Summary 1. Ectomycorrhizal (EM) fungi play key roles in forest ecosystems, but the potential effects of invasive plants on EM fungal communities have not been assessed. In this study, we tested whether the non-mycorrhizal herbaceous plant Alliaria petiolata (garlic mustard) can alter the abundance of EM fungal communities in North America. 2. In three forests in New England, USA, we compared EM root tip abundance in soils where A. petiolata had invaded to adjacent areas without a history of A. petiolata invasion. At one site, we also intensively sampled EM root tip abundance across the edges of A. petiolata patches to determine the spatial pattern of A. petiolata effects on EM fungi. In a glasshouse experiment, we experimentally invaded soils with A. petiolata and Impatiens capensis , a native species and compared EM fungal colonization of white pine ( Pinus strobus ) seedlings grown in both soils. We also measured the effect of the A. petiolata allelochemical benzyl isothiocyanate on the growth of three species of EM fungi in pure culture. 3. In the field, EM fungal root tip biomass was lower in invaded soils, with the strongest reductions observed in forests dominated by conifers. Alliaria petiolata invasion did not have a significant effect on total root biomass. The influence of A. petiolata on EM fungal abundance in the field was localized, with the strongest inhibition observed within 10 cm of the edge of A. petiolata patches. 4. Pine seedlings growing in soils that were experimentally invaded with A. petiolata also had lower EM fungal root tip biomass compared to uninvaded soils. The native species I. capensis caused similar reductions in EM fungal colonization. Growth of pure cultures of all three species of EM fungi was completely inhibited by benzyl isothiocyanate. 5. Synthesis. Alliaria petiolata inhibits the growth of EM fungi in forests of its introduced range. Changes in EM fungal communities caused by the invasion of A. petiolata may influence tree seedling establishment and biogeochemical cycling.

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The invasive plant Alliaria petiolata (garlic mustard) inhibits ectomycorrhizal fungi in its introduced range

Controlling species invasions is a leading problem for applied ecology. While controlling populations expanding linearly or exponentially is straightforward, intervention in systems with complex dynamics can have complicated, and sometimes counterintuitive, consequences. Most invasive plant populations are stage-structured and density-dependent--a recipe for complex dynamics--and yet few population models have been created to explore the effects of control efforts on such species. We examined the demography of the invasive biennial plant Alliaria petiolata (garlic mustard) on the front of its spread into a natural area and found evidence of strong density dependence in vital rates of first-year rosette and second-year adult stage classes. We parameterized a density-dependent, stage-structured projection model using field-collected data. This model produces two-point cycles with alternating years in which adults vs. rosettes are more prevalent. Such population dynamics match observations in natural populations, suggesting that these complicated population dynamics may result from deterministic rules. We used this model to evaluate simulated management strategies, including herbicide treatment of rosettes and clipping or pulling of adult plants. Management of A. petiolata by inducing mortality of either rosettes or adults will not be effective at reducing population density unless the induced mortality is very high (>95% for rosettes and >85% for adults) and repeated every year. Indeed, induced mortality of rosettes can be counterproductive, causing increases in the stationary distribution of A. petiolata density. This species is typical of many invasive plants (stage-structured, short-lived, high fertility) and exhibits common forms of density dependence. Thus, the management implications of our study should apply broadly to other species with similar life histories. We suggest that management should focus on managing adults rather than rosettes, and on creating efficient control in targeted areas of the population, rather than spreading less efficient efforts widely.

Complex population dynamics and control of the invasive biennial Alliaria petiolata (garlic mustard).

Biological invasions are a fixture in our landscapes, with consequent losses in endemic biota and shifts in ecosystem function. Despite the historical recognition of exotic species success in novel environs, this phenomenon lacks a holistic-descriptive framework. Recent attempts to explain biological invasions are based largely on identifying the inherent invasive qualities of successful exotic species (i.e., invasiveness), or characterizing the susceptibility of a habitat to an introduced species (i.e., invasibility), with few studies examining their interaction or additional contributing factors (e.g., time since introduction). We propose unifying the ’points of entry’ into biological invasions with a state factor model that incorporates all contributing variables—not just species or habitats—into a quantifiable, factorial model amenable to hypothesis testing. State factors are phenomenological variables describing the state of a system—historically used in soil and vegetation science. Our state factor equation relates any quantifiable property of an invasion (i) as a function of propagule pressure (p), introduced habitat (h), invader autecology (a), source environment (s), and time since introduction (t). By manipulating state factors singly, or in interaction, targeted variation can be related to quantifiable properties of exotic species while controlling, or at minimum accounting for, remaining factors contributing variation to the system. This holistic factor-function paradigm extends research on invasions from beyond the limits imposed by current theory, fosters novel empirical approaches, elucidates knowledge gaps in our understanding of resident invasions, and allows for variable accounting via a factor matrix. Here we briefly outline the ontogeny of state factors in soil and vegetation science, detail our proposed ’phast’ framework for biological invasions, including notation, and examine a case study in state factor utility.

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A unifying framework for biological invasions: the state factor model

Restoration often includes control of invasive plants, but little is known about how native plant communities respond to this control. The biennial Alliaria petiolata (M. Bieb.) Cavara and Grande (garlic mustard) is one of the most prevalent invasive plants in forests of eastern North America. We investigated the effects of the herbicide Round-up (glyphosate) on Alliaria and the response of the forest floor plant community to the herbicide and the subsequent decline of Alliaria .I n an old-growth Acer–Fagus stand and a second-growth Liriodendron-dominated stand in Hueston Woods State Nature Preserve, Ohio, United States, we spot applied Round-up in November 2000 and 2001 in 25 1 · 1–m plots and maintained 25 control plots. Herbicide decreased Alliaria density in both stands and reduced the density of other species in leaf during treatment (mostly exotic winter annuals) in the old-growth stand. Treatment did not affect the initial density of the Alliaria cohort that germinated in the spring of 2001, but decreased the 2002 cohort. Community differences were found in the old-growth stand after Alliaria reduction, specifically greater cover of spring ephemerals in the herbicide treatment. In the second-growth stand, herbicide treatment increased reproduction of the late-summer perennial, Phryma leptostachya. These results indicate that glyphosate reduces Alliaria without negatively impacting native species and that some native species respond positively to a single-year reduction in this invasive biennial.

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Effects of Herbicide on the Invasive Biennial Alliaria petiolata (Garlic Mustard) and Initial Responses of Native Plants in a Southwestern Ohio Forest

The invasive grass Microstegium vimineum is widely distributed in closed-canopy forests, but often is patchily distributed under uniform canopy conditions. We hypothesized that the occurrence of patches of invasion may be related to two interacting factors, the presence of dense understory shrub layers and the presence of thick litter layers on the forest floor. Seeds of M. vimineum were sown in plots located under or distant from Lindera benzoin (spicebush) shrubs, and with or without litter manipulations (none, half of natural amount, naturally occurring amount, double the natural amount) in a mature forest in central New Jersey, USA. Populations were monitored for germination and survivorship, as well as growth and fecundity of surviving plants. Neither shrub-associated shade nor litter depth affected seed germination, but both factors affected survivorship, growth and reproduction. The presence of shade from the shrubs reduced survivorship and seed set. Seeds germinating on top of the litter layer also experience higher mortality than seeds germinating under litter and in contact with soil. These results suggest that the interacting effects of shade from understory strata and deep litter layers may limit the spread of M. vimineum. The loss of shrub layers due to intense deer browse and other factors may thus accelerate the spread of this highly invasive species.

Leaf litter and understory canopy shade limit the establishment, growth and reproduction of Microstegium vimineum

Garlic mustard (Alliaria petiolata) is an invasive, spring-germinating, obligately biennial plant. In a central Pennsylvania forest, two distinctive plant patch types exist simultaneously: patches with mixed juveniles and adults and patches that are exclusively juvenile. We experimentally tested two hypotheses to explain this peculiar distribution. The habitat limitation hypothesis proposes that garlic mustard, like other biennials, is limited to rarely available habitats and can only reach maturity in a subset of the patches where its seeds germinate. The intraspecific competition hypothesis proposes that juveniles only survive to maturity in patches lacking adults. We mapped the natural distribution of ten juvenile and ten mature patches and used transects to assess juvenile and adult density in 2 y. We found that patches cycled annually from juvenile to mixed, supporting the intraspecific competition hypothesis. We looked for biotic (interspecific competition) and abiotic (shading) factors tha...

Jeffrey W. Brennan

Papers

Spatial and Temporal Segregation of Juvenile and Mature Garlic Mustard Plants (Alliaria petiolata) in a Central Pennsylvania Woodland