Garlic mustard (Alliaria petiolata) is an aggressive invasive plant of temperate forests, and constitutes the single greatest threat to forest health throughout much of the northern US. Garlic mustard produces chemicals within its tissues that are toxic to insects that would normally consume forest plants. This plant has also been found to be toxic to soil bacteria and fungi normally responsible for making nutrients available to other plants. The goal of the study is to examine a particularly aggressive garlic mustard infestation in pine plantations located in Sand Ridge State Forest (IL). The objectives are to identify changes that garlic mustard makes to soil organisms, as well as soil nutrient availability to other plants. Should garlic mustard result in decreases in soil nutrient availability in forest soils, forest productivity could decrease, which would in turn alter harvestable biomass and the amount of carbon that could be stored in forests. The researchers will investigate changes in soil chemistry and microbial communities in response to garlic mustard invasion across a range of forest sites using a combination of lab and field studies including: experimental site manipulations (e.g., additions of garlic mustard or nitrogen); site and experimental plot characterization of general soil chemistry (pH, C, N, cations, P) and microorganisms (biomass, diversity, enzyme activities). These analyses should provide information as to whether garlic mustard alters forest health and suggest changes that will occur to stands as this invasive plant moves into new areas.

As garlic mustard is currently invading forests at high rates, infestations that result in reduced nutrient availability in North American forests can lead to serious long-term consequences for forest health. The long-term goal of this project will be to evaluate the impacts of garlic mustard across a broad range of forest soils, and to improve our understanding of the mechanisms by which garlic mustard invades and subsequently acts as an ecological change agent in these forests. All research on this project to date has been a collaborative effort involving Bradley University undergraduate students during the academic year, expanding to include local community college students, local high school students and K-12 educators during the summer, including large numbers of women and members of underrepresented groups in science. Funds acquired through this grant will support continued collaborations among these groups.

Project Report

Garlic mustard (Alliaria petiolata) is an invasive biennial herb that was brought over from Europe in the 1800s for food and medical purposes. Although uninvasive in its native range, it has spread in North America to 34 states and four Canadian provinces. Garlic mustard is known to have negative impacts on ecosystems including decreased native plant diversity and growth, but its effects on soil microbes is less well studied. The main objectives of the grant were to 1) examine how garlic mustard impacts sandy soils at Sand Ridge State Forest (SRSF) in Mason County, Illinois with a recent history of invasion, 2) design and implement experiments to help establish that garlic mustard can alter below-ground processes that facilitate its invasion, such as effects on nutrient cycling and microbial community characterization, 3) examine how light characteristics and forest structure affect size inequalities in garlic mustard first- and second-year plants, 4) engage underrepresented groups of high school and undergraduate students in the scientific process and help them design and implement original experiments, and 5) increase public awareness of this invasive species. Information acquired by these studies has important implications for eradication, management, and long-term health of formerly invaded areas following eradication of garlic mustard. The results of the initial studies at SRSF on pine plantations recently invaded with garlic mustard, supported that garlic mustard invasion altered certain soil properties including increased pH, greater plant available nitrogen, and increased production of carbon dioxide by soil microbes. Overall the study suggested that 1) garlic mustard may leave long-lasting effects on environments even after eradication and 2) garlic mustard may alter soil properties once established or may be attracted to areas with high plant-available nitrogen. These findings were published in 2012 in Applied Soil Ecology. To address the latter finding and the second research objective, another experiment was implemented at SRSF in 2009, in which nitrogen was altered and garlic mustard was added to simulate invasion, with the prediction that garlic mustard alters the soil properties creating a more favorable environment for its continued spread. Data from the first two years of study indicated that garlic mustard altered nitrogen availability and did not preferentially grow in areas of high nitrogen, as more seeds did not germinate in those treatments. Garlic mustard releases chemicals called glucosinolates which affect native plant growth and seed germination by decreasing densities of important fungal mutualists. However, in this experiment, these compounds did not appear to alter microbial function. These plots will be monitored long-term, but the data from the first three years of the study are expected to be published shortly. Other studies from this project identified decreased densities of ectomycorrhizal fungi and alterations to microbial community structure in areas invaded with garlic mustard. Associated with environmental cues for growth, the ability of garlic mustard to grow under different light conditions also likely contributes to its invasivity. In another local area (Glen Oak Park, a temperate deciduous forest) size inequality formation of first- and second-year garlic mustard was monitored under shade and sunlit gap conditions. Greater size differences in first-year plants were found in shaded areas, whereas in sunlit areas second-year plants exhibited larger size inequalities. This has the potential to affect management as often-limited resources may be better geared towards removal of larger second year plants in disturbed/sunlit areas. This data is also in the process of being published. Overall, garlic mustard is a growing problem in local forested ecosystem, and more information on how it functions in resource poor soils is necessary to determine its long-term consequences on ecosystem health. The research program described above engaged high school, undergraduate and graduate students in authentic scientific research experiences. Students were involved in establishing research site and data collection, and many had the opportunity to create original research projects on an aspect of garlic mustard under the guidance of faculty researchers and peer-mentors. Topics included effects of garlic mustard tissues on native plant germination, characterization of soil and soil bacterial characteristics in invaded areas, and quantification of garlic mustard characteristics such as seed mass and seed output. Student researchers presented their research at symposiums and meetings to faculty, peers and other researchers. These data contributed greatly to the knowledge of local garlic mustard populations. The research experience also gave the youth transferable knowledge including a background in ethics, laboratory and field safety practices, and scientific writing and communication skills. Public awareness of garlic mustard invasiveness will always be a major goal of invasive plant research. An outreach program at the local zoological state park (Wildlife Prairie State Park) is in the works to engage park guests about this invader, with the goal of creating citizen science networks to map garlic mustard populations. Increasing community scientific literacy will ensure that new policies created regarding environmental management practices are scientifically sound.

National Science Foundation (NSF)
Division of Environmental Biology (DEB)
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Henry L. Gholz
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Bradley University
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