Biogeographic Variation in Resistance of the Invasive Plant, Alliaria Petiolata, to a Powdery Mildew Fungus and Effect of Resistance on Competitive Dynamics

Garlic mustard is an invasive Eurasian biennial that has spread throughout the eastern United States and southern Canada. Populations of this plant vary in their susceptibility to Erysiphe cruciferarum, a causal agent of powdery mildew disease in Brassicaceous plants. We examined whether there were biogeographic patterns in the distribution of resistance in invasive North American and native European populations of this plant. We grew plants from 78 invasive and 20 native populations and screened them for powdery mildew resistance in the greenhouse. We found that populations were mostly monomorphic for either resistance or susceptibility but that some polymorphic populations were found from both continents. The proportion of populations showing resistance versus susceptibility was similar in both Europe and North America. Within continents, the spatial distribution of resistant and susceptible populations did not deviate significantly from random. We also examined whether the possession of the resistance trait alter intraspecific competitive dynamics. In two trials, we competed plants from resistant and susceptible populations in a target-neighbor design in the presence and absence of powdery mildew inoculum and examined the growth of the target plant. Target plants from resistant populations were overall larger than target plants from susceptible populations. Target plants were overall larger when grown in competition with susceptible neighbors. Further, resistant target plants showed a greater degree of release from competition when grown with a susceptible neighbor versus a resistant neighbor than the degree of release shown by susceptible target plants. This suggests a benefit of possessing the resistance trait with little apparent costs which should promote selection for this trait within plant populations

Variation in Resistance of Experienced and Naïve Seedlings of Jewelweed (Impatiens capensis) to Invasive Garlic Mustard (Alliaria petiolata)

Author Institution: Wilmington CollegeThe invasive species garlic mustard, Alliaria petiolata, has negative impacts on understory forest species in the Midwest. Plants that coexist with A. petiolata in the field may show resistance to its negative effects as a result of natural selection. In a growth room experiment, we investigated if naïve and experienced seedlings of impatiens capensis vary in their response to the presence of A. petiolata. Impatiens capensis individuals from areas without A. petiolata (i.e., naïve plants) and from nearby areas with A. petiolata (i.e., experienced plants) were collected from the field and were then grown with A. petiolata in pots for 16 weeks. We measured height, stem diameter, reproduction and biomass of I. capensis and biomass of A. petiolata. There was a significant (P < 0.05) negative correlation between biomass and height of naïve I. capensis and biomass of A. petiolata, while there was no significant correlation between these variables for experienced I. capensis. Our results indicate the potential for the evolution of resistance to the presence of A. petiolata in I. capensis and point toward the need for further studies

Comparing Allelopathic Effects of Root and Leaf Extracts of Invasive Alliaria petiolata, Lonicera maackii and Ranunculus ficaria on Germination of Three Native Woodland Plants

Author Institution: Department of Biology, Wilmington College, Wilmington, OhioInvasive plant species can exhibit allelopathic effects on native plant species. The strength of this allelopathic effect can vary with invasive species, with target species and with type of plant tissue extract. The purpose of this study was to determine the direct effects of extracts from roots or leaves of three Midwestern US invasive plants (Alliaria petiolata, Lonicera maackii and Ranunculus ficaria) on the germination success of three native target species (Anemone virginiana, Blephilia hirsuta and Elymus hystrix) in a fully factorial experiment. Leaf extract treatments overall showed more germination inhibition compared to root extract treatments. As concentration increased, effects of extracts increased. Extracts of leaves of A. petiolata had the greatest inhibition of germination across all other treatments. Effects of root and leaf extracts of each invasive species varied with each target species. While E. hystrix showed little response to extracts of roots or leaves of L. maackii and R. ficaria, B. hirsuta and A. virginiana germination were reduced by leaf extracts of these two invasive species. This study confirms the strong direct allelopathic effects of A. petiolata, though the strength of the effect varies with target species and with type of tissue used to make extracts. This study is the first to directly compare the effects of these invasive species on a suite of native, ecologically-relevant target species

Habitat Assessment and Conservation Status of Endangered Northeastern Bulrush

Scirpus ancistrochaetus (Northeastern Bulrush) is a federally endangered sedge that grows in temporary wetlands. We performed surveys of 90 wetlands in Pennsylvania, Maryland, West Virginia, and Virginia, measuring areal extent, stem density, and number of flowering stems of Northeastern Bulrush. We also measured percentage of tree canopy closure, presence of threats, and size of wetland. Percentage of tree canopy closure was negatively correlated with wetland area, percentage of wetland area occupied by North-eastern Bulrush, total number of stems, stem density, and percentage of flowering stems. Wetland area was positively related to percentage of flowering stems and had a tendency to be positively related to stem density, likely in part due to larger wetlands having lower tree canopy closure. Invasive Phalaris arundinacea (Reed Canarygrass) and Microstegium vimineum(Japanese Stiltgrass) were present at 7% and 21% of the wetlands, respectively. Odocoileus virginianus (White-tailed Deer) and Ursus americanus (Black Bear) damage were present in 38% and 17% of wetlands, respectively. Modification of habitat was noted at 27% of wetlands. For wetlands with previous data on population size, 14% had increased, 34% were stable, 25% had decreased, and 27% were absent or had severely decreased. Our recommendations for management include reducing tree canopy closure with control of invasive species and White-tailed Deer where needed

A Review of Garlic Mustard (Alliaria petiolata) as an Allelopathic Plant

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

Habitat Assessment and Conservation Status of Endangered Northeastern Bulrush

Scirpus ancistrochaetus (Northeastern Bulrush) is a federally endangered sedge that grows in temporary wetlands. We performed surveys of 90 wetlands in Pennsylvania, Maryland, West Virginia, and Virginia, measuring areal extent, stem density, and number of flowering stems of Northeastern Bulrush. We also measured percentage of tree canopy closure, presence of threats, and size of wetland. Percentage of tree canopy closure was negatively correlated with wetland area, percentage of wetland area occupied by North-eastern Bulrush, total number of stems, stem density, and percentage of flowering stems. Wetland area was positively related to percentage of flowering stems and had a tendency to be positively related to stem density, likely in part due to larger wetlands having lower tree canopy closure. Invasive Phalaris arundinacea (Reed Canarygrass) and Microstegium vimineum(Japanese Stiltgrass) were present at 7% and 21% of the wetlands, respectively. Odocoileus virginianus (White-tailed Deer) and Ursus americanus (Black Bear) damage were present in 38% and 17% of wetlands, respectively. Modification of habitat was noted at 27% of wetlands. For wetlands with previous data on population size, 14% had increased, 34% were stable, 25% had decreased, and 27% were absent or had severely decreased. Our recommendations for management include reducing tree canopy closure with control of invasive species and White-tailed Deer where needed

A Review of Garlic Mustard (Alliaria petiolata) as an Allelopathic Plant

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

Amur Honeysuckle (Lonicera maackii) Management Method Impacts Restoration of Understory Plants in the Presence of White-Tailed Deer (Odocoileus virginiana)

Management methods for invasive species vary in their restoration success in the presence or absence of herbivores. We investigated the performance of understory plants after management of the invasive shrub Amur honeysuckle using two herbicide-based methods (cut/paint and basal application) in fenced and unfenced plots. The cut/paint method resulted in the removal of above-ground stems, while the basal application method resulted in the dead stems remaining in place. Light level in the cut/paint treatment was higher than in the basal application treatment, which was higher than in the control (no management) treatment. Across fencing treatments, fruit production, height, and subsequent recruitment of transplanted jewelweed were greater in the cut/paint treatment. Across management treatments, jewelweed plants were taller in the fenced treatment. Native species richness was generally higher in the cut/paint and basal application treatments than in the control treatment. There were more jewelweed recruits, more jewelweed fruits, and greater native species richness in the cut/paint treatment than in the basal application treatment in fenced plots, but these measures were similar in both management treatments in unfenced plots. Thus standing dead stems of Amur honeysuckle offered protection from damage in the presence of herbivores, offsetting the advantage of the cut/paint method seen in the fenced plots. There was a trend for more leaves of transplanted wild ginger in the basal application treatment. There were more invasive garlic mustard and more Amur honeysuckle seedlings in the cut/paint treatment than in the control treatment. Our results illustrate the complexities involved in selecting appropriate restoration management techniques given herbivore pressure, differential species response, and presence of multiple invasive species. In our study, we demonstrated that the basal application honeysuckle management method (and therefore perhaps similar methods that leave the dead stems standing) led to increased understory plant restoration success when compared to the cut/paint method

Contrasting Effects of Allelochemicals from Two Invasive Plants on the Performance of a Nonmycorrhizal Plant

In addition to resource competition, allelopathy is believed to contribute to the invasiveness and impact of several plant invaders of North America. In this study, we examined whether aqueous leaf extracts of Alliaria petiolata and Lonicera maackii, two invaders of deciduous forests in North America, affected growth and reproduction of a target nonmycorrhizal plant and whether effects varied across a soil fertility gradient. While nutrient addition substantially improved the performance of Arabidopsis thaliana grown in field soils in pots in a growth room, addition of A. petiolata extracts to these soils had no significant independent or interactive effects on growth or reproduction. In contrast, addition of L. maackii extracts both directly reduced growth and reproduction of A. thaliana and greatly constrained increases in growth and reproduction of A. thaliana in response to increasing nutrient availability. Use of a nonmycorrhizal target plant revealed that effects of L. maackii were independent of allelopathic effects on mycorrhizae, an effect attributed to A. petiolatain other studies

Amur Honeysuckle (Lonicera maackii) Management Method Impacts Restoration of Understory Plants in the Presence of White-Tailed Deer (Odocoileus virginiana)

Management methods for invasive species vary in their restoration success in the presence or absence of herbivores. We investigated the performance of understory plants after management of the invasive shrub Amur honeysuckle using two herbicide-based methods (cut/paint and basal application) in fenced and unfenced plots. The cut/paint method resulted in the removal of above-ground stems, while the basal application method resulted in the dead stems remaining in place. Light level in the cut/paint treatment was higher than in the basal application treatment, which was higher than in the control (no management) treatment. Across fencing treatments, fruit production, height, and subsequent recruitment of transplanted jewelweed were greater in the cut/paint treatment. Across management treatments, jewelweed plants were taller in the fenced treatment. Native species richness was generally higher in the cut/paint and basal application treatments than in the control treatment. There were more jewelweed recruits, more jewelweed fruits, and greater native species richness in the cut/paint treatment than in the basal application treatment in fenced plots, but these measures were similar in both management treatments in unfenced plots. Thus standing dead stems of Amur honeysuckle offered protection from damage in the presence of herbivores, offsetting the advantage of the cut/paint method seen in the fenced plots. There was a trend for more leaves of transplanted wild ginger in the basal application treatment. There were more invasive garlic mustard and more Amur honeysuckle seedlings in the cut/paint treatment than in the control treatment. Our results illustrate the complexities involved in selecting appropriate restoration management techniques given herbivore pressure, differential species response, and presence of multiple invasive species. In our study, we demonstrated that the basal application honeysuckle management method (and therefore perhaps similar methods that leave the dead stems standing) led to increased understory plant restoration success when compared to the cut/paint method