2007 NORTHEASTERN NATURALIST 14(1):73–88
Impacts of Garlic Mustard Invasion on a Forest
Kristina Stinson1,*, Sylvan Kaufman2, Luke Durbin3,
and Frank Lowenstein4
Abstract - To assess the community-level responses of a New England forest to
invasion by the Eurasian biennial Alliaria petiolata (garlic mustard), we conducted a
vegetation census at twenty-four plots ranging from low to high invasive cover, and
experimentally removed 0, 50, or 100% of garlic mustard from adjacent highly
invaded plots at the same study site. Species richness did not respond to natural or
experimental levels of invasion, but the Shannon diversity and equitability indices
declined with increasing in situ densities of garlic mustard, and increased in response
to removal of garlic mustard at the experimental plots. Individual species demonstrated
variable responses to high-, intermediate-, and low-level invasion. Of all
plant functional groups, tree seedlings declined most notably with increasing in situ
levels of invasion. This functional group, and seedlings of three key canopy tree
species within the group, increased in response to partial, but not full eradication of
garlic mustard. Our results demonstrate that the effectiveness of full or partial
removal depends on management priorities for promoting overall diversity, species
richness, native species composition, and/or individual species performance within
Although it is often assumed that exotic plant species alter the structure
and biodiversity of the resident communities they invade, we know surprisingly
little about their effects on native flora (e.g., Alvarez and Cushman
2002). Relationships between native plant diversity and invasion have been
well studied within the context of invasibility (Brown and Peet 2003, Davis
et al. 2000, Dukes 2002, Kennedy et al. 2002, Knops et al. 1999, Levine
2000, Lyons and Schwartz 2001, Naeem et al. 2000, Prieur-Richard and
Lavorel 2000a), drawing upon Elton’s (1958) hypothesis that higher native
species diversity confers greater community-level resistance to invasion. It
is now well accepted that resource availability and disturbance regimes also
determine resistance or invasibility of communities (Brooks 2003, Dukes
and Mooney 1999, Gordon 1998, Vitousek 1986). Very few studies have
focused on whether increasing levels of invasion have negative relationships
with native community richness, diversity, and species composition (see
Alvarez and Cushman 2002, Levine et al. 2003 and references therein, Vlok
1988). The majority of these use observational data to compare invaded
1Harvard University, Harvard Forest, 324 North Main Street, Petersham, MA 01366.
2Adkins Arboretum, Ridgely, MD 12610. 3Illinois Wesleyan University,
Bloomington, IL 61701. 4Forest Conservation Program, The Nature Conservancy,
Sheffield, MA 02157. *Corresponding author - firstname.lastname@example.org.
74 Northeastern Naturalist Vol. 14, No. 1
communities to their uninvaded counterparts (Levine et al. 2003) and focus
on heavily invaded versus uninvaded systems (D’Antonio and Kark 2002,
Mooney and Drake 1986, Parker et al. 1999). Thus, there is a need for
observational and experimental data on the effects of invasive plants on
native communities across a range of infestation levels. Experiments that
use both full and partial eradication methods can empirically answer fundamental
ecological questions about how invasion levels affect native
communities, while simultaneously demonstrating how well a given removal
strategy can achieve specific management goals.
Here, we join two years of correlative data with experimental field
manipulations to assess the effects of increasing levels of infestation by an
exotic plant on a New England forest understory community. Alliaria
petiolata Bieb., Cavara, and Grande (garlic mustard) is a Eurasian forb
increasing in density within forest-edge and understory habitats throughout
much of North America (Nuzzo 1993, Welk et al. 2002). Because invasion
by exotic plants into intact communities, such as the forest understory, is
less common than invasion into disturbed sites (Von Holle et al. 2003),
garlic mustard’s unusual capacity to enter and proliferate within the intact
forest community has prompted research on its interaction with understory
vegetation (McCarthy 1997, Meekins and McCarthy 2001, Nuzzo 2000).
While it has been implicated as a cause of reduced native plant diversity and
native plant performance (Anderson et al. 1996, McCarthy 1997, Nuzzo
1993, Welk et al. 2002), only one other study has directly tested this idea in
the field (McCarthy 1997). A recent greenhouse study suggested that garlic
mustard competes with seedlings of some but not other canopy tree seedlings
in the midwestern United States (Meekins and McCarthy 1999). No
data exist about this species’ impact on forest communities in New England,
where it ranges from low to very high densities in both disturbed and
wooded areas throughout New York, Connecticut, Massachusetts, and Vermont
(Nuzzo 2000). At a forested site in western Massachusetts, we tested:
(1) whether different degrees of invasion by garlic mustard resulted in
declines in indices of community-level diversity; (2) whether and how full
and partial eradication of garlic mustard affected indices of diversity and
community structure; and (3) whether native species, particularly tree seedlings,
differed in their responses to garlic mustard.
Garlic mustard is an obligate biennial, sexually reproducing forb. Flowers
are typically borne on one or more stalks (Nuzzo 2000). Plants breed
primarily via self-pollination, but outcrossing via insect-pollination can also
occur (Anderson et al. 1996). Seeds germinate in early spring, and seedlings
progress to an evergreen rosette form during the first year of growth. Second
year plants begin flowering in early summer, regardless of size, and subsequently
die after seed production (Byers and Quinn 1998, Cavers et al.
2007 K. Stinson, S. Kaufman, L. Durbin, and F. Lowenstein 75
1979). Seeds are borne in linear capsules (siliques), are dispersed by gravity,
water, and soil disturbance, and require overwintering prior to germination
(Cavers et al. 1979). Some flowering stalks can reach over 100 cm in height,
and a single plant can produce hundreds of seeds. Garlic mustard was
introduced into the northeastern United States from Europe in the mid-1800s
(Nuzzo 1993), and its native habitat consists of hedges, semi-shaded floodplain,
and forest-edge sites, preferring intermediate light and soil moisture
levels to extremely shaded or dry sites (Dhillion and Anderson 1999,
Meekins et al. 2001). In the last two decades, garlic mustard has undergone
rapid population explosions throughout North America, and has become
increasingly widespread in forest-edge, riparian, and forest habitats (Nuzzo
2000, Welk et al. 2002).
The Berkshire Taconic Landscape (BTL) is a 120,000-acre area located
on the borders of Connecticut, Massachusetts, and New York, and is
representative of mixed hardwood forests throughout the Lower New England-
Northern Piedmont ecoregion. Our study site is situated in a largely
unfragmented forest of about 36,000 acres at the heart of BTL, on a lower
slope of the Taconic Mountains in rich maple woods (42º07'20"N,
73º42'38"W, approx 250 m elevation). The canopy is dominated by native
species, primarily Acer saccharum Marsh. (sugar maple), with a mixture of
Fraxinus americana L. (white ash), Acer rubrum L. (red maple), and Prunus
serotina Ehrh. (black cherry). Soils generally consist of patchy quaternary
sediments and glacial till with calcareous bedrock. Mean annual precipitation
is 114 cm, mean monthly temperatures range from -6 ºC (Jan) to 21 ºC
(July), with freezing temperatures between November and March and typical
growing seasons from March–November (www.weather.com/weather/
climatology/monthly/01257). Garlic mustard has begun to successfully invade
both forest edges and woodland habitats in this area. At some local
sites, it has attained between thirty and sixty percent cover in the understory
(K. Stinson and S. Kaufman, pers. observ.).
We implemented a stratified random sampling design to establish sixteen
4- x 4-m study plots throughout the study area, where our overall estimates of
garlic mustard cover ranged from 0–35%. We established four strata consisting
of areas with: high (over 30%), medium (10–30%), low (less than 10%),
and uninvaded (0%) cover of garlic mustard. Within each of these areas, a
random-number chart was used to determine the midpoint locations of four 4-
x 4-m plots, for a total of 16 plots. We conducted vegetation censuses at each
plot in June 2002, August 2002, and June 2003 for the following functional
group categories: herbaceous plants, tree seedlings, and shrubs. Using a 1-m2
grid for accuracy, we recorded percentage cover on a square-meter basis for
each functional group and for garlic mustard. Percentage-cover levels were
averaged across grids to generate plot-level percentage-cover means. The
76 Northeastern Naturalist Vol. 14, No. 1
plot-level percentage-cover levels of garlic mustard ranged from 0–37%. We
identified to species all stems within the functional group categories mentioned
above. We measured species abundance as the number of stems for
each species in each 1-m2 grid cell and tallied the cell counts to generate plotlevel
species abundance totals for each plot.
In order to segregate effects of physical environment from those of garlic
mustard invasion on the native plant community, we collected environmental
data at each census plot during midsummer 2002. We measured soil moisture
at six locations per plot using a Delta-T type ML2x Theta Probe (Delta-T
Devices Ltd, Burwell, Cambridge, UK) on a single, overcast day in July. On
the same day, eight soil samples were taken from directly beneath the loose
litter layer and then combined into one sample per plot and sent to the
University of Massachusetts Soil and Plant Tissue Testing Laboratory in
Amherst, MA for analysis. Soil nutrients were determined in ppm along with
soil pH and cation exchange capacity (MEQ/100g). We measured average
incident light intensity in volts at 100 cm from the forest floor at eight
locations within each plot using a LI-COR 1600 photometer (LI-COR Inc.,
Lincoln, NE) on a single, overcast day in August. We also recorded slope and
aspect for each plot, and measured litter depth at eight randomly selected
points at each plot. Measurements for each plot were pooled into plot-level
average values for analysis.
To experimentally test the effects of varying degrees of garlic mustard
invasion on the understory community, we imposed full and partial removal
treatments in a heavily invaded forest understory area (30–35%
garlic mustard cover levels). Within this area, twelve 4- x 4-m plots,
consisting of nine 1-m2 subplots and a 0.5-m wide buffer, were randomly
selected as described above in June 2002. All plots were fenced so that
potential herbivory by local deer populations would not disrupt the treatments.
Four plots were each subjected to three treatments in 2002: full
removal of garlic mustard, partial removal of garlic mustard, and no removal.
Full removal was achieved by pulling all first- and second- year
plants from each of the nine subplots. Partial removal was achieved by
thinning subplots to 50% of original cover of garlic mustard. Additional
garlic mustard plants were pulled in May 2003 to maintain the treatment
levels. We conducted a diversity census in June 2003, in which we identified
and counted all plants per plot, as described above. We calculated
species richness, Shannon equitability index, and Shannon diversity index
for each experimental and control plot. We compared diversity indices and
environmental variables in the removal treatments to those within four
unfenced, heavily-invaded plots and four unfenced, uninvaded plots in
order to detect possible differences between sites with and without natural
deer herbivory. The uninvaded plots also allowed us to assess similarities
between our experimental removals and intact, non-manipulated sites and
to evaluate management potential of our treatments.
2007 K. Stinson, S. Kaufman, L. Durbin, and F. Lowenstein 77
Using the methods described above, we measured soil moisture and light
during June and August 2003 within each 1-m2 subplot of all fenced and
unfenced study plots. To capture vertical light profiles along different strata of
the understory vegetation, we measured incident light intensity at three
heights in the forest understory (0, 50, and 100 cm). We also recorded daytime
soil temperature using a Tenma dual-input thermometer with K-type thermocouples
(MCM Electronics, Centerville, OH) at each point. From these data,
we generated plot-level means for early and late summer conditions.
Plants were classified at the species level and by functional groups (tree
seedlings, shrubs/vines, graminoids, and forbs) for analysis. Species richness
(S), Shannon diversity (H') and Shannon equitability (J), of native
species were calculated from vegetation census data at our observation and
experimental plots as follows: S = the total number species per plot; H' =
Pi ln Pi , where Pi = # individuals per species/total number of individuals
in the community; and J = H'/ln S.
We used stepwise regression methods (SAS REG procedure) to test for
effects of environmental variables and garlic mustard presence on the density
(# individuals per m2), percent cover (% cover per m2), relative abundance of
species (# stems within single species/total # stems for all species), relative
abundance of functional groups (% cover for a given functional group/total %
cover for all functional groups), and diversity indices (S, H', and J) at our
census observation plots. This approach allowed us to determine the effects of
garlic mustard on community responses while accounting for environmental
variation among our observation plots. We employed linear regression methods
(SAS REG) to test for relationships between garlic mustard density and
the abundances of tree seedlings, graminoids, forbs, and shrubs.
We tested for the effects of our removal treatments on native plant
species diversity, species equitability, species richness, and abundances of
functional groups and key species, as defined above, using an analysis of
variance (ANOVA) model with treatment (full, partial, or no removal) as a
fixed effect. We analyzed differences between treatments in the percent
cover of garlic mustard at the time of the census. We tested for differences in
environmental variables and community diversity (H', J, and S) between our
experimental plots and those undergoing natural deer herbivory using a twoway
ANOVA, with invasion level (high or low) and fencing (fenced or
unfenced) as the main effects. By comparing relative abundances with and
without garlic mustard, we separated the effects of our treatments on proportional
changes in invader abundance from those on proportional changes in
the native flora itself. Means for early (June) and late (August) summer
environmental measurements (light, soil moisture, and soil temperature)
were estimated separately on each date of observation for each plot. The
effects of removal treatment and time on these non-independent observations
were analyzed using repeated measures ANOVA with treatment as the
fixed main effect. For light measurements, the effect of treatment was tested
78 Northeastern Naturalist Vol. 14, No. 1
against the height x treatment variance. The treatment x time interaction was
tested against the height x treatment x time effect.
Overall species richness (S) did not change with respect to garlic mustard
abundance (Fig. 1A). However, species diversity (H') and species equitability
(J) declined with increasing garlic mustard abundance (Figs. 1B–C). The linear
regressions between garlic mustard cover and species diversity were negative,
as were those between garlic mustard density and the dependent variables
species equitability, tree seedling relative abundance, and graminoid relative
abundance (Table 1). The cover and density of graminoids were both negatively
correlated with increasing light levels. Graminoid relative abundance
was negatively correlated with soil K content. Percent cover of graminoids was
positively correlated with soil moisture and Mg and was nonrandom with
respect to aspect. The abundance, cover, and relative abundance of forbs were
not affected by any of the variables measured. Due to very low numbers of
individuals on the census plots, shrub and vine species were not included as a
response variable in our analysis of functional group responses to increasing
field densities of garlic mustard . In addition to native shrubs, we observed
small numbers of the non-native shrubs Berberis thunbergii D.C.(Japanese
barberry) and Celastrus orbiculata Thunb. (oriental bittersweet) at several
plots, but their presence was not correlated with garlic mustard. All other
species observed in our census were native species. Thus, after accounting for
heterogeneity in the abiotic environment, we found evidence for negative
effects of garlic mustard densities on graminoids and tree seedlings, but not on
the herbaceous or shrub layers of the community.
Table 1. Stepwise linear regression results for effect of environmental variables and garlic
mustard invasion on community measurements (species diversity, species richness, density, %
cover, and relative abundance). Only those variables that contributed significantly to the model
are shown. The parameter estimate indicates whether there was a positive or negative effect on
the dependent variable. The model R2 is the cumulative R2 value for the model. * = P < 0.05, **
= P < 0.01, and *** = P < 0.001.
Dependent variable Independent variable estimate Model R2
Species diversity Garlic mustard % cover -0.252 0.64***
Species equitability Garlic mustard density -0.092 0.46**
Mg 0.115 0.62*
Tree seedling relative abundance Garlic mustard density -0.434 0.75***
Graminoid density Light -0.219 0.49**
North aspect 0.428 0.77***
Soil moisture 3.19 0.94***
Graminoid % cover Light -1.696 0.61***
North aspect 0.271 0.74*
Mg 1.06 0.88**
Graminoid relative abundance Garlic mustard density -0.813 0.59**
K -0.035 0.75*
2007 K. Stinson, S. Kaufman, L. Durbin, and F. Lowenstein 79
The change in absolute tree seedling abundance was negatively correlated
with total garlic mustard density for four canopy tree species. Regression
Figure 1. Species richness (S), Shannon diversity index (H'), and Shannon
equitability index (J) as a function of: abundance of Alliaria petiolata (garlic mustard)
at in situ observation plots (A–C), and response to experimental A. petiolata
removal treatments (D–F). Error bars represent ± 1 std error of the mean.
80 Northeastern Naturalist Vol. 14, No. 1
equations demonstrated significantly more negative changes in abundance in
sugar maple, white ash, and black cherry (Table 2). Negative changes in red
maple abundance were also observed with increasing density of first year
garlic mustard seedlings, but the correlation was not significant.
The percent cover of garlic mustard was reduced by 57% on average
in the partial-removal treatment and by 99% on average in the fullremoval
treatment (ANOVA for main effect of removal treatment: F =
20.22, P < 0.001), and the garlic mustard canopy ranged from 10 to 100 cm
in height. Average plot-level light intensity from 0–100 cm was significantly
lower in the highly invaded plots compared to the full- and partialremoval
plots, indicating that both medium and high garlic mustard density
decreased the amount of light available to surrounding forest vegetation
(Ftreatment = 7.59, P < 0.001). As expected, more light was available at higher
understory strata than on the forest floor (Fheight = 7.41, P < 0.01). In June,
average light levels were highest in the full-removal treatment and lowest
in the high-density control plots. In August, light levels were lowest in the
partial-removal treatment (Ftime x treatment = 5.02, P < 0.01), most likely because
leaf senescence of garlic mustard occurred later at these sites (K.
Stinson, pers. observ.). Soil moisture did not differ among treatments, but
was higher in August than in June (main effect in repeated measures
ANOVAs: Fmoisture = 0.78, P = 0.46; Ftime = 68.43, P < 0.001). Mean soil
temperature was higher in the high-density garlic mustard plots compared
to those with full and partial removal, perhaps due to insulating effects of
dense garlic mustard stands, but did not differ throughout the summer
(Ftemp = 3.99, P = 0.02; Ftime = 0.13, P = 0.72).
Analysis of variance demonstrated that the total number of species (S) did
not differ among removal treatments at the P = 0.05 level (Fig. 1D). In
contrast, H' significantly increased in the full-removal treatment compared to
the other two treatments (Fig. 1E), indicating that garlic mustard negatively
affects diversity of native species in this community. The Shannon
equitability index (J) was also greater in the full-removal plots than in the
other treatments (Fig. 1F). Thus, a more even representation of species, rather
than a higher total number of species, contributed to the short-term enhancement
of diversity by removal of garlic mustard in our experimental plots.
We found no difference in H' between our full-removal plots and our
uninvaded observation plots (F = 0.2294, P = 0.6489). Comparisons between
our fenced treatment plots and unfenced controls demonstrated a significant
Table 2. Results from linear regression equations relating the change in absolute tree seedling
abundance 2002–2003 to increasing total garlic mustard density at observation plots in the field.
* = P < 0.05. NS = not significant.
Species Beta R2
A. saccharum (sugar maple) -23.49 0.42 *
F. americana (white ash) -04.53 0.18 *
P. serotina (black cherry) -57.88 0.45 *
A. rubrum (red maple) NS NS
2007 K. Stinson, S. Kaufman, L. Durbin, and F. Lowenstein 81
effect of invader density on H', but there were no detectable effects of
fencing on S (ANOVA: Ffence trt = 2.46, P = 0.14; Fgm density = 0.67, P = 0.43;
Ffence trt x density = 1.42, P = 0.26) or H' (ANOVA: Ffence trt = 0.003, P = 0.951; Fgm
density = 28.881, P < 0.001; Ffence trt x density = 2.35, P = 0.15). There were no
differences in light (F = 0.77, P = 0.38), soil moisture (F = 0.1.83, P = 0.18),
or soil temperature (F = 0.46, P = 0.50) between our full-removal plots and
our uninvaded observation plots.
The relative abundance of the tree seedling functional group increased in
response to partial, but not full removal of garlic mustard (Fig. 2A). The
relative abundances of graminoids, shrubs, and forbs did not change in
response to either treatment. When garlic mustard was included in the
analysis, relative abundance of all native taxa increased in response to the
full-removal treatment, but did not change in response to partial removal
(Fig. 2B), indicating that the invader was dominant at both medium- and
high-invasion levels. The native flora showed no differences in absolute
abundances of individuals in the native functional groups. Thus, the major
change in functional-group composition following removal of the invader
was to release native plants from dominance by the invader.
The absolute abundances of individual native species varied with full or
partial removal of garlic mustard (Fig. 3). In general, tree seedling abundances
did not change in response to the removal treatments. Both ash and sugar
maple showed slight declines in relative abundance in the full-removal plots,
while increasing in the partial-removal plots, but the response was only
significant (post hoc P < 0.05) for ash in the full-removal plots. Typical
understory species—such as Viola papilionacea Pursh. (common violet),
Aster divaricatus L. (white wood aster), and Carex appalachica J. Webber
and S. Ball (Appalachian sedge)—increased in response to full removal,
whereas others—such as Prunus serotina Ehrh. (black cherry), Geum
triflorum Pursh. (old man’s whiskers), and Carex deweyana Schwein (Dewey
sedge)—become less abundant after partial and full removal of garlic mustard
(post hoc tests between treatments for each species: P < 0.05). Thus, there
were species-specific changes in abundance in response to the invader.
Effects of invasion severity on native diversity
Most studies that examine relationships between native plant diversity
and invasion focus on the invasibility of communities (Brown and Peet
2003, Davis et al. 2000, Dukes 2002, Elton 1958, Kennedy et al. 2002,
Knops et al. 1999, Levine 2000, Lyons and Schwartz 2001, Naeem et al.
2000, Prieur-Richard and Lavorel 2000a), but effectively controlling invasions
that have already occurred requires an understanding of their impacts.
Here we present correlative and experimental evidence that increasing invasion
by garlic mustard directly reduces native plant diversity as measured by
the Shannon index (H'), providing critical data on the incremental impacts of
sparse to severe invasions (cf. Manchester and Bullock 2000). Other studies
have focused on species richness (S) as an indicator of community-level
82 Northeastern Naturalist Vol. 14, No. 1
Figure 2. (A) Relative abundances of native functional groups (trees, shrubs, forbs,
and graminoids) in response to control, partial-removal, and full-removal treatments
(A. petiolata [garlic mustard] excluded). (B) Relative densities of A. petiolata with
respect to native trees, shrubs, forbs, and graminoids in response to control, partialremoval,
and full-removal plots. Bars represent proportional status of groups within
2007 K. Stinson, S. Kaufman, L. Durbin, and F. Lowenstein 83
response to invasion (Alvarez and Cushman 2002, Manchester and Bullock
2000, Meiners et al. 2002), and in some cases both, S and H' are negatively
correlated with invader presence (Fairfax and Fensham 2000). Since we did
not observe a corresponding decline in S, we conclude that garlic mustard
does not reduce the number of native taxa in the understory as it increases in
abundance, at least within the time frame of this study. Instead, the decline
in H' was related to reduced overall species equitability (J), which indicates
Figure 3. Mean number of individuals (mean absolute abundance) observed for the
ten most common species in the experimental plots. Bars indicate control (darkest
fill), partial removal (mid-tone fill), and full removal (light-gray fill) of A. petiolata
(garlic mustard). Error bars indicate ± 1 standard error of the mean.
84 Northeastern Naturalist Vol. 14, No. 1
invasion-driven changes in the proportional representation of species. As
discussed below, we also show that individual plant taxa respond differently
to increasing invasion severity, leading to species compositional changes
that are not reflected by measures of diversity or richness. Similarly, Nuzzo
(1999) found a decline in the percent cover of perennial plants with increasing
garlic mustard invasion over time, but no change in overall species
richness at a long-term monitoring study in Illinois.
Because environmental variation was not generally associated with garlic
mustard invasion levels at our sites, it is unlikely that abiotic factors control the
negative relationship between invasion and native diversity. Also, since environmental
variables, diversity, and species composition at our full-removal
plots were similar to those at our uninvaded, unfenced observation plots, deer
exclusion does not appear to influence our results, and the native community
appears to “recover” to diversity levels that are similar to those at uninvaded
locations within two growing seasons following full eradication. Based on our
combined experimental and correlative data, we conclude that declines in
native diversity with increasing garlic mustard densities reflect compositional
responses of the native flora to increasingly severe invasion. Our combined
field and experimental data further suggest that short-term community-level
recovery of H' and J can be achieved by eradication methods.
Responses of native taxa to increasing severity of invasion
Other studies have suggested that garlic mustard invasion has negative
impacts on native plant performance (McCarthy 1997, Meekins et al. 2001,
Nuzzo 2000, Yost et al. 1991), but none have tested this idea in the field. Here
we provide both correlative and experimental evidence that high levels of
garlic mustard presence reduce native graminoid and tree seedling abundance.
The significant decline in the percent cover of tree seedlings shown in Table 1
provides in situ evidence that increasing garlic mustard invasion may interfere
with recruitment of this functional group. At the species level, the significantly
more negative changes in the abundance of sugar maple, white ash, black
cherry, and red maple seedlings at higher garlic mustard densities shown in
Table 2 provide correlative evidence that seedlings of these key canopy trees
favor less invaded areas. The distribution of graminoids, which was correlated
with microsite factors as well as invasion levels, is probably due to environmental
heterogeneity as well as the presence of garlic mustard .
The removal of garlic mustard has been indicated for short-term “release”
periods to allow recovery of native species (McCarthy 1997). Despite
natural declines with increasing invader presence at our observation plots,
our experimental data show that removal of garlic mustard did not alter the
abundance of native functional groups after two growing seasons. However,
the relative abundance of native tree seedlings responded positively to
partial but not full removal (Fig. 2A), suggesting that this functional group
recovers quickly in dominance after moderate control measures to remove
the dominant invader. To determine whether or not negative effects of the
eradication treatment itself may in part explain the lack of native plant
responses to full removal requires further investigation.
2007 K. Stinson, S. Kaufman, L. Durbin, and F. Lowenstein 85
Variable responses of individual species were masked at the functionalgroup
level, indicating the need for management plans that do not necessarily
target a generalized component of native vegetation. Ash seedling abundance
contributed most to the release of tree seedlings in response to partial removal,
and sugar maple showed a similar trend, but the responses of the other tree
seedlings were not as pronounced. It is important to note that the longer-term
recovery of individual species of tree seedlings will be dependent on seedling
dynamics, including seed crop and seed production, which can vary
interannually within a given species. Forbs as a functional group did not
respond to the treatments, but full removal of garlic mustard clearly releases the
understory species V. papilionacea and C. appalachica, while reducing abundance
of weedy species such as G. triflorum and C. deweyana (Uva et al. 1997).
In herbaceous plants, much of the recovery we observed may have been due to
short-term vegetative growth, while population increases resulting from sexual
reproduction may take longer than the period of our experiment. As with tree
seedlings, recovery times may differ across herbaceous species according to
seedling demography, dispersal, and other factors affecting growth. One
hypothesis that emerges from our observations is that species-specific responses
to garlic mustard invasion are related to variation in dependence of
native plants on mycorrhizal fungi at our study sites. Stinson et al. (2006)
recently demonstrated that garlic mustard disrupts native plant-mycorrhizae
mutualisms, having the strongest effect on highly dependent tree species,
including those in the present study. Differential responses to shading may be
another driver of species-specific responses to high and moderate invasion
levels, as well as other environmental and demographic factors (Byers and
Quinn 1998, Meekins and McCarthy 2000). In another experimental eradication
study for this species, removal of garlic mustard from a wooded floodplain
in western Maryland resulted in the release of different plant functional groups,
and its effect on species diversity was not distinguishable from effects of
environmental variability among heavily and sparsely invaded sites through
time (McCarthy 1997). Similarly, in a potted competition experiment in Ohio,
garlic mustard negatively affected the growth and survival of Quercus prinus
L. (chsetnut oak) seedlings, but not Acer negundo L. (box elder) or the annual
plant Impatiens capensis Meerb. (jewelweed) (Meekins and McCarthy 1999).
Thus, garlic mustard invasion appears to impact native plants, including the
recruitment of tree seedlings, via individualistic species-level responses at our
experimental site and in other forests.
Short-term, individualistic species responses to high-, intermediate-, and
low-level invasion contribute to variable effects of garlic mustard on native
forest understory flora at our site in New England. Both full and partial
removal of garlic mustard can rapidly increase diversity and species
equitability in the forest understory, but neither affects native-species richness.
The costs of full eradication may outweigh the benefits, since full
removal does not appear to serve immediate goals for maintaining overall
native-species richness or encouraging release of the overall native flora.
86 Northeastern Naturalist Vol. 14, No. 1
Seedlings of two important canopy tree species may, in fact, initially benefit
more from partial than from full removal, although individual species losing
dominance may become more vulnerable to other stochastic factors that
could influence their abundance over time (Smith and Knapp 2003). Conversely,
when conservation priorities emphasize species diversity and
equitability, rather than overall richness or individual native taxa, then fulleradication
efforts may be warranted even where garlic mustard abundance
is currently moderate. Managers should also consider that re-colonization by
garlic mustard may occur, in which case repeated eradications may be
necessary. In practice, removal via herbicide may be necessary at many sites
where manual removal is not viable, and the secondary effects of herbicide
treatments on native species' responses to removal are not addressed here.
We urge longer-term removal studies to assess community-level responses
to this and other exotic species to help develop strategic control plans for
specific management priorities.
Funding was provided by a grant from the Berkshire-Taconic Office of The
Nature Conservancy (TNC), with supplementary assistance from the Harvard Forest
NSF LTER Program. An award from the National Science Foundation Research
Experience for Undergraduates Program provided support to L. Durbin. We thank F.
Bazzaz for hosting K. Stinson and S. Kaufman as postdoctoral fellows; D.R. Foster
for collegiality and logistical support; T. Seidler for input on analysis; G. Motzkin, B.
DeGasperis, and two anonymous reviewers for comments on the manuscript; and B.
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