Garlic mustard's (Alliaria petiolata, Brassicaceae) invasive success is attributed in part to its release of allyl isothiocyanate (AITC) into the soil. AITC can disrupt beneficial arbuscular mycorrhizal fungi (AMF) associated with native plant roots, which limits their soil resource uptake. However, AITC and its precursor, sinigrin, have never been detected in garlic mustard-invaded forest soils. Here, we use high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) to assess the concentration and bioactivity of these putative allelochemicals in paired forest plots uninvaded or invaded by garlic mustard. Our methods detected AITC and sinigrin only where garlic mustard was present and our recovery of AITC/sinigrin coincided with adult senescence. A bioassay of in situ fungal hyphae abundance revealed significantly reduced hyphal abundance in the presence of garlic mustard relative to uninvaded soils. Finally, the lowest concentration of AITC measured in the field (*0.001 mM) is highly inhibitory to the spore germination of a forest AMF species, Glomus clarum. Together, our data provide the first direct evidence of garlic mustard-produced sinigrin and AITC in forest soils and demonstrate that even low levels of these chemicals have the potential to significantly suppress AMF growth and spore germination, strengthening their status as allelopathic novel weapons.

Abstract. Soil resources derived from mutualistic arbuscular mycorrhizal fungi (AMF) play a critical role in the physiological function of many native plant species. Allelopathic plant invasion studies have revealed declines in AMF inoculation potential of invaded soils, and lost opportunities for plants to form new AMF associations. Yet, if allelochemicals also kill AMF external hyphae already associated with plant roots, this mutualism disruption should result in physiological stress for native plants. We previously demonstrated that forest soils infested with garlic mustard (Alliaria petiolata), an allelopathic invader, exhibit reduced fungal hyphal abundance. Here, we demonstrate for the first time that treatment with garlic mustard tissue reduces soil respiration rates and diminishes physiological function of false Solomon's seal (Maianthemum racemosum), an AMF-dependant forest understory native. Treated plants exhibited reduced stomatal conductance and photosynthesis relative to controls, consistent with the proposed loss of AMF function. Such physiological declines, if sustained over several growing seasons, could decrease native understory perennials' growth rates and increase their susceptibility to environmental stresses. These data provide an explicit mechanism that can help explain the loss of established native perennials from invaded mature forests. We propose that the physiological costs of mutualism disruption may be a widespread but previously untested mechanism enhancing the invasion of undisturbed ecosystems by allelopathic species.

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SummaryInvasive plants impose novel selection pressures on na€ ıve mutualistic interactions between native plants and their partners. As most plants critically rely on root fungal symbionts (RFSs) for soil resources, invaders that disrupt plant-RFS mutualisms can significantly depress native plant fitness. Here, we investigate the consequences of RFS mutualism disruption on native plant fitness in a glasshouse experiment with a forest invader that produces known antifungal allelochemicals.Over 5 months, we regularly applied either green leaves of the allelopathic invader Alliaria petiolata, a nonsystemic fungicide to simulate A. petiolata's effects, or green leaves of nonallelopathic Hesperis matronalis (control) to pots containing the native Maianthemum racemosum and its RFSs. We repeatedly measured M. racemosum physiology and harvested plants periodically to assess carbon allocation.Alliaria petiolata and fungicide treatment effects were indistinguishable: we observed inhibition of the RFS soil hyphal network and significant reductions in M. racemosum physiology (photosynthesis, transpiration and conductance) and allocation (carbon storage, root biomass and asexual reproduction) in both treatments relative to the control.Our findings suggest a general mechanistic hypothesis for local extinction of native species in ecosystems challenged by allelopathic invaders: RFS mutualism disruption drives carbon stress, subsequent declines in native plant vigor, and, if chronic, declines in RFS-dependent species abundance.

Most plants form mutually beneficial relationships with microorganisms in their roots, especially fungi. Invasive plants can release substances toxic to other species known as allelochemicals. Allelochemicals from the invasive species garlic mustard can inhibit beneficial soil fungi, thereby disrupting the plant-fungal mutualism. We demonstrate that treatment with garlic mustard leaves reduces the ability of a native plant, False Solomon's seal, to store carbohydrates. Additionally, we demonstrate that weeding garlic mustard from forest plots allows this native to grow larger, flower more frequently, and enter long-term dormancy less often relative to plots where garlic mustard occurs.