the invader, that the native plant community will recover. Often, the focus of management in a natural area is on removing a single invasive species from the community without taking into account the suite of other invasive species present. In the absence of a quantitative understanding of a species' purported negative effect on the overall plant community, management efforts may be largely wasted or unsuccessful (D'Antonio and Meyerson, 2002).

Removal experiments are useful to determine the influence that a given species has in a community (Diaz et al., 2003). They have been used historically in plant ecology to examine the response of the remaining community to the loss of one or more of its components (Aarssen and Epp, 1990; Diaz et al., 2003). This type of experiment is a direct way of understanding the effect of an invasive species on the remaining plant community (Stinson et al., 2007). Removal experiments have been used to examine the interactions of invasive species with their surrounding community (Alexander and D'Antonio, 2003; Biggerstaff and Beck, 2007). Interestingly, these studies suggest a mixed response of the native species, ranging from an increase in native richness and diversity (McCarthy, 1997; Brandon et al., 2004; Harms and Hiebert, 2006; Hejda and Pysek, 2006; Hulme and Bremner, 2006; Stinson et al., 2007) to no change in native richness and diversity (Morrison, 2002; Erskine Ogden and Rejmanek, 2005). In addition to native species responses, exotic species also present in the community may also increase (Alvarez and Cushman, 2002; Erskine Ogden and Rejmanek, 2005; MacDougall and Turkington, 2005; Hulme and Bremner, 2006).

While many studies focus on management of late-successional communities (Woods, 1997) information on disturbed, early-successional forest communities that are typical in many urbanized areas is lacking. Since these areas are often inhabited by nonnative species, they are often a lesser priority for land managers compared to higher quality areas. However, sites for restoration are often highly degraded in urbanized areas. In this study, we examined effects of the exotic invasive species Hesperis matronalis (dame's rocket) on a plant community it has invaded. Although it is a common invasive plant (Mitchell and Ankeny, 2001; Rothfels et al., 2002; Murphy et al., 2007), there are few studies examining the ecology of H. matronalis. In comparison, more than 50 studies have focused on the negative effects of the confamilial Alliaria petiolata (garlic mustard) (e.g., Nuzzo, 1993; McCarthy, 1997; Nuzzo, 1999; Stinson et al., 2007). In this study, we examined: (1) the effects of the removal of Hesperis matronalis on plant community composition; (2) response of species richness and diversity to removal of H. matronalis; and (3) differences in response between native and exotic species. We hypothesized that native species will increase in cover with the removal of the rosette forming H. matronalis.

METHODS

STUDY SPECIES

Hesperis matronalis L. (Brassicaceae) is a short-lived perennial or biennial species native to Eurasia (Gleason and Cronquist, 1991). Populations include seed, seedlings, juvenile rosettes and flowering plants. Plants that flower die back by early Jul. while rosettes persist later into the growing season. The white, pink or purple flowers are often confused with Phlox species, because of their spring blooming period and the occasional appearance of five petals rather than the four that are typical for the mustard family. In the United States, this species is present in 42 states, excluding those states in extreme southern portions of the United States, and is present in all southern provinces of Canada (NatureServe, 2007; USDA NRCS, 2007). This species was likely introduced in the 1700s (Adams, 2004) as a garden plant and is often included in "native" wildflower mixes (Mehrhoff et al., 2003). A single plant growing in the open habitats has the potential of producing up to 5000 seeds per year (Stevens, 1932), although this can vary greatly with differing environmental conditions. This species is found along roadsides, in open woods and moist bottomlands (Gleason and Cronquist, 1991).

EXPERIMENTAL SETTING AND DESIGN

In May 2005, 10 pairs of 1 x 1 m plots were located randomly in 10 3 m segments of a north to south transect in second-growth mesic-woods at Indiana Dunes National Lakeshore (41[degrees]37'N, 87[degrees]05'W). Indiana Dunes has a mean annual precipitation of 998 mm, mean temperature of 12 C, and a temperate, humid, continental climate (Furr, 1981). The transect was on a disturbed 12,000 y old Glenwood sand dune (Olson, 1958). The transect was bounded by a road on the east, an ephemeral wetland on the west and abandoned homesites on the north and south suggesting the presence of human impacts on the vegetation. Hesperis matronalis was apparently introduced many years previously to a homesite nearby. Between 1995 and 1996, the population expanded rapidly into the woods along the road and beyond. Thus, the study population was approximately 10 y old at the start of the study (N. Pavlovic, pers. obs.). Within each pair of plots, one plot was randomly designated as the control plot, wherein H. matronalis was left undisturbed, and the other was designated as the treatment plot, wherein all H. matronalis plants were pulled and removed from the site in both spring and summer over the 3 y. In Jun. 2005, the initial biomass of seedling and adult of H. matronalis pulled from the treatment plots was measured, after drying at 70 C for 24 h.

Subsequently, we determined percent cover for each plant species present in each plot, for bare ground and for litter in two seasons: spring (late May or early Jun.) and summer (Aug.) from 2005-2007. Species were classified as native or exotic, the latter being non-indigenous to North America (USDA Plants database: http://plants.usda.gov/). For Hesperis matronalis, cover in both the control and treatment plots was determined before the current season's plants were pulled. Cover values were assessed using ocular estimates.

ANALYSIS

Species cover data were relativized based on the total cover across all species on a plot basis. We analyzed the community Sorenson similarity data matrix using both nonmetric multidimensional scaling (NMS) (McCune and Mefford, 2006) and perMANOVA (Anderson, 2001). Nonmetric multidimensional scaling is a nonparametric ordination method based on the rank orders of plot similarity. PerMANOVA is a nonparametric multivariate analysis method using similarity (Sorenson) to compute F-ratios based on within- and among-group similarity defined in n species multivariate space. For statistical testing, probability values are calculated from a permutation procedure (Anderson, 2001). Using the Sorenson distance matrix, we examined the similarities of the plant communities for each paired control and pulled plot within seasons and years. These data were examined using one-way ANOVA followed by a Benjamini-Hochberg adjustment for the P-values to avoid Type-I error from multiple tests (Benjamini and Hochberg, 1995). We used paired t-tests on several sets of data, which were also followed by the same adjustment for the P-values. These t-tests were done at each survey date (i.e., spring 2005, summer 2005, etc.) because we were interested in the vegetation changes at each time point, and because we wanted to examine seasonal effects on the vegetation. Thus, we examined how total vegetation cover, litter, bare ground and cover of Hesperis matronalis changed across seasons and years. Using paired t-tests, we also examined how treatment affected diversity measures, including species richness (S), evenness (E), Shannon's diversity index (H') and Simpson's diversity index (D) (McCune and Mefford, 2006). Additionally, we categorized our plant data into cover of exotic and native woody and herbaceous species as well as annuals, perennials and biennials to examine how the treatment influenced these measures within years and seasons. We correlated diversity, bare ground, litter and native and exotic life form measures with NMS ordination axis scores to examine trends in diversity across major compositional gradients (P < 0.05) (Norusis, 1992). We looked at how certain dominant (i.e., common species present at all survey times) species changed within the different treatments. These species were: Celastrus orbiculatus (Oriental bittersweet), Circaea lutetiana (enchanter's nightshade), Euonymus alatus (burning bush), Lindera benzoin (spicebush), Osmorhiza claytonia (sweet cicely), Parthenocissus quinquefolia (Virginia creeper), Polygonum virginianum (jumpseed), Rosa multiflora (multiflora rose) and Sassafras albidum (sassafras). Finally, using linear regression we looked at the relationship between initial biomass of H. matronalis pulled and the diversity measures (S, E, H', D) at each survey date in the H. matronalis removal plots.

[FIGURE 1 OMITTED]

RESULTS

Fifty-one native and exotic species were recorded over the 3 y of this study (Appendix 1). Exotic species comprised 24% of the species, 35% of the woody plants and 16% of the forbs. NMS ordination showed a clear separation of plots by season along axis 2 and by treatment along axis 1 (Fig. 1). Together, the two axes explained 93% of species compositional variation. The first axis (57% of compositional variation) was negatively correlated with diversity (E: r = -0.87, P = 0.001; H': r = -0.64, P = 0.025; D: r = -0.70, P = 0.012) and exotic woody plant cover (r = -0.64, P = 0.026) and the second axis (36% of the variation) was negatively correlated with species richness (r = -0.60, P = 0.038), native forbs (r = -0.73, P = 0.007), annuals (r = -0.92, P < 0.001) and exotic forbs, exclusive of Hesperis matronalis (r = -0.79, P = 0.002) and positively correlated with bare ground cover (r = 0.81, P = 0.001). Thus, summer vegetation had lower richness than spring vegetation. Each of the season/treatment combinations clustered together except for the initial removal treatment in spring 2005 (Sp5R). The close proximity of the points spring 2005 H. matronalis control (Sp5C) and spring 2005 H. matronalis removed (Sp5R) prior to H. matronalis removal shows that the paired plots were similar initially in composition (Fig. 2). The summer 2005 control (Su5C) and summer 2005 removal plots (Su5R) are not in close proximity because H. matronalis was removed prior to the summer sampling. The removal of H. matronalis from treatment plots produced a shift to the left along axis 1. The longer trajectory lines of the plots with H. matronalis removed that were sampled in spring, compared to plots sampled in summer, indicated greater compositional changes in the spring (late May/early Jun.) flora than in the summer (Aug.) flora associated with H. matronalis removal. The vectors for growth forms indicated which forms produced the observed changes in ordination space. Removal of H. matronalis increased exotic woody cover over the 3 y of this study. Native annuals increased from 2005 to 2006 in both the control and removal plots (Fig. 1 and Fig. 2D).

[FIGURE 2 OMITTED]

PerMANOVA indicated that there was a significant difference in species composition between the control and removal plots in summer for all years and in spring only in 2007 (Table 1). Similarity matrices indicated that in the spring surveys, dissimilarity between the control and removal plots increased significantly from 2005-2007 (Table 2). In the summer surveys, there were no statistically significant differences in dissimilarity values across the years, but the values were higher than those in spring except in 2007. Total vegetation cover in the control and removal plots did not generally differ across years and seasons, except in summer 2006 (Fig. 2A). However, treatment did significantly affect the cover of Hesperis matronalis at all season/year time steps except for spring 2005, when the experiment began (Fig. 2A).

Whether Hesperis matronalis was present or removed had no statistically significant effect on species richness or diversity measures. Cover of native woody species did not significantly differ between the control and removal treatments. However, exotic woody species had greater cover in plots with H. matronalis removed compared to control plots on all sampling dates except for summer 2006 (Fig. 2B). There was no significant difference in cover of native and exotic forbs between the two treatments (Fig. 2C) nor for cover of annuals, perennials or biennials (Fig. 2D). The peak in annual cover in spring of 2006 was independent of treatments and may have represented a response to the greater rainfall in 2006 after the end of a 2005 drought (Fig. 2D). Total rainfall in 2005 was 663 mm compared to 1275 mm in 2006 and 1143 in 2007. In terms of specific native and exotic woody species responses, only cover of Rosa multiflora showed a significant response to the pulling of H. matronalis in any year (2005: [F.sub.1, 38] = 4.08, [P.sub.adj] = 0.05; 2006: [F.sub.1, 38] = 9.34, [P.sub.adj] = 0.01; and 2007: [F.sub.1, 38] = 5.33, [P.sub.adj] = 0.04). Two species, Alliaria petiolata and Berberis thunbergii (Japanese barberry) appeared in the removal plots but not in the control plots in the last survey year (2007).

After examining all of the relationships between biodiversity indices and initial Hesperis matronalis biomass in treatment plots, we found that the only significant regression was a positive one between H. matronalis biomass in spring of 2005 and species richness in 2006 (r = 0.67, P = 0.036) and summer of 2007 (r = 0.65, P = 0.044). Annual cover (spike in Fig. 2D) and richness increased in spring of 2006, which was probably due to the annual forb response to increased precipitation in 2006 compared to 2005. The positive relationship between biomass and species richness in summer of 2007 suggested that the greater the biomass of H. matronalis removed the greater was the increase in richness. Looking at new species occurrence across all experimental plots from 2005 to 2007, eight species were gained in the H. matronalis removal plots compared to only two species in the control plots.

DISCUSSION

Removal of Hesperis matronalis at the beginning of the experiment resulted in decreased H. matronalis cover for the remaining two years of the study. An increase in exotic woody plant cover following removal of H. matronalis significantly changed community composition. The response was greatest in spring when more species were present and early growing exotic woody plants were at an advantage compared to native woody plants that sprout later (Harrington et al., 1989; Xu et al., 2007). The smaller positive difference in summer cover of exotic woody plants compared to the native woody plants likely arose from this difference in seasonal growth patterns (Fig. 2B). Despite the shift in the overall community composition, neither species richness, evenness nor diversity on a plot basis differed significantly between control and treatment plots. This illustrates that the effect of a species on a plant community may not be reflected in species richness or diversity measures (Yurkonis et al., 2005). That is, invasive species do not always directly replace species in the communities they invade, as is commonly suggested (Levine et al., 2003). Changing cover or abundance of species can result from an invader decreasing the recruitment of native species by either saturating the area with invasive propagules (Brown and Fridley, 2003; Yurkonis et al., 2005) or competing more effectively for light and other resources (Tilman, 1993; Davis et al., 2000). In our study, the number of species did not change with removal of H. matronalis, but relative covers of species within removal plots did. Because our study site was a previously disturbed community, it is possible that recruitment of missing spring and summer herbaceous species was limited. A few studies have also reported no change in richness and diversity of species after an invasive species was removed from a habitat (Meiners et al., 2001; Morrison, 2002; Erskine Ogden and Rejmanek, 2005).

Despite phylogenetic and life-history similarities, removal of Hesperis matronalis yielded different community responses (richness, diversity) compared to other studies involving removal of Alliaria petiolata. In two studies, removal of A. petiolata decreased diversity of native species present (McCarthy, 1997; Stinson et al., 2007). One possibility for the lack of decrease in native richness here was the 3 y length of our study; however, the A. petiolata studies ran for 3 and 1 y, respectively. Because these durations were similar or less than ours, this possibility is unlikely. Another possibility is that due to reported allelopathic (Prati and Bossdorf, 2004) and anti-mycorrhizal (Roberts and Anderson, 2001; Stinson et al., 2006) effects of A. petiolata, this species is better able to displace native vegetation compared to H. matronalis. However, it is not known if H. matronalis has similar effects on surrounding vegetation or soil communities.

Hesperis matronalis removal did significantly increase woody exotic species cover. Two species in particular benefited from removal of H. matronalis: Rosa multiflora and Euonymus alatus. The difference in R. multiflora cover between control and treatment plots increased with time after H. matronalis removal. The relatively greater R. multiflora cover in removal plots suggests the release of this species from a limiting resource, such as light. We did not observe, however, new R. multiflora seedlings (S. Leicht-Young, pers. obs.). Although changes were not statistically significant, cover of E. alatus increased in response to H. matronalis removal. Unlike R. multiflora, however, there was an increase in E. alatus seedlings as the study progressed, perhaps indicating that H. matronalis competed with E. alatus for resources (S. Leicht-Young, pers. obs.). Although their addition did not make exotic species richness significantly increase, A. petiolata and Berberis thunbergii appeared in the removal plots in the last survey year (2007) when they had not been in the plots previously. Similar studies examining the effects of invasive species removal on the remaining plant community have found that other exotic species often expand or recruit into the newly disturbed or bare areas (Marrs and Lowday, 1992; Alvarez and Cushman, 2002; Erskine Ogden and Rejmanek, 2005; Hulme and Bremner, 2006; Murphy et al., 2007). This is particularly true in degraded areas where much of the recruitment or existing community is made up of exotic species, as was the case at this study site. In addition, the result of a removal experiment can depend on the location. Marrs and Lowday (1992) found that after removing Pteridium aquilinium (bracken fern) from two different locations in heathlands, one area returned to a diverse plant community, while the other became a monoculture of clonal graminoids, even with native plant seeding. Thus, the results of species removal may be site-dependent.

Another issue to consider when conducting removal studies is the scale of the actual restoration in contrast to the scale of our experimental treatments and management. Our study was done on a small, 1 x 1 m plot basis; however, restoration areas would be much larger. Erskine Ogden and Rejmanek (2005) found in the case of Foeniculum vulgare (fennel) that the scale of the treatment influences whether or not a treatment is successful. In their study, the small scale treatment effects were more pronounced than when the treatments were applied on a landscape scale. When F. vulgate was removed from larger areas, Mediterranean grasses increased, most likely because at this large scale there was more propagule pressure from these grasses. Given the amount of Euonymus alatus, Rosa multiflora and other invasive species in the areas surrounding our plots, it was not surprising that these species would increase with the removal of a species having large basal leaf rosettes.

It is a frustrating reality for land managers that restoration efforts often result in further exotic species invasion (Alvarez and Cushman, 2002; Erskine Ogden and Rejmanek, 2005; MacDougall and Turkington, 2005; Hulme and Bremner, 2006). However, realism is also needed when choosing an area for restoration. If a site is at the urban end of an urban-to-rural gradient, the propagule pressure from invasive species may be very great, and it could be unrealistic to expect this area to be as resistant to invasion as larger higher quality sites or as a rural site having fewer invasive species (Luken, 1997). Our study showed the importance of treating more than one exotic species while conducting restoration, because other exotic species may respond positively to the removal of the target exotic invasive species in such areas. Target plants must be removed repeatedly to prevent them from returning (Murphy et al., 2007), other invasive species must also be removed, and successfully removing invasive species from a small area surrounded by other invasive species may not be the best choice for a conservation goal. Instead, areas in which pressure from neighboring invasives can be curtailed and repeated removals of invasive species can be carried out while introducing native species may prove to be a better strategy for land managers seeking to restore degraded habitats.
APPENDIX 1.--Species list of plants recorded during the 3 y study
and how they were characterized for the native/exotic analysis

Native woody

Carya cordiformis                     bitternut hickory
Fraxinus americana                    white ash
Lindera benzoin                       spicebush
Parthenocissus quinquefolia           Virginia creeper
Prunus serotina                       black cherry
Quercus rubra                         red oak
Ribes cynosbati                       prickly gooseberry
Rubus flagellaris                     dewberry
Sassafras albidum                     sassafras
Sambucus nigra ssp.                   common elderberry
  canadensis
Smilax tamnoides                      bristly greenbriar
Toxicodendron radicans                poison ivy
Vitis riparia                         riverbank grape

Native forb

Acalypha rhomboidea                   three-seeded mercury
Arisaema dracontium                   green dragon
Bidens discoidea                      small beggarticks
Campanulastrum americanum             American bellflower
Chaerophyllum procumbens              spreading chervil
Circaea lutetiana                     enchanter's nightshade
Claytonia virginica                   spring beauty
Cryptotaenia canadensis               Canadian honewort
Dioscorea villosa                     wild yam
Floerkea proserpinacoides             false mermaidweed
Galium aparine                        cleavers
Geum canadense                        white avens
Hackelia virginiana                   stickseed/beggar's lice
Osmorhiza claytonii                   sweet cicely
Osmorhiza longistylis                 aniseroot
Pilea pumila                          clear-weed
Polygonatum biflorum                  smooth Solomon's seal
Polygonum virginianum                 jumpseed
Sanicula odorata                      clustered black
                                        snakeroot
Solidago ssp.                         goldenrod
Valerianella chenopodiifolia          Great Lakes corn-salad
Viola ssp.                            violet

Exotic woody

Berbens thunbergii                    Japanese barberry
Celastrus orbiculatus                 Oriental bittersweet
Euonymus alatus                       burning bush
Ligustrum vulgare                     European privet
Lonicera maackii                      Amur honeysuckle
Lonicera ssp.                         honeysuckle
Rosa multiflora                       multiflora rose

Exotic forb

Alliaria petiolata                    garlic mustard
Hesperis matronalis                   dame's rocket
Lamium purpureum                      purple deadnettle
Lysimachia nummulmia                  creeping jenny
Stellaria media                       common chickweed


Acknowledgments.--Field assistants included David Zaya and Gary Glowacki. We thank the National Park Service for permission to conduct this work at the Indiana Dunes National Lakeshore. Helpful reviews were provided by Walter Loope, Ellen Jacquart, Douglas Wilcox and two anonymous reviewers. This is contribution #1490 of the Great Lakes Science Center. Use of trade, product or firm names does not imply endorsement by the U.S. Government.

SUBMITTED 29 JANUARY 2008

ACCEPTED 29 JULY 2008

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NOEL B. PAVLOVIC, (1) STACEY A. LEICHT-YOUNG, KRYSTALYNN J. FROHNAPPLE AND RALPH GRUNDEL

United States Geological Survey, Lake Michigan Ecological Research Station, 1100 North Mineral Springs Road, Porter, Indiana 46304

(1) Corresponding author: Telephone: (219) 926-8336 x428; FAX: (219) 929-5792; email: npavlovic@ usgs.gov
TABLE 1.--Results of perMANOVA based on within and between
Sorenson dissimilarity of control vs. removal plots by year and season

Season/Year   Source      DF    SS      MS       F       P

Spring 2005   Treatment    1   0.066   0.066   0.768   0.642
              Residual    18   1.558   0.087
              Total       19   1.624
Summer 2005   Treatment    1   0.464   0.464   2.526   0.002
              Residual    18   3.306   0.184
              Total       19   3.77
Spring 2006   Treatment    1   0.245   0.245   1.542   0.099
              Residual    18   2.864   0.159
              Total       19   3.109
Summer 2006   Treatment    1   0.504   0.504   2.536   0.004
              Residual    18   3.576   0.199
              Total       19   4.079
Spring 2007   Treatment    1   0.515   0.515   2.818   0.005
              Residual    18   3.293   0.183
              Total       19   3.808
Summer 2007   Treatment    1   0.393   0.393   1.82    0.056
              Residual    18   3.884   0.216
              Total       19   4.277

TABLE 2.--Mean ([+ or -] SE) Sorenson dissimilarity values between
control and Hesperis removal plots by season and year. Means followed
by the same superscript letter do not differ significantly (P > 0.05),
using the Benjamini-Hochberg adjustment

Year                Spring                      Summer

2005       0.338 [+ or -] 0.034 (a)    0.608 [+ or -] 0.034 (c)
2006       0.479 [+ or -] 0.032 (b)    0.606 [+ or -] 0.038 (c)
2007       0.613 [+ or -] 0.036 (c)    0.618 [+ or -] 0.038 (c)
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Author:Pavlovic, Noel B.; Leicht-Young, Stacey A.; Frohnapple, Krystalynn J.; Grundel, Ralph
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2009
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