An International Journal ofEcology, Evolution and Environment

The Population Dynamics and Ecological Effects of GarlicMustard, Alliaria petiolata, in a Minnesota Oak Woodland

MARK A. DAVIS,1 ABBY COLEHOUR, JO DANEY, ELIZABETH FOSTER, CLAREMACMILLEN, EMILY MERRILL, JOSEPH O’NEIL, MARGARET PEARSON, MEGAN

WHITNEY, MICHAEL D. ANDERSON AND JERALD J. DOSCHDepartment of Biology, Macalester College, Saint Paul, Minnesota 55105

The Population Dynamics and Ecological Effects of GarlicMustard, Alliaria petiolata, in a Minnesota Oak Woodland

MARK A. DAVIS,1 ABBY COLEHOUR, JO DANEY, ELIZABETH FOSTER, CLAREMACMILLEN, EMILY MERRILL, JOSEPH O’NEIL, MARGARET PEARSON, MEGAN

WHITNEY, MICHAEL D. ANDERSON AND JERALD J. DOSCHDepartment of Biology, Macalester College, Saint Paul, Minnesota 55105

ABSTRACT.—Garlic mustard, Alliaria petiolata (M. Bieb.) Cavara & Grande, is an introducedbiennial forb that has commonly been referred to as highly invasive and as having substantialnegative effects on other plants in the eastern deciduous forests of North America. However,several recent studies have documented only modest effects on other plant species, raisingquestions as to the extent of the threat really posed by A. petiolata. Alliaria petiolata oftenexhibits an alternating two-year life-history cycle, with high rosette years alternating with highflowering stem years. It has been proposed that this cycle is partly driven by intraspecificcompetition between the stems and the rosettes. In a two-year study, we extensively sampledA. petiolata in a Minnesota woodland at two spatial scales, including 6.5 km of belt transects ina 6.8 ha study grid (20 3 20 m cells) and 90 small sampling quadrats (1.0 3 0.5 m) within thegrid. At the large scale, we compared seed bank abundance and diversity of other herbaceousplants with A. petiolata abundance. Using the monitoring data we also investigated whetherthis population was exhibiting an alternating two-year life-history cycle, consistent with theintraspecific competition hypothesis for this phenomenon. At the small scale, we comparedA. petiolata abundance with the abundance of other plants, including herbs, ferns, shrubs, andtree seedlings. We also conducted an ex-situ pot experiment in which we planted seeds of sixtree species in soil collected from dense A. petiolata patches and soil collected where A.petiolata was absent and recorded emergence rates and seedling growth over an 8 wk period.Overall, we found little evidence that A. petiolata was negatively affecting other plant species.This is consistent with other recent studies and indicates that, despite earlier claims to thecontrary, A. petiolata seems to be more a product than an agent of change in eastern NorthAmerican deciduous forests. We also documented an alternating two-year life-history cycle,providing additional evidence to support the hypothesis that this cycle is at least partly beingdriven by intraspecific competition.

INTRODUCTION

Garlic mustard, Alliaria petiolata (M. Bieb.) Cavara & Grande, is a European forb that wasintroduced into North America in the 1800s and has spread widely throughout forestedhabitats in the central and eastern U.S. Alliaria petiolata is typically described as a highlyinvasive species, which negatively impacts native herbs and tree seedlings in a variety of ways,including outcompeting native herbs and reducing their abundance and diversity (Nuzzo,1993; Stinson et al., 2007; Rodgers et al., 2008) and inhibiting the growth of tree seedlingsand herbs through an allelopathic effect on mutualistic mycorrhizae (Roberts andAnderson, 2001; Stinson et al., 2006; Callaway et al., 2008; Wolfe et al., 2008; Andersonet al., 2010). In recent years, control and eradication programs have been commonthroughout its range, including the use of fire, herbicides, and manual removal (Nuzzo,1991), the latter often carried out by community groups mobilized to engage in garlicmustard ‘pullathons.’

1 Corresponding author: Telephone: (651) 696-6102; FAX: (651) 696-6443; e-mail: davis@macalester.edu

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Despite this common portrayal of Alliaria petiolata, recent studies have been inconsistentin their findings, and even when negative effects have been documented, they often havebeen quite modest. For example, in a multiple-site study in Minnesota hardwood forests,Van Riper et al. (2010) reported that ‘‘a negative correlation was found between A. petiolatacover and native species in 2006, but not in 2007 or 2008.’’ In addition, the authors reportedthat ‘‘there were also consistent negative correlations between total A. petiolata cover in thespring (adults + seedlings) and native species percent cover in the spring, although P valueswere nonsignificant. (emphasis added).’’ In their study of A. petiolata and its effects, Stinsonet al. (2007) reported that while the Shannon diversity index was negatively associated withgarlic mustard the ‘‘species richness (of other plants) did not respond to natural orexperimental levels of invasion.’’ Nuzzo et al. (2008) studied three non-native species, one ofwhich was A. petiolata in fifteen forests in New York and Pennsylvania (five forests for eachspecies) in order to test, among other things, the hypothesis that ‘‘increased non-nativeplant cover is associated with reduced diversity and cover of native plant species.’’ Theauthors concluded, ‘‘Invasive plants are conspicuous and are often assumed to be the agentsof change; however, our study in conjunction with the work of others indicates that …invasive plants are the beneficiaries, rather than the agents, of change.’’ Rogers et al. (2008)and Rooney and Rogers (2011) reached similar conclusions regarding A. petiolata and twoother non-native plants (Rhamnus cathartica and Lonicera 3 bella) based on analyses ofvegetation change in more than ninety Wisconsin forest stands.

A biennial, Alliaria petiolata has been documented as often exhibiting an alternating two-year life-history cycle, in which the first year rosette stage and the second year flowering stemstage alternate dominance year to year (Van Riper et al., 2010; Pardini et al., 2009). It hasbeen proposed that intraspecific competition may partly drive this cycle, with rosettesexperiencing intense shoot competition during years when stems are very abundant,resulting in few rosettes amassing enough energy to produce a stem the following year(Winterer et al., 2005). While a reasonable hypothesis, and supported via modeling (Pardiniet al., 2009), clear field evidence in support of the driving role played by intraspecificcompetition has been only recently reported (Winterer et al., 2005; Bauer et al., 2009;Herold et al., 2011).

The first purpose of our study was to evaluate at multiple scales the relationships betweenAlliaria petiolata and other plants in an oak woodland in eastern Minnesota in order todetermine whether the data are more consistent with A. petiolata being an agent orbeneficiary of change at this site. In addition we wanted to document whether or not A.petiolata at this site is exhibiting an alternating two-year life-history cycle, and if so, whetherthe findings are consistent with the intraspecific competition hypothesis.

METHODS

Study site.—We conducted this study at Macalester College’s field station, the KatharineOrdway Natural History Study Area, located in Inver Grove Heights in eastern Minnesota(44u489280N, 93u019310W). Situated on the bluffs of the Mississippi River, the field stationcontains a variety of habitats, including grasslands, several wetlands, riparian habitat, abackwater lake, and a woodland, the latter of which was the site of this study. The woodlandis dominated by oaks (Quercus rubra, Q. ellipsoidalis, Q. alba, Q. macrocarpa); however, theabsence of fire in most of the woodland during the past half century has resulted in theestablishment of a number of fire susceptible tree species, which now make up the majorityof the understory, the most common being Ulmus americana, U. rubra, Acer negundo, Prunusserotina, Tilia americana, and Fraxinus pennsylvanica. A 2001 vegetation survey of the

2012 DAVIS ET AL.: GARLIC MUSTARD DYNAMICS AND EFFECTS 365

woodland recorded only very few small patches of Alliaria petiolata (Karen Schik, pers.comm.). Although one cannot rule out the possibility that A. petiolata may have been presentin the woodland at very low densities for a number of years prior to 2001, it is clear that anysignificant spread throughout the woodland has occurred only during the past decade.Today, the two most abundant herb species (based on cover) in the woodland study areaare Desmodium glutinosum and A. petiolata, with other common herbs being Amphicarpaeabracteata, Geranium maculatum, Circaea lutetiana, Ageratina altissima, and Parthenocissus spp.Several species of Rubus and Ribes are common shrubs and common fern species includeAthyrium filix-femina, Osmunda claytoniana, and Adiantum pedatum. Besides A. petiolata, othernon-native species in the woodland include Rhamnus cathartica and Lonicera tatarica.

Large scale data collection.—In summer 2010, we set up a 2.84 ha study grid (seventy-one 203 20 m cells with vertices defined by UTM (Universal Transverse Mercator) coordinates. Weexpanded the grid to 6.08 ha in 2011 (152 total cells). In 2010, we estimated Alliaria petiolatacover in the 71 contiguous cells using the line-intercept method, with three parallel lines foreach cell: two running north-south along opposite sides of the cell and a third running inthe same direction through the middle of the cell. For each cell, the total lengths of the linesegments intersecting with A. petiolata plants in the three transects were summed anddivided by 60 (total of 60 m of lines) to yield A. petiolata cover. In addition to cover, thenumber of flowering stems intercepted by the three transects of a cell was recorded andused as a measure of flowering stem frequency for the cell. In summer 2011, we increasedthe sampling portion of the grid to 152 cells and we used the same line-intercept method tosample A. petiolata in all 152 cells, for a total of 6.5 km of transects. JMP software (v. 9.0) wasused to calculate the Spearman’s rank correlation (r) to assess associations among variablesand between years.

To document possible large-scale spatial relationships between Alliaria petiolataabundance and the abundance and diversity of other plant species, we conducted a seedbank study in the grid. In late May 2011 (after A. petiolata germination and first-yearemergence), we collected soil samples from all 152 cells, with five standardized soilcollection sites in each cell. At each collection site, approximately 1 L of soil was removed toa depth of 15 cm. (Soil samples collected in prior studies of A. petiolata have been made atsimilar depths, e.g., 10–13 cm, Anderson et al., 2010). The soil from the five sampling areaswas mixed together and then 2.5 L of the mixed soil was spread in a 2 cm layer over a 2.5 cmlayer of autoclaved potting soil in 52.5 3 25.5 cm trays. The soil was kept moist and the traysexposed to fluorescent lights using a day-night cycle that was appropriate for early summerat the study site. After 8 wk, the number of emerged seedlings in each tray (cell) wascounted as well as the number of species in the tray. Species were identified when possible.If a seedling could not be identified, it was considered a unique morphospecies andcontributed to the species richness of the tray. The number and species richness of theseedlings in soil collected from the cells were compared with the estimated total A. petiolatacover (in 2010 and 2011), as well as with the change in the abundance of A. petiolata from2010 to 2011 in the respective cells using Spearman’s rank correlation.

Small scale data collection.—In summer 2010, using the transect data, we identified ten cellsthat encompassed a wide range of Alliaria petiolata abundance (from zero to 85% cover). Ineach of these ten cells, we established nine permanent quadrats (1.0 3 0.5 m), one in eachcorner, one at the midpoint of each of the four sides, and one in the middle. In both 2010and 2011, all species in the quadrats were identified and their percent cover recorded.Recorded species included A. petiolata, other herbs, shrubs, ferns, and tree seedlings. Inaddition, the number of A. petiolata rosettes and stems was recorded. The three A. petiolata

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abundance metrics for each year were compared with the same year’s cover and richness ofeach of the other groups: herbs (excluding A. petiolata), shrubs, ferns, and tree seedlings.Comparisons also included change in these variables between 2010 and 2011.

Because effects of Alliaria petiolata on individual species are not likely to be evident inanalyses of community level variables (e.g., species richness and total cover of other plants),we also examined correlations between all three A. petiolata metrics in both years and thecover of the five most abundant (highest total cover) herb species in the quadrats:Desmodium glutinosum, Circaea lutetiana, Geranium maculatum, Amphicarpaea bracteata, andAgeratina altissima. Herold et al. (2011) describe Geranium maculatum as a spring dominant, aguild which was documented to be sensitive to presence of A. petiolata in their study. Toexamine the possibility of effects on this guild, we also included the only other springdominant species from Herold et al. (2011) that occurred in our quadrats, Geum canadense,in the single-species analysis.

All comparisons were conducted using nonparametric bivariate regressions and Spear-man’s rank correlation in JMP version 9.0, with Bonferroni-adjusted P-values to correctfor multiple comparisons (240 separate bivariate regressions for community data; 36comparisons for individual species).

Ex-situ experiment.—Alliaria petiolata has been reported to negatively affect tree seedlingsvia allelochemical effects on mycorrhizae (Roberts and Anderson, 2001; Callaway et al., 2008;Wolfe et al., 2008). In order to test whether this effect might be occurring in the woodland,we collected soil from one of two types of patches (minimum size 5 10 m2): patches whereA. petiolata was abundant (GM), and patches where A. petiolata was not present (NGM). Thetwo types of collecting sites were spatially interspersed with one another. Since A. petiolata isa biennial, it is possible that in the past it did grow in what are currently non-A. petiolatapatches. However, given our observations in the ninety quadrats, flowering stems are likelyto produce large numbers of first-year rosettes in their immediate vicinity, some of whichwould produce new flowering stems, which would produce more rosettes, etc. Alliariapetiolata removal experiments have similarly shown that once the species has occupied anarea several square meters in size, it continues to persist at the site, even in spite of removalefforts (Anderson et al., 2010). Thus, while we cannot be sure A. petiolata has never grown inour designated non-A. petiolata sites, it is unlikely that the non-A. petiolata sites had beenrecently occupied by A. petiolata. Moreover, given that A. petiolata likely arrived only ten yearsago and is currently in the midst of spreading throughout the woodland, it is quite likelythat the species never occupied the majority of the NGM soil collection patches.

Four seeds of six different tree species (Quercus rubra, Q. macrocarpa, Ulmus Americana,Fraxinus pennsylvanica, Prunus serotina and Betula papyrifera) were planted into 0.95 L pots(one species per pot) of each kind of soil (GM or NGM). To determine the minimum fieldsoil concentration necessary to detect any difference between the soil types, both the GMand the NGM soil were diluted into four different levels using autoclaved potting soil, sothat the pots consisted of GM soil at levels of 25%, 50%, 75%, and 100%. Forty-eight potswere used for each species (12 replicates for each soil concentration level) for a total of 288pots. These pots were placed on tables in the field station laboratory next to a room-lengthbank of windows, which faced southeast. The pots were rotated daily to control for varyinglight conditions at different locations on the tables. Pots were regularly watered and werechecked every Monday, Wednesday, and Friday for any seedling emergence over the courseof 8 wk. At the end of the 8 wk experiment, seedling stem height, number of leaves, andaverage leaf length were measured, counted, and recorded for each seedling. Because wehad multiple dependent variables which could be intercorrelated, data were analyzed using

2012 DAVIS ET AL.: GARLIC MUSTARD DYNAMICS AND EFFECTS 367

MANOVA with soil type (GM and NGM) and field soil concentration as independentvariables. Separate analyses were performed for each seedling species using JMP version 9.0,with the identity matrix set as the m-matrix option. We examined all possible models (maineffects of both independent variables plus the interaction term, both main effects together,and each main effect separately). When a MANOVA model was significant, univariateANOVA was performed for each response variable, with Tukey’s HSD as a post-hoc analysis,when necessary. Prior to analysis, response variables were examined graphically fornormality, and equality of variances was tested for both independent variables (O’Brien,Brown-Forsythe, Levene, and Bartlett tests in JMP version 9.0) to ensure the data met theassumptions of MANOVA. Separate analyses were run with all data, and with a smallerdataset which excluded possible outliers (determined as points lying outside the quantileplot), to which MANOVA is sensitive (Quinn and Keogh, 2002). In no case did analysis ofthe two datasets lead to different conclusions.

RESULTS

Large scale population dynamics of Alliaria petiolata.—Alliaria petiolata cover in most cells inthe study grid increased between 2010 and 2011 (Fig. 1). At the same time, the number offlowering stems declined dramatically throughout the study area (Fig. 2), meaning that theincrease in cover in 2011 was due to a substantial increase in the cover of rosettes. The 2011cover in a cell, which consisted largely of rosettes, was very strongly correlated with thenumber of stems in the cell in 2010 (r 5 0.726, P , 0.0001, n 5 71, Fig. 3). The change inthe number of stems in a cell between 2010 and 2011 was inversely correlated with thenumber of stems in cells in 2010 (r 5 20.932 P , 0.0001, n 5 71, Fig. 4). As shown inFigure 4, only cells with relatively few number of stems in 2010 exhibited an increase in thenumber of stems in 2011.

FIG. 1.—The percent cover of Alliaria petiolata in the 71 cells sampled in 2010 compared to the coverof A. petiolata in the same cells in 2011, showing a marked increase in cover in 2011 compared to 2010.(The 10 cells with cross-hatching are the cells containing the 1.0 3 0.5 m quadrats)

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Large scale seed bank study.—The species richness of the plants emerging from a cell’s soilwas positively correlated with the change in the cover of Alliaria petiolata from 2010 to 2011(r 5 0.325, P 5 0.024, n 5 71, Fig. 5). The number of seedlings from all species thatemerged from a cell’s soil in 2011 was marginally positively correlated with the cell’s 2011 A.petiolata cover (P 5 0.096, n 5 152).

Small scale quadrats.—Out of 240 bivariate regressions between the three Alliaria petiolatametrics and other plant species, including cover and richness of the different plant groupsand changes in these variables from 2010 to 2011, none were significant when adjusted formultiple comparisons. (Unadjusted P-values were only significant for four of the 240regressions, considerably fewer than the 12 that would be expected by chance.) Single-species comparisons yielded significant correlations (adjusted for multiple comparisons) foronly one of the six species examined: Ageratina altissima, which was significantly positivelycorrelated with A. petiolata cover (r 5 0.348, P 5 0.029) and rosette number (r 5 0.341, P 5

0.036) in 2011, and marginally positively correlated with number of A. petiolata stems (r 5

0.325, P 5 0.065), rosettes (r 5 0.303, P 5 0.13), and cover (r 5 0.319, P 5 0.079) in 2010.Pot seedling experiment.—Seedling emergence for Prunus serotina and Betula papyrifera was

too low to permit any analysis. There was no difference between seedlings grown in GM andNGM soil in time to emergence for any of the other four species. For the three post-emergence seedling metrics (height of seedlings, number of leaves, mean leaf length)MANOVA indicated no significant effects of either soil type (GM vs. NGM) or soil dilution

FIG. 2.—Change in the number of Alliaria petiolata flowering stems recorded in the belt transects from2010 to 2011 for all 71 cells. As shown, there were far fewer flowering stems in 2011 than in 2010

FIG. 3.—The number of rosettes recorded in the 90 quadrats (1.0 3 0.5 m) in 2011 shown as afunction of the number of flowering stems recorded in the respective quadrats in 2010. The data weresignificantly correlated based on a Spearman Rank analysis (r 5 0.726, P , 0.0001) as well as based onthe regression analysis shown

2012 DAVIS ET AL.: GARLIC MUSTARD DYNAMICS AND EFFECTS 369

for Quercus rubra, Q. macrocarpa, and Ulmus americana. In Fraxinus pennsylvanica MANOVA alsoindicated no effect of soil dilution, but a marginally significant soil type effect (exact F 5 2.72,P 5 0.056, Fig. 6), in which the performance of F. pennsylvanica seedlings in Alliaria petiolatasoil was reduced by 9% in terms of mean leaf length (2-tailed P , 0.04), by 12% in terms ofstem height (2-tailed P 5 0.02, and by 12.5% in terms of leaf number (2-tailed P , 0.05).

DISCUSSION

We found very little evidence that Alliaria petiolata is negatively affecting other plantsgrowing in this oak woodland. When we did find statistically significant relationshipsbetween A. petiolata abundance and the abundance and species richness of other plants(when all species growing under natural conditions), the associations were more likely to bepositive. This is just one in a series of recent studies that have documented very small,inconsistent, or no negative effects of A. petiolata on other plant species in the eastern NorthAmerican forests and woodlands (Stinson et al., 2007; Nuzzo et al., 2008; Rogers et al., 2008;Bauer et al., 2010; Van Riper et al., 2010; Herold et al., 2011; Rooney and Rogers, 2011).

FIG. 4.—The percent relative change in the number of Alliaria petiolata flowering stems recorded inthe line-intercept transects from 2010 to 2011 for the 71 cells shown as a function of the number ofstems in the respective transects in the cells in 2010. The data were significantly correlated based on aSpearman Rank analysis (r 5 20.923, P , 0.0001) as well as based on the regression analysis shown

FIG. 5.—The number of species that emerged from the soil collected in each of the 71 cells shown as afunction of the change in the cover of garlic mustard (GM) in the respective cells between 2010 and2011. The data were significantly correlated based on a Spearman Rank analysis (r 5 0.325, P 5 0.024)as well as based on the regression analysis shown

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It is not clear why earlier studies of Alliaria petiolata seemed to consistently documentsignificant negative effects on other plant species (e.g., McCarthy, 1997; Meekins andMcCarthy, 1999; Prati and Bossdorf, 2004; Stinson et al., 2006). One possibility is that resultsshowing minimal effects were seldom published. Another is that the effects of A. petiolatamay have changed over time. Diez et al. (2010) found increasingly negative soil feedbacks onnon-native plants the longer a non-native plant species had been established, and also thatthe negative soil effects were stronger for more widespread species. It is possible that A.petiolata populations may become similarly compromised over time, thereby reducing itsability to negatively affect other species. Lankau et al. (2009) documented a decline inallelochemical concentrations (glucosinolate and alliarinoside) over time in A. petiolatapopulations, and suggested that while high glucosinolate concentrations may be favored byinterspecific competition, natural selection may switch during periods of high intraspecificcompetition (e.g., when A. petiolata is very abundant in an area) and favor reducedproduction of allelochemicals. This may then translate into reduced impacts on native plantspecies dependent on soil mutualists (Lankau et al., 2009). While the development of negativesoil feedbacks and/or a decline in allelochemical concentrations over time might explain the

FIG. 6.—The growth metrics of green ash (Fraxinus pennsylvanica) seedlings (stem height, no. ofleaves, and mean leaf length) shown for seedlings grown in soil collected from dense Alliaria petiolatapatches (GM) and from areas where A. petiolata was absent (NGM)

2012 DAVIS ET AL.: GARLIC MUSTARD DYNAMICS AND EFFECTS 371

paucity of documented adverse effects of A. petiolata at sites inhabited by A. petiolata for a longperiod of time, e.g., many decades, it is unlikely that this phenomenon explains the findingsreported here since A. petiolata established in the study site only recently.

It is important to recognize that outside of the seedling pot experiment, it is not possibleto attribute any causal relationships between the patterns of abundance and richness wedocumented in the seed bank study. Thus, the positive associations between Alliaria petiolataand other species cannot be interpreted as A. petiolata facilitating or benefiting in some waythe other plant species. For example, the fact that an increase in A. petiolata abundance in acell between 2010 and 2011 was positively associated with the species of other species thatemerged from the cell’s soil in no way means that A. petiolata is actually benefitting theseother species. The positive association may simply be due to the fact that site suitabilityvaries across the landscape and that A. petiolata and most other plant species all respondsimilarly to the changes in site suitability.

Similarly, although we found little evidence that other plant species are being adverselyaffected by Alliaria petiolata, this cannot be interpreted to mean that A. petiolata has, or hashad, no negative effects on any species. For example, while the pot experiment showed verysmall to no effects of A. petiolata soil on recently emerged tree seedling performance, theseedlings were grown under ideal conditions, with plenty of water and sunlight. In a similarstudy (Davis et al., 2005), Quercus rubra seedlings grown in native grass soil and non-nativegrass soil performed equally well if the soils were well watered. However, if the plants weredrought stressed, the seedlings performed much worse in the non-native grass soil (Daviset al., 2005). In a similar way, it is possible the soil effects of A. petiolata on tree seedlingsmight be greater if the seedlings are stressed in some way.

With respect to the lack of evidence of adverse effects in the naturally growing plants, it ispossible that species negatively affected by Alliaria petiolata already had been excluded fromthe site, leaving only species more resistant to A. petiolata. However, this seems unlikely inthis case, given that A. petiolata arrived at the site only slightly more than a decade ago and isstill spreading throughout the woodland.

Our data clearly showed that Alliaria petiolata exhibited the alternating two-year life-historycycle in the study site during the 2 y of study, with stems dominating in the first year of thestudy and rosettes in the second year. At the same time, the data showed that small areasmay be cycling out of sync with the larger scale dynamics. If intraspecific selection is helpingto drive this cycle, one would expect that the cycle would be more strongly exhibited in highstem density areas than in low stem density areas (Pardini et al., 2009). As we reported, thispattern was exhibited by our data, which showed that the number of A. petiolata stems in acell in 2010 is inversely associated with the change in stem number in the cell between 2010and 2011 (Fig. 4). These data are consistent with other recent findings (Bauer et al., 2009;Herold et al., 2011) and support the hypothesis that intraspecific competition helps to drivethe alternating two-year life-history cycle in A. petiolata.

Acknowledgments.—We thank associate editor Roger Anderson and two anonymous reviewers for theirvery helpful comments. Dan Hornbach assisted with some of the statistical analyses. This research wassupported by the Duane Roberts Field Biology Study Fund, the Louis Daniel Frenzel Jr. EndowedScholarship, the Andrew W. Mellon Foundation and Macalester College.

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SUBMITTED 15 AUGUST 2011 ACCEPTED 31 JANUARY 2012

374 THE AMERICAN MIDLAND NATURALIST 168(2)