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(American Journal of Botany. 2008;95:1416-1425.) doi: 10.3732/ajb.0800184 © 2008 Botanical Society of America, Inc.

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Effects of Alliaria petiolata (garlic mustard; Brassicaceae) on mycorrhizal colonization and community structure in three herbaceous plants in a mixed deciduous forest¹

The Holden Arboretum, 9500 Sperry Road, Kirtland, Ohio 44094 USA

Received for publication 3 June 2008. Accepted for publication 9 September 2008.

ABSTRACT

Herbaceous plant species are important components of forest ecosystems, and their persistence in forests may be affected by invasive plant species that reduce mycorrhizal colonization of plant roots. I examined the effect of the invasive plant Alliaria petiolata on arbuscular mycorrhizal fungi (AMF) colonizing the roots of three forest plant species. AMF root colonization and community structure was examined from plants that were growing either in the absence or presence of Alliaria under natural forest conditions. AMF root colonization varied among the plant species but was not significantly affected by Alliaria. With molecular methods, 12 different taxa of AMF could be distinguished among the root samples, and these taxa belonged to the genera Acaulospora and Glomus, with Glomus dominating AMF communities. There were significant differences between the community of AMF colonizing roots of Maianthemum racemosum and Trillium grandiflorum, but only AMF communities of Maianthemum roots were significantly affected by Alliaria. Indicator species analysis found that an Acaulospora species type was a significant indicator of Maianthemum plants grown in the absence of Alliaria. These results suggest invasive plants like Alliaria may selectively suppress AMF fungi, and this suppression can affect AMF communities colonizing the roots of some native plant species.

Key Words: Alliaria petiolata • arbuscular mycorrhizal fungi • Arisaema triphyllum • diversity • Maianthemum racemosum • terminal restriction fragment length polymorphisms • TRFLP • Trillium grandiflorum

Herbaceous plants may comprise as much as 80% of the total plant richness in northern hardwood forests (Gilliam,2007). These plant communities provide resources for animal and insect populations, can affect regeneration of forest trees, and affect soil microbes responsible for maintaining soil fertility (Zak et al., 1990; Westover et al., 1997; George and Bazzaz, 2003; Costa et al., 2006; Gilliam, 2007). In addition, herbaceous plants can mediate both carbon and nutrient cycling and effect the export of nutrients from forest ecosystems (Muller and Bormann, 1976; Tessier and Raynal, 2003; Gilliam, 2007). Consequently, herbaceous plant communities are important components of northern hardwood forests, and changes to these communities could have ecological significance. The composition, structure, and persistence of these communities are governed by the extent to which the physical limitation of the environment and biotic interactions promote or hinder individual plant growth and reproduction (Gleason, 1926; Crawley, 1997). One especially important biotic interaction is the one that forms between plants and mycorrhizal soil fungi.

Arbuscular mycorrhizal fungi (AMF) are soil fungi that form mutualistic relationships with plant roots. AMF colonize plant roots and extensively explore soil, acting as an extension of the plant root system. Therefore, AMF play a crucial role in plant nutrient acquisition, especially the acquisition of nitrogen and phosphorus (Smith and Read, 1997 and references therein). Numerous studies have demonstrated the importance of AMF colonization to plant growth and survivorship, including the survival of young life stages of forest trees (Hetrick et al., 1988; Smith and Read, 1997; Stinson et al., 2006). Plant root systems can be colonized by a diverse group of AMF (Helgason et al., 1999; Vandenkoornhuyse et al., 2003; Santos-González et al., 2007), each of which differs in functional attributes (Helgason et al., 2002). These differences in AMF functional attributes could lead to specialization in resource acquisition among different portions of the plant root system. The effects of AMF on maintaining plant community diversity and productivity (van der Heijden et al., 1998; Klironomos et al., 2000; Vogelsang et al., 2006) may, in part, be attributed to differences in the functional attributes of AMF taxa that increase resource acquisition and survivorship under natural conditions.

A number of human disturbances, including nitrogen enrichment (Egerton-Warburton and Allen, 2000; Egerton-Warburton et al., 2001), soil disturbance (Merryweather and Fitter, 1998), changes in land use and management (Vandenkoornhuyse et al., 2003), and changes in plant community composition (Vandenkoornhuyse et al., 2003; Santos-González et al., 2007) have been implicated in reducing AMF diversity and root colonization of plants and soil. Of increasing interest is the effect of invasive plant species on AMF in soil and how invasive plants can affect AMF diversity and the growth of native plants. Alliaria petiolata (garlic mustard; Brassicaceae) is a biennial herb native to Eurasia that is becoming a rampant invader in woodlands throughout eastern North America (Nuzzo, 1999). Whole-plant extracts of Alliaria contain secondary compounds, including glucosinalates and other allelopathic chemicals that have reported antifungal properties, that are exuded through roots and leaf litter (Roberts and Anderson, 2001). Previous studies have demonstrated that aqueous extracts of Alliaria tissue can reduce plant growth and mycorrhizal colonization (Roberts and Anderson, 2001; Stinson et al, 2006) under greenhouse conditions.

In addition, soil microbial communities in North America may be more sensitive to Alliaria than soil microbial communities in the plants native range that have coevolved with Alliaria secondary compounds (Callaway et al., 2008). Communities of soil fungi in North America may constitute a "naïve" community of organisms especially susceptible to the secondary chemistry of Alliaria (Callaway et al., 2008). The success of Alliaria in invading native plant communities may depend on allelopathy, which acts as a novel weapon to suppress AMF mutualists of resident plant species (Callaway and Ridenour, 2004; Vogelsang et al., 2004; Inderjit et al., 2006; Callaway et al., 2008). Here, I describe a study conducted in a mature, mixed hardwood forest to assess the impact of Alliaria on AMF colonization and community structure on the roots of three important native forest wildflowers: Arisaema triphyllum L. (Jack-in-the-pulpit; Araceae), Maianthemum racemosum (L.) Link subsp. racemosum (false solomon’s seal; Liliaceae), and Trillium grandiflorum Michx. Salisb. (large-flowered trillium; Liliaceae). I chose to focus this study on these three species because they are extensively colonized by mycorrhizal fungi, are important components of many forest plant communities, and populations of these plants are on the decline in many places in eastern North America. I hypothesized that the roots of plants growing in the presence of Alliaria would have both reduced colonization and reduced diversity of AMF. Consequently, I used a combination of standard staining methods to assess root length colonized by AMF as well as molecular methods to describe how Alliaria affected the community of AMF colonizing plant roots in the field.

MATERIALS AND METHODS

Site description and plant sampling
The study site is a 16-ha forest known as Trillium Trail located in Allegheny County, Pennsylvania. This forest is a mature mixed mesophytic forest dominated by white and red oak (Quercus alba L. and Q. rubra L.), sugar maple (Acer saccharum Marshall), American beech (Fagus grandifolia Ehrh.), and tulip poplar (Liriodendron tulipifera L.), and the forest contains a diverse woody and herbaceous understory. The study site is characterized by moderately sloping ground (8–15% slope) and silt loam soils. Garlic mustard (Alliaria petiolata Bieb.) began invading Trillium Trail from road edges in the early 1990s and has since become an important component of the forest understory. Alliaria is uniformly distributed throughout the site and grows in discrete patches within the forest. Sampling was conducted during early summer (mid-June) of the 2007 growing season. Native plant species were sampled from within patches of Alliaria and also from within adjacent areas that did not contain Alliaria. Areas that did not contain Alliaria were never more than 5 m from Alliaria patches. In this fashion, we were able to collect 10 plant pairs (Alliaria+ and Alliaria–) for each of three plant species: Jack-in-the-pulpit (Arisaema triphyllum L.), false solomon’s seal [Maianthemum racemosum (L.) Link subsp. racemosum], and large-flowered trillium (Trillium grandiflorum Michx. Salisb.) (3 species x 2 treatments x 10 replicates per species). Plants were gently dug from the soil to limit disturbance of the root system. Soil was gently shaken from the plants, which were then placed in a cooler and transported to the laboratory. In the laboratory, all roots attached to the plant rhizome were excised 1.0 cm from the rhizome (approximately 500 mg wet mass root tissue), and rinsed in sterile distilled water to remove all visible soil. Roots were then placed in 70% ethanol and subsequently stored at 4°C until analysis.

Arbuscular mycorrhizal colonization of roots
Half of each plant’s root system (250 mg wet mass, 50-cm total root length) was used to estimate AMF colonization. Roots were stained with trypan blue in lactic acid using a modification of the procedure outlined by Kormanik and McGraw (1982). Roots were cleared in 5% KOH for 6 h at room temperature, stained at room temperature for 2 h, then destained overnight. Trypan blue is a common nonvital stain used to estimate AMF colonization (Dickson and Smith, 1998), and the no-heat modification of the standard procedure has been successfully used (Burke et al., 2002, 2003). Root samples were mounted on microscope slides, and colonization was determined using the slide mount method of McGonigle et al. (1990). A total of 150 intersections were scored for each plant sample except for Arisaema triphyllum for which only 100 intersections were scored because roots were short. The root length colonized by mycorrhizal hyphae, arbuscules/coils, and vesicles was determined according to the method of Brundrett et al. (1994), who considered hyphae to be mycorrhizal only if they were visually connected to vesicles or arbuscules. Because both Trillium and Maianthemum tended to form hyphal coils in roots more than arbuscules, we considered arbuscules and coils as the same structure for scoring AMF colonization.

DNA extraction and TRFLP analysis
DNA was extracted from the remaining root tissue of each sample. Roots weighing more than 250 mg fresh mass were placed in a 1.5-ml bead beating tube containing 300 mg of 400 µM glass beads (VWR, West Chester, Pennsylvania, USA), 300 mg 1-mm glass beads (Chemglass, Vineland, New Jersey, USA) and 750 ml 2% CTAB (cetyltrimethylammonium bromide). Samples were beaten for 90 s on a Precellys homogenizer (Bertin Technologies, Montigny-le-Bretonneux, France) at 6500 rpm. Approximately 500 ml of the extractant was then removed from the tube, and nucleic acids were purified by phenol/chloroform extraction, then precipitated with 20% polyethylene glycol 8000 in 2.5 M NaCl (Burke et al., 2005). All extracted DNA was suspended in Tris-EDTA buffer and stored at –20°C. DNA was diluted in PCR grade water, and a 20-fold dilution of the DNA was used for PCR. PCR was made using universal eukaryotic primer NS31 (Simon et al., 1992) and AMF-specific primer AM1 following general procedures in Helgason et al. (1998) except that only 30 cycles of PCR were performed. Primers were labeled with the fluorochromes 6-carboxyfluorescein (6FAM) (AM1) and 4,7,2',4',5',7'-hexachloro-6-carboxyfluorescein (HEX) (NS31). I empirically determined that the restriction enzymes AluI and Hsp92 (Promega, Madison, Wisconsin, USA) provided the largest number of distinct terminal restriction fragments (TRFs), so these enzymes were used to cut the labeled PCR product. Terminal restriction fragment length polymorphisms (TRFLPs) were analyzed at the Cornell Bioresource Center using an Applied BioSystems (Foster City, California, USA) 3730xl DNA Analyzer. Because Arisaema plants contained substantially less root tissue than either Trillium or Maianthemum, only six composite root samples for Arisaema (N = 3/treatment) could be analyzed with molecular methods as compared to 20 samples for both Trillium and Maianthemum.

AMF TRFLP sequence database
The TRFLP sequence database (was created from two sources of environmental DNA: (1) roots harvested from Trillium and Maianthemum; and (2) forest soil collected at the Trillium Trail site. DNA was extracted and AMF amplified with unlabeled primers as noted before. The PCR product was separated by gel electrophoresis on a 1% agarose gel in Tris-acetate-EDTA (TAE) buffer. The PCR products were excised from the gel and purified using a PCR purification kit (Promega, Madison, Wisconsin, USA) using the manufacturer’s instructions. PCR product was then cloned using a pGEM-T Easy Vector System (Promega) using the manufacturers instructions. Fifty randomly selected colonies from each type of environmental DNA were incubated overnight at 37°C in Luria-Bertani (LB) medium, and plasmids were harvested using a Wizard Plus SV Minipreps DNA Purification System (Promega). A total of 150 clones (50 clones each from Trillium and Maianthemum roots, 50 clones from soil DNA) were recovered and screened for distinct TRFs. Clones were used for PCR using labeled NS31 and AM1 as noted, and the PCR product was then used for TRFLP. Clones that represented distinct TRFs were sequenced using a Dye Primer Cycling Kit (Applied Biosystems). A total of 100 clones were sequenced following this procedure to capture distinct TRFs. Sequencing was completed through the Cornell Bioresource Center using an Applied BioSystems 3730xl DNA Analyzer. Generated sequences were compared to EMBL/GenBank/DDBJ database entries using the FASTA program (European Bioinformatics Institute, Hinxton, UK, http://www.ebi.ac.uk) to determine the putative identity of AMF.

Data and statistical analysis
Differences in the percentages of root length colonized among the samples were analyzed by two-way ANOVA using procedures in the program SigmaStat version 3.5 (Systat Software, Richmond, California, USA). All data were arcsine transformed prior to statistical analysis. I used TRFLP fingerprints (both AluI and Hsp92 profiles) for determining species type identity and proportional abundance within root samples. AMF species types were considered present in a root system if peaks distinct to that species type were identified in both the AluI and Hsp92 TRFLP profiles. The areas under these distinct peaks were used to calculate the proportional abundance of AMF species types in the core (Burke et al., 2005, 2006). In some instances, AMF species types shared TRFs. This fact necessitated lumping together some AMF species types based on TRFLP patterns. However, this should provide a more conservative estimate of species richness and community diversity among the samples. Evenness (equitability) and Shannon diversity (H') were calculated using procedures available through the program PC-ORD 4 (MjM Software, Gleneden Beach, Oregon, USA). Differences in root colonization and community diversity among samples were analyzed by a two-way ANOVA using procedures in the program SigmaStat 3.5. To determine if differences existed in AMF root communities among the plant species and in the presence of Alliaria, I used multiresponse permutation procedures (MRPP) and indicator species analysis available through PC-ORD 4. MRPP is a nonparametric, multivariate procedure for testing the hypothesis of no difference between groups. The test statistic (T) explains the separation between the groups in multidimensional space; the more negative the T-value, the greater the separation between the groups. The P-value describes how well the separation between groups is likely due to chance alone, and A represents the chance-corrected within group agreement and is a measure of the effect size (McCune and Grace, 2002). For both MRPP and indicator species analysis, the proportional peak area of each identified species type was used as an indicator of abundance within each sample. A total of 1000 randomizations were used for the Monte Carlo test during indicator species analysis. Because AMF communities can be affected by a number of interacting environmental variables, we considered P < 0.1 significant for comparison of communities among plants from Alliaria+ and Alliaria– sites. Because of the low number of root samples available for community analysis, Arisaema roots were not used for MRPP or indicator species analysis. Similarities of recovered AMF sequences to known sequences were obtained by comparison to EMBL/GenBank/DDBJ database entries using the FASTA program. The taxonomic affinity of some recovered sequences was shown using a multiple sequence alignment created with the program Clustal X 1.83 (Thompson et al., 1997), and this alignment was used to construct a distance tree by the neighbor-joining method (Appendix S1, see Supplemental Data with the online version of this article). All sequence data between primers NS31 and AM1 were used for alignment purposes.

RESULTS

Root colonization
Total root length colonized ranged from a mean of 39 ± 9% in Arisaema to 94 ± 1% in Maianthemum (Fig. 1). Total root length colonized by arbuscules or coils ranged from a mean of 29 ± 7% in Arisaema to 71 ± 3% in Trillium (Fig. 1). Total vesicular colonization was always less than 1% for all plant species. ANOVA indicated that there were significant differences among the plant species for AMF root colonization (Table 1), but that the presence of Alliaria had no significant effect on AMF root colonization. Roots of all species were heavily colonized, with extensive hyphal coiling in Trillium (Figs. 1 and 2).

View larger version (19K): [in this window] [in a new window]   Fig. 1. Arbuscular mycorrhizal colonization of roots of three forest plants growing in the absence or presence of Alliaria. Plants growing in the presence of Alliaria are denoted by a (+). Mean colonization and SE bars are shown.

View larger version (67K): [in this window] [in a new window]   Fig. 2. Root sections stained with trypan blue showing arbuscular mycorrhizal structures. Roots were observed at a magnification of 200x for assessment of AMF colonization. (A) Trillium, hyphal coiling within cortical cells. (B) Maianthemum, vesicles and darkly stained cells containing arbuscules/coils. (C) Arisaema, lightly stained arbuscules in cortical cells.

View larger version (9K): [in this window] [in a new window]   Fig. 3. Terminal RFLP (TRFLP) profiles of the arbuscular mycorrhizal community found in the root system of an individual Maianthemum plant growing in the absence of Alliaria. The x-axis represents the size of the TRF in base pairs, while the y-axis represents integrated fluorescence units on a scale of 0–3000. (A) Profile generated with the restriction enzyme AluI showing major peaks contributed by AMF taxa identified through sequence cloning. (B) Profile generated with the restriction enzyme Hsp92 showing major AMF taxa.

View larger version (22K): [in this window] [in a new window]   Fig. 4. Neighbor joining tree showing the taxonomic affinities of selected arbuscular mycorrhizal fungi (AMF) sequences recovered in this study. An Acremonium-like hyphomycete (AB108787) was used an outgroup (not shown). The number at each branch indicates percentage of bootstrap replicates of 1000 that support that branch. Values less than 50% are not shown. All clones used for the alignment were recovered from Trillium Trail except for clone series 3811, 3812, and 516, which were recovered from beech–maple forests at the Holden Arboretum.

View larger version (31K): [in this window] [in a new window]   Fig. 5. Arbuscular mycorrhizal community diversity in roots of three forest plants growing in the absence or presence of Alliaria. Plants growing in the presence of Alliaria are denoted by a (+). Means and SE bars are shown. Species richness is the number of taxa detected in the root systems.

View larger version (21K): [in this window] [in a new window]   Fig. 6. Distribution of arbuscular mycorrhizal fungi (AMF) taxa among plant–treatment combinations. AMF taxa that are significant indicators of the plant–treatment combinations are denoted with an asterisk (P-value) (also see Table 4). Mean relative abundance (peak area) is shown with the SE bar. (A) Relative abundance of AMF taxa between Maianthemum and Trillium grown in the absence of Alliaria. (B) Relative abundance of AMF taxa between Maianthemum grown in the absence and presence (+) of Alliaria.

The roots of herbaceous forest plants are commonly colonized by mycorrhizal fungi (Brundrett and Kendrick, 1990a; Demars, 1996), and these fungi benefit the plants they colonize through increased nutrient gain, improvement in plant water status, and improved plant growth and fitness (Merryweather and Fitter, 1996; Lapointe and Molard, 1997; Helgason et al., 2002). Consequently, any biotic or abiotic factor that reduces mycorrhizal colonization or diversity in soil can have a negative impact on the growth and persistence of herbaceous plants in forest ecosystems. In this study, I examined the effect of Alliaria petiolata on mycorrhizal colonization and mycorrhizal community structure in three important forest plants under natural conditions. Alliaria did not significantly affect the root length colonized by mycorrhizal fungi although I was able to detect some effects of Alliaria on mycorrhizal community structure in at least one plant species, Maianthemum racemosum.

The levels of AMF encountered in all three plants species were comparable to levels reported by other investigators under natural conditions (Brundrett and Kendrick, 1990a; Demars, 1996) with extensive hyphal coiling in Maianthemum and Trillium (Brundrett and Kendrick, 1990b). Although I did not observe any effects of Alliaria on root colonization, I did observe significant differences in root colonization among the three plant species, with Arisaema having the lowest levels of root colonization. These differences may be due, in part, to the different growth cycle of Arisaema roots which grow and senescence seasonally, whereas Maianthemum and Trillium have a perennial root system with longer-lived roots (Brundrett and Kendrick, 1990a); however, sampling was conducted in mid-June, during which time the Arisaema root system should have reached maximum levels of AMF colonization, so sampling time is unlikely to have affected these results (Brundrett and Kendrick, 1990a). On the other hand, sampling was made prior to Alliaria death and senescence and could have affected our root colonization results. Although secondary chemicals (i.e., allelochemicals) have been reported from the roots of Alliaria, senescence and degradation of Alliaria leaf litter may be an important source of AMF inhibiting substances under natural conditions (Roberts and Anderson, 2001; Stinson et al., 2006). Concentrations of glucosinalates, one of the primary secondary chemicals produced by Alliaria and implicated as an important AMF inhibitory substance (Roberts and Anderson, 2001; Stinson et al., 2006), vary at the field site with highest detectable levels in late summer following plant senescence (Data not shown). AMF colonization could be more impacted later in the season when the highest levels of Alliaria secondary chemicals are expected to be present in soil. Other studies have reported on the seasonal production of plant secondary compounds and how the effects of these compounds on other plant species may vary temporally and spatially (Blair et al., 2005; Inderjit et al., 2006). In addition, previous greenhouse studies using Alliaria plant extracts (Roberts and Anderson, 2001; Stinson et al., 2006) may have subjected soil communities to much higher levels of secondary chemicals than present under field conditions. Lower concentrations and seasonal variation in secondary chemicals under field conditions could partly explain the lack of difference in root colonization in the presence of Alliaria in this study.

Molecular analysis of plant roots and soil allowed for the construction of a database of AMF present at the field site that enabled me to identify AMF in plant roots. The majority of recovered sequences belonged to the genus Glomus with some sequences representing the genus Acaulospora also present. As was the case in our study, the genus Glomus was found to dominate clone libraries from tropical forest tree seedlings and grassland plant communities with as many as 95% of recovered clones belonging to this genus (Husband et al., 2002; Santos-González et al., 2007). On the other hand, only 27% of AMF sequences recovered from temperate forest soils belonged to the genus Glomus with the majority of sequences showing similarity to the genus Acaulospora (Helgason et al., 1998). Together, these studies suggest that forests may have site-specific communities of AMF, which are determined by biotic and abiotic conditions. For example, soils at our field site are finely textured, which may explain the lack of Gigaspora in our samples because this genus may prefer soils with low clay content (Lekberg et al., 2007). In total, I was able to detect 12 different AMF species types, but I was not able to distinguish clearly among the species types using TRFLP. Consequently, some species types that clearly differed in sequence alignment had to be combined into one species type for analytical purposes. However, my results compare favorably with the nine AMF species types that were colonizing the roots of Hyacinthoides nonscripta and identified through sequencing (Helgason et al., 1999). In a more recent study, Santos-González et al. (2007) identified 19 AMF species types colonizing the roots of two grassland forbs, and Vandenkoornhuyse et al. (2003) found much higher levels of AMF richness (between 15 and 30 based on TRF richness) than in this study. Nonetheless, my data may not be atypical for a forest herbaceous community. Coadaptational specificity between plants and AMF taxa could result in a correspondence between plant species richness and AMF richness in natural systems and partly explain the higher AMF richness in grassland as compared to forests (van der Heijden et al., 2004).

I found that AMF community structure was significantly different between Maianthemum and Trillium and that Maianthemum communities were significantly impacted by Alliaria. Previous studies have observed differences in the AMF community among different plant species (Helgason et al., 2002; Vandenkoornhuyse et al., 2003; Santos-González et al., 2007), and in forest trees of different ages (Husband et al., 2002). Diversity measures were generally unaffected by either plant species or Alliaria, suggesting that AMF community structure changes between the plant species, but that diversity itself (e.g., AMF richness) remains unchanged. It is important to note, however, that standard diversity measures using TRFs may be insensitive to all but the largest changes in community structure (Hartmann and Widmer, 2006), and this insensitivity of TRFLP could explain the lack of differences in diversity in this study.

Indicator species analysis suggested that the community changes I observed may, in part, be related to changes in the presence of some AMF species types. AMF species types Acaulospora 2 and Glomus 1 were significant indicators of Maianthemum, and the Acaulospora 2 species type was also affected by Alliaria when colonizing Maianthemum. AMF taxa are more prevalent in some plant species than in others (Santos-González et al., 2007), but the associations between forest plants and AMF could also be controlled by the composition of the forest overstory community (Helgason et al., 1998), the season, and soil characteristics (Husband et al., 2002; Lekberg et al., 2007; Santos-González et al., 2007). In this study where we sampled in one forest at one time of the year, changes in the community of AMF are more likely specific to the plant species examined and to the presence of Alliaria. Although only one of Maianthemum’s AMF colonists may have been affected by Alliaria, AMF taxa can provide specific benefits to the host plant (van der Heijden et al., 1998; Hart and Klironomos, 2002; Helgason et al., 2002; Hart and Reader, 2002a, b; Koch et al., 2004). Therefore, any change in the community of AMF on Maianthemum could have important ramifications for root system function, especially given that Acaulospora 2 was a very important colonizer of the Maianthemum root system.

To my knowledge, this study is the first to address the potential effects of Alliaria on the mycorrhizal communities of native forest plants under field conditions, but my results come with some caveats. Although care was taken to collect plant pairs growing in the presence and absence of Alliaria within a short distance of each other, it is likely that the distribution of AMF is determined in part by the heterogeneity of the soil environment. Microsite differences in abiotic conditions between collection patches could have masked the effects of Alliaria on AMF communities especially given the small number of plants examined. In addition, my study was also confined to one particular forest with generally uniform overstory vegetation and land use history. Future studies should include a more comprehensive examination of AMF communities from many forest types invaded by Alliaria, especially because overstory vegetation can affect AMF communities. Nevertheless, although my results may be conservative, they do suggest that Alliaria can affect AMF community composition in the roots of some forest plants and that these effects may be specific to the AMF species present in the forest soil.

FOOTNOTES

¹ The author thanks S. Kalisz and the Kalisz laboratory for use of their field site and for providing the plants for this study and J. C. López-Gutiérrez and S. Carrino-Kyker for reviewing an earlier version of this manuscript. This work was funded by the Holden Arboretum Trust and the Corning Institute for Education and Research.

² E-mail: dburke{at}holdenarb.org; phone: 440-602-3858; Fax: 440-602-8005; present address: Department of Biology, Case Western Reserve University, Cleveland, OH 44106 USA

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