Alliaria petiolata is a widespread biennial herb from Eurasia that is one of the most recognizable invasive plants of forests in the eastern United States and southern Canada. After two decades of intensive study on its physiology, ecology, and impacts, this plant has come to be known in both the scientific and gray literature as an allelopathic plant capable of exerting negative, chemically mediated effects on plants and microbes in its environment. A critical review of the literature reveals that there is evidence both supporting and failing to support this assertion, and that conclusions can be affected greatly by the experimental approaches taken, the target species examined, the sources of allelopathic inputs, and environmental factors. The objective of this review is to provide a history of allelopathy research in A. petiolata, describing the various approaches that have been taken and conclusions drawn, and to summarize the current standing of A. petiolata as an allelopathic plant using the most ecologically relevant data on this phenomenon. Finally, we discuss the degree to which allelopathy, versus other mechanisms, may contribute to the invasive success of this plant.

Invasive plants impact native plant and animal species, communities, and ecosystems. We explore the role of treefall gaps in facilitating invasive species growth. We measured the frequency and density of different life stages of three invasive species: Rubus phoenicolasius, Berberis thunbergii, and Rosa multiflora in an eastern deciduous mature forest. Gaps were important for the establishment, growth, and reproduction of R. phoenicolasius, growth of R. multiflora and reproduction of B. thunbergii. We present several hypotheses as to why we observed a species-specific response of invasive plants to treefall gaps, including propagule rain as a driver of invasive plant establishment.

Invasive plant species are believed to decrease biodiversity and local abundances of native species. Examining the mechanisms of invasive species impacts can help direct restoration efforts, especially where species employ multiple mechanisms to increase their relative dominance and negatively affect native species. Ranunculus ficaria is an invasive species in many temperate deciduous forests in the northeastern United States, and is especially dense in highly disturbed urban riparian habitats. This species may prevent establishment of native species in invaded areas directly through competition or allelopathy. Alternatively, its success may simply be a consequence of a modified disturbance regime. We tested for direct effects of R. ficaria on the growth of a native riparian grass phytometer, Elymus riparius. We also examined three potential mechanisms by which R. ficaria could directly impact native plants: nutrient competition, light/space competition, and allelopathy. We tested for nutrient competition by adding fertilizer to the soil in selected plots. To test for allelopathy, we added a tea made from R. ficaria leaves to selected plots. We also quantified plant community responses to R. ficaria removal based on volunteer seedling abundances. We found 99% higher phytometer biomass in removal plots than in invaded plots. Removal plots also had 493% more volunteer seedlings. There was no evidence for nutrient competition or allelopathy. Ranunculus ficaria is directly responsible for reducing volunteer sprout abundance and the biomass, but not necessarily diversity, in our study sites, probably through competition for space and/or light. This suggests that R. ficaria has some role in driving ecosystem change, and that removal of these populations will help restore some native species.

Invasive species are of global importance because of their impacts on ecological communities, habitat structure, native community dynamics, and ecosystem processes and function. Scientists and conservation managers are increasingly focusing on the biological impacts of invasive species and on devising management practices that emphasize the health of ecosystems based on measured biological processes. Amur honeysuckle (Lonicera maackii (Rupr.) Herder) is a highly successful invasive shrub in forests of eastern North America. The scientific literature surrounding this species has grown in the past several decades as researchers have investigated L. maackii impacts across multiple ecological scales. In this review we synthesized literature on (a) the key traits related to this species' invasion success, (b) the impacts this invasive species has at various ecological scales, (c) the outcomes of restoration efforts for this species, and (d) the connections of this weed to invasion ecology theories. Lonicera maackii impacts are complex and vary across ecosystems and spatial scales; we report findings from studies demonstrating a wide range of effects on species composition, community structure, ecosystem function, and successional trajectories. We end by providing a working ecological framework that may help guide future research and conservation efforts.

Invasion of deciduous forests by woody shrubs such as Lonicera maackii is known to negatively impact the abundance and richness of native herbs, but effects on recruitment of seedlings and saplings of native trees, such as Acer saccharum, are less well known. Ultimately, these impacts could alter the species composition of forest trees. Our study was designed to evaluate the impact of L. maackii and environmental factors on recruitment of A. saccharum seedlings and on the transition of seedlings to saplings and saplings to mature trees. We selected four study sites in southwestern Ohio where we censused L. maackii and A. saccharum seedlings, saplings, and trees in 16 plots (eight with and eight without L. maackii) at each site. We measured L. maackii abundance and maximum diameter of the primary stem (an indication of shrub size), edaphic factors (soil density, pH, soil percentage of nitrogen [%N], soil percentage of carbon [%C]), topographic factors (elevation, slope, aspect), biotic factors (overall tree abundance), and human influences (proximity to roadways). Using a generalized linear mixed model with model comparison techniques, we found that in plots with larger L. maackii, seedling recruitment was lower, the transition from seedlings to saplings was greater, and the transition from saplings to trees was unaffected. Seedling recruitment increased with increasing soil %C, but decreased with increasing soil %N. Slope was positively associated with a greater transition from seedlings to saplings, and soil density and soil %C negatively affected the transition to trees. The transition to trees was higher with greater tree abundance, and the transition to saplings and trees was greater away from roadways. Overall, these results indicate that environmental factors have positive and negative impacts on recruitment of A. saccharum, and large L. maackii have both a negative and positive relationship to recruitment. Further studies will be necessary to uncover the mechanisms involved in the negative and positive relationships of L. maackii to recruitment of A. saccharum.

Although many invasive plant species negatively impact native plants in natural communities, their relationships with other nonnative plants remain relatively unexplored. In some cases, invasive plant species may be capable of facilitating the invasion of other nonnative species into natural areas, thereby exacerbating their invasive ecosystem effects. We examined whether Lonicera maackii (Rupr.) Herder (Amur honeysuckle), a woody shrub from Asia that is rapidly spreading throughout the midwestern USA, is associated with other invasive plant species, compared to locations where L. maackii is not yet present. Lonicera maackii is known to detrimentally impact native plant communities and to alter soil nutrients and light levels in invaded areas, indicating that it has the potential to act as an invasion facilitator. Using plots with and without L. maackii in four study sites across southwestern Ohio, we quantified species richness (S), relative abundance (RA), proportion of total species (PR), and diversity (H) of invasive species, compared to native and nonnative species that were not invasive. The presence of L. maackii was significantly associated with an increased number, proportion, and diversity of other invasive plant species, but the relative abundance of invasive individuals did not differ between plots with and without L. maackii. Presence of L. maackii and also distance to roads were explanatory variables that predicted S, PR, RA, and H for invasive species. Overall, the association of L. maackii with other invasive plant species in natural areas indicates the need for continued investigation into the potential role of L. maackii as an invasion facilitator in eastern deciduous forests in the midwestern USA.

Many exotic invasive plants exhibit plasticity in form and function across a range of environmental conditions, optimizing available resources in a manner that frequently outcompetes native organisms. The invasive shrub Lonicera maackii is one of the most prominent invasive plant species in the Midwestern United States. The objectives of this research were to investigate the morphological and physiological plasticity of this invasive shrub across light environments, and to study allometric parameters that will help estimate the aboveground biomass of L. maackii of all size classes. Shrubs were selected from open, forest edge, and understory habitats. Photosynthetic responses to light and leaf nitrogen content were measured throughout the growing season in 2003 and shrubs were harvested in October 2003. The maximum photosynthetic assimilation rates for open grown shrubs were more than double the values measured in the edge and understory. Maximum photosynthesis rates were strongly correlated with leaf nitrogen content, yet the photosynthetic nitrogen use efficiency was uniform across habitats. Open-grown shrubs had the highest values of total and partitioned biomass, although shrubs from all locations showed a proportional distribution to leaf, branch, trunk, and fruit. Although reproductive shrubs can produce copious amounts of fruits and seed in high light environments, fruit production still occurred in forest interior environments and is a direct source of seeds in the understory. Results suggest that because L. maackii exhibits physiological and morphological plasticity coupled with prolific fecundity (even in the understory), this species can persist in all habitats.

Recent studies have found that allelopathy can be an important mechanism of plant invasions. Alliaria petiolata, Lonicera maackii, Ranunculus ficaria, Celastrus orbiculatus, and Microstegium vimineum are invasive species found in the Midwestern USA. We investigated the comparative direct and indirect allelopathic effects of these five species in a laboratory setting using leaf extracts in a germination experiment and a growth experiment. Results illustrate that the effect of each invasive species varied with target species and with life stage. Extracts of L. maackii and R. ficaria had the largest overall effects (∼50% reduction) on germination across both species, but effects of extracts varied by target species. Extracts of A. petiolata and C. orbiculatus had a greater effect on germination of E. hystrix than on C. fasciculata. Extracts of L. maackii, M. vimenum, and R. ficaria had larger inhibitory germination effects on C. fasciculata than on E. hystrix. For growth of E. hystrix after 8 wk, C. orbiculatus and L. maackii extracts had the largest (>80%) reduction of biomass, followed by treatment with extracts of R. ficaria (∼ 20% reduction). Treatment with L. maackii and C. orbiculatus extracts reduced height, with extracts of L. maackii having greater effects (60% versus 40% reduction, respectively). Plants treated with L. maackii extracts allocated significantly less biomass to roots. We also found evidence of indirect impacts. Mycorrhizal inoculation overall was most negatively affected by treatment with L. maackii extracts (∼70% reduction), even at low concentrations, and least by treatment with C. orbiculatus, with intermediate effects of A. petiolata, M. vimineum, and R. ficaria. Overall, L. maackii had the strongest effects, followed closely by C. orbiculatus, which was then followed in turn by R. ficaria. These results provide further support for the allelopathic potential of L. maackii and add to the growing body of evidence on the allelopathic potential of R. ficaria and C. orbiculatus. Effects of A. petiolata and M. vimineum were less strong than in other studies, suggesting the importance of variation between populations and in experimental venue.

In 1989, Edward Frankel recorded the distribution of Pueraria montana var. lobata (kudzu) in and around New York City, motivated by the concern that kudzu was extending its range northward from the southeastern USA, where it is an aggressive invasive species. Understanding species persistence is important in determining the dynamics of the distribution of invasive species as they spread, particularly since harsh environmental conditions at the periphery of a species range might result in frequent extinctions. Long-term species persistence data are difficult to obtain; the Frankel (1989) data thus represent a valuable contribution to documenting population persistence. In this study, we were able to identify unambiguously 22 of Frankel's original 34 sites of kudzu in the New York City metropolitan area. After more than 20 yr, we found only 32% of kudzu populations persisted in and around New York City. In contrast, we used herbarium records to identify 19 sites in Georgia and South Carolina where P. montana var. lobata was documented to occur in the same approximate time period as Frankel's data, and found that 95% of these populations persisted. Even accounting for the difference between urban and rural sites, populations of P. montana var. lobata go extinct more often near the periphery of the range than at the core of the invasion.