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Invasion of Phragmites australis (common reed) in wetlands throughout North America, and particularly the Laurentian Great Lakes Basin, poses significant ecological problems. The extended period of low Great Lakes water levels from 2000 to 2013 created conditions for large expansions of Phragmites in the Great Lakes coastal zone. The following extended period of high water in the Great Lakes during late 2010's, culminating in record high lake levels in 2020 allowed managers to take advantage of high water by using a cut-to-drown management strategy (i.e., cutting plants below the water surface to stop the flow of atmospheric gases) to control Phragmites populations. To examine the efficacy of a cut-to-drown control strategy, we conducted a controlled-greenhouse study that tested the effect of submergence and timing of cutting (early or late in growing season) on Phragmites growth and viability post treatment. To evaluate Phragmites growth and viability, we measured belowground biomass, rhizome non-structural carbohydrate content (NSC), and rhizome viability following a cut-to-drown treatment. Applying a cut-to-drown treatment reduced average belowground biomass production up to 99%, limited rhizome NSC content up to 83%, and inhibited rhizome viability, regardless of timing of cutting treatments. These results suggest that under high-water conditions, utilizing a cut-to-drown strategy has potential for being a useful control mechanism for Phragmites. However, further research is needed to determine to what extent these results will lead to sustained reductions in growth and viability under field conditions, where rhizome belowground biomass and storage capacity are much larger. [ABSTRACT FROM AUTHOR]

Increased mobility of people around the globe has facilitated transferring species to new environments, where some have found suitable conditions and even become invasive. False indigo‐bush (Amorpha fruticosa L.) is a plant native to North America but has intentionally or unintentionally spread over the Northern Hemisphere, where it often becomes invasive. The plant is especially easily dispersed within the watersheds of large rivers, where seasonal flooding is regular. Seeds and other propagules are buoyant, and when the water recedes, new plants emerge, forming dense thickets where only a few other species can co‐exist. In order to sustain native biodiversity, spread control is needed. However, mechanical control and eradication measures currently in use are labor demanding and costly, while application of herbicides is limited. On the other hand, the plant possesses a number of beneficial properties, such as phytochemical applications (medical and insecticidal effects), biocoenotic uses (honey plant, ornamental features), and ecosystem services (soil stabilization, provision of food for animals, and fiber and biomass for industry, e.g., nanocellulose). For the reasons above mentioned, the plant is considered quite controversial, and the paper discusses both aspects: potential detrimental effects when introduced to new habitats and its beneficial uses for human society. In addition, the paper presents alternative measures of spreading control (e.g., grazing) and argues that exploiting it for beneficial purposes might help spread control, thus covering the expenses of controlling its distribution. [ABSTRACT FROM AUTHOR]

Herbivores play a critical role in plant invasions either by facilitating or inhibiting species establishment and spread. However, relatively few studies with invasive plant species have focused on the role of plant tolerance and how it varies geographically to influence invasion success. We conducted a common garden study using two lineages (native and invasive) of the grass Phragmites australis that are prevalent in North American wetlands. Using 31 populations collected across a broad geographic range, we tested five predictions: 1) the invasive lineage is more tolerant to simulated folivory than the native lineage, 2) tolerance to herbivory decreases with increasing latitude of origin of the populations, 3) estimates of tolerance are correlated with putative tolerance traits and plasticity in those traits, 4) a tradeoff exists between tolerance and resistance to herbivory and 5) tolerance has a fitness cost. Response to folivory varied substantially among populations of P. australis, ranging from intolerance to overcompensation. Our model selection procedure deemed lineage to be an important predictor of tolerance but, contrary to our prediction, the native lineage was 19% more tolerant to folivory than the invasive lineage. Tolerance for both lineages exhibited a u‐shaped relationship with latitude. A tolerance–resistance tradeoff was evident within the invasive but not the native lineage. Also, tolerance was positively correlated with belowground biomass allocation, leaf silica concentrations, specific leaf area and plasticity in stem density, and negatively correlated with the relative growth rate of the population and plasticity in putative resistance traits. Lastly, although we did not detect costs of tolerance, our results highlight that fast growth rates can maintain high fitness in the presence of herbivory. Herbivory and plant defense strategies for P. australis lineages in North America exhibit complex biogeographic patterns that cause substantial heterogeneity in enemy release and biotic resistance and, consequently, invasion success. [ABSTRACT FROM AUTHOR]

Aim: Recent research has highlighted that the relationship between species interactions and latitude can differ between native and invasive plant taxa, generating biogeographical heterogeneity in community resistance to plant invasions. In the first study with foliar pathogens, we tested whether co‐occurring native and invasive lineages of common reed (Phragmites australis) exhibit non‐parallel latitudinal gradients in foliar fungal communities, pathogen susceptibility and damage, and whether these biogeographical patterns can influence the success of invasion. Location: North America. Time period: 2015–2017. Major taxa studied: Perennial grass P. australis. Methods: We surveyed 35 P. australis field populations, spanning 17° latitude and comprising four phylogeographical lineages, including one endemic to North America and one invasive from Europe. For each population, we quantified the percentage of leaf pathogen damage and cultured fungi from diseased leaves, which we identified using molecular tools. To assess whether latitudinal gradients in pathogen damage had a genetic basis, we inoculated plants from 73 populations with four putative pathogens in a complementary common garden experiment and measured P. australis susceptibility (i.e., diseased leaf area). Results: We isolated 84 foliar fungal taxa. Phragmites australis lineage influenced fungal community composition but not diversity. Despite the invasive European P. australis lineage being the least susceptible to three of the four pathogens tested in the common garden experiment, pathogen damage in the field was similar between native and invasive lineages, providing no evidence that release from foliar pathogens contributes to the success of invasion. Genetically based latitudinal gradients in pathogen susceptibility observed in the common garden were isolate specific and obscured by local environmental conditions in the field, where pathogen damage was threefold higher for northern compared with southern populations, regardless of lineage. Main conclusions: Our results highlight that host plant lineage and genetically based biogeographical gradients strongly influence foliar fungal communities and pathogen susceptibility, but do not translate to patterns of pathogen damage observed in the field. [ABSTRACT FROM AUTHOR]

We studied the reproduction and dispersal of giant hogweed ( Heracleum mantegazzianum) at its northern distribution limit in North America (Québec, 2014-2016) to better understand the invasion dynamics of the species along rivers. Seeds were collected from a riparian population to conduct germination, floatation, and dispersal experiments. Data were analysed in comparison with a real invasion case that was initiated about 10 years ago along a river system. In Québec, giant hogweed individuals produce on average 14,000 to 16,000 seeds with a germination rate of 75-85%. Seeds with endosperm that fall in water likely sink within 5 hr. Along a small brook, most disperse over short distances (<40 m) in summer, although some can travel 100-300 m. These data suggest that late-summer or early-fall water dispersal of seeds would not explain the magnitude and rapidity of the invasion patterns observed along streams. We suggest that late-fall and, especially, spring floods are the most efficient dispersal vectors for giant hogweed seeds and are likely responsible for the establishment of populations kilometres downstream from introduction points along river systems. The spread of giant hogweed would consequently be less influenced in the near future by a rise in temperature than by a change in the magnitude or timing of flood events. [ABSTRACT FROM AUTHOR]

PREMISE OF THE STUDY: Using phylogeographic analyses to determine the geographic origins of biological invaders is important for identifying environmental adaptations and genetic composition in their native range as well as biocontrol agents among indigenous herbivores. Eurasian watermilfoil (Myriophyllum spicatum) and its hybrid with northern watermilfoil (M. sibiricum) are found throughout the contiguous United States and southern Canada, forming one of the most economically costly aquatic plant invasions in North America, yet the geographic origin of the invasion remains unknown. The objectives of our study included determining the geographic origin of Eurasian watermilfoil in North America as well as the maternal lineage of the hybrids. METHODS: DNA sequence data from acpDNAintron and the nrDNA ITS region were compiled for accessions from 110 populations of Eurasian watermilfoil and hybrids from North America and the native range (including Europe, Asia, and Africa). Datasets were analyzed using statistical parsimony and Bayesian phylogenetics to assess the geographic origin of the invasion. KEY RESULTS: The two Eurasian watermilfoil cpDNA haplotypes in North America are also found from China and Korea, but not elsewhere in the native range. These haplotypes did not overlap and were limited in native geographic range. The ovule parent for hybrids can come from either parental lineage, and multiple haplotypes from both parental species were found. CONCLUSIONS: The geographic origin of this prolific aquatic plant invasion of North America is in Asia. This provides critical Information to better understand the invasion pathway and inform management into the future. [ABSTRACT FROM AUTHOR]

Aim Canada thistle ( Cirsium arvense- Cardueae, Asteraceae) is one of the worst invasive plants world-wide. Native to Eurasia, its unintentional introduction into North America now threatens the native flora and is responsible for enormous agricultural losses. The goals of this study are to: (1) reconstruct the evolutionary history of C. arvense and estimate how often it may have colonized North America, (2) compare the genetic diversity between European and North American populations to detect signs of demographic bottlenecks and/or patterns of population admixture, and (3) conduct bioclimatic comparisons to infer eventual niche shifts following this species' introduction into North America. Location Europe and North America. Methods A total of 1522 individuals from 58 populations were investigated with six microsatellite markers. Estimates of heterozygosity ( HE) and allelic richness ( RS) were quantified for each population, and population structure was inferred via analyses of molecular variance (AMOVAs), principal components analyses (PCAs), Mantel tests and Bayesian clustering analyses. Climatic niche spaces were based on 19 bioclimatic variables extracted from approximately 32,000 locations covering the entire range, and compared using PCA and hierarchical cluster analysis. Results Although there is evidence of multiple introductions from divergent European lineages, North American populations of C. arvense exhibited significantly lower levels of genetic diversity than their putative ancestors. Bioclimatic comparisons pointed to a high degree of niche conservatism during invasion, but indicated that genotypes from the former USSR and Central European mountain chains were probably best adapted to invade North America upon entry into the continent. Main conclusions Genetic and historical data suggest that C. arvense first entered North America from Western Europe with the first European settlers, and was later introduced from Eastern Europe into the prairie states during the agricultural boom. The species went through a significant bottleneck following its introduction into the New World, but the level of genetic diversity remained high owing to admixture between genetically differentiated lineages and to a highly efficient outcrossing breeding system. [ABSTRACT FROM AUTHOR]

We studied the effects of hand weeding of second-year plants of the biennial garlic mustard ( Alliaria petiolata) on first-year plants (seedlings) and native ground layer vegetation. Garlic mustard is a Eurasian species that has invaded deciduous forest ground layers in eastern North America. Treatments consisted of a control and an early or late weeding of second-year garlic mustard. The early treatment (early March) was applied before garlic mustard seeds had germinated and when most native species were dormant. The late treatment (mid-May) occurred after plants had bolted, flowering was occurring, and most native species and new garlic mustard seedlings were actively growing. Pre-treatment data were obtained in 2004 and treated and control plots were sampled in 2005, 2006, and 2007. No significant treatment effects were observed in 2004 or 2005. In 2006, mean cover of first-year plants was higher in the early weeding treatment than in the late weeding treatment and control. In 2007, mean cover of first-year garlic mustard was higher in the control than in either of the two weeding treatments. There were no significant treatment effects in any year on native vegetation cover, bare ground, or the five most abundant native species. Our data indicate that (1) late weeding of garlic mustard provided more effective control than early weeding because late weeding allows second-year plants to compete with garlic mustard seedlings for a longer period of time and (2) competition between first- and second-year plants is responsible for alternating dominance of first-year and second-year garlic mustard plants. [ABSTRACT FROM AUTHOR]

Garlic mustard (Alliaria petiolata) is a nonnative, shade-tolerant forb that was introduced into North America in the mid-1800s. Currently, garlic mustard is spreading across the landscape at a rate of 6400 square kilometers per year. In this article, we synthesize the current state of knowledge on the mechanisms underlying garlic mustard's widespread success and the ecological impacts of its invasion. Although no single mechanism appears to explain the success of garlic mustard, a combination of plant traits--all slightly different from those of native plants--seems to confer garlic mustard with tremendous success in the new habitats it invades. The domination of this new species in eastern forests is clearly changing the ecology of these systems. The consequences of garlic mustard invasion include the loss of biological diversity, ripple effects through higher trophic levels, and changes in the function of soil microbial communities. [ABSTRACT FROM AUTHOR]

• Biological control agents reduce spotted knapweed fitness via multiple pathways. • Seed head insect attack rates exhibited inter-annual variability but no directional change. • Plant responses to biological control can require multiple years to detect. • Plant competition facilitates and can be essential for effective biological control. Spotted knapweed (Centaurea stoebe) is an invasive plant introduced into North America from Europe that has colonized diverse plant communities across the temperate zone of North America. The impacts of classical biological control insects on this plant have been identified as harmful (benefiting the plant), inconsequential, or exhibiting effective controls. Here we present results that measured C. stoebe densities in Colorado subjected to small numbers of insect releases in 2001 and monitored for density effects on the plant from 2007 to 2020. Seed head weevils (Larinus spp), gall flies (Urophora spp) and root weevils (Cyphocleonus achates) were also measured. In one monitoring transect with ample native perennial and introduced grasses, knapweed stem densities declined over a 14-year interval from about 50 stems/m2 to 0/m2. At a second site that was disturbed by flood scour midway through the study, stems declined over a 13 year interval from about 40 stems/m2 to under 20 stems/m2. At a site lacking C. achates , stem densities were measured at 40 stems per m2 in 2020, the same densities reported at other sites in 2008. No statistically detectable directional shifts in growing season precipitation or temperatures have occurred across the 2001–2020 interval, and the strongest relationships among variables measured here were directional declines in stem and rosette densities of the knapweed. Seeds/seed head showed positive relationships with precipitation but negative relationships with seed head Larinus spp densities, which in turn showed no directional trends in abundance. The outcome of biological control releases at these Colorado sites for C. stoebe ranged from excellent to negligible, and our work supports the context specific model of biological control, i.e., biological control in conjunction with plant competition are the essential components for reduction of C. stoebe densities. [ABSTRACT FROM AUTHOR]

• Planted grass turfs inhibit invasive Frangula alnus establishment from seed. • Grass turf reduces seedling emergence, height growth, reproduction, and survival. • Soil compaction and leaf litter have little effect on Frangula alnus establishment. Perennial invasive plants can be controlled by damaging established vegetative individuals or by preventing establishment of new plants from seed. Less information is available about the latter. We examined how seedling establishment of glossy buckthorn (Frangula alnus), a non-native shrub that invades forests in eastern North America, is affected by four soil surface treatments – grass-dominated turf, oak leaf litter, pine leaf litter, soil compaction – relative to an untreated control (tilled soil). Data on seedling emergence, survival, growth, and reproduction were collected from 1.5 m2 plots (six per treatment) over two years at the Kingman Research Farm, Madbury, NH, USA. Overall, buckthorn success from seed was inhibited by grass turf and facilitated by oak litter, while pine litter effects were complex and soil compaction had no effect. Specifically, relative to control, the turf treatment reduced buckthorn seedling emergence (year 2 only), increased mortality over two years, reduced density of living buckthorn, and prevented fruiting. Oak litter resulted in greater buckthorn seedling height relative to control, while pine litter, although reducing seedling emergence (year 1 only), increased both seedling height and the proportion of plants fruiting in year 2. Across all treatments and control, buckthorn seedling emergence was greatest in June, declining thereafter. Of the four treatments, only grass turf offered some promise of resisting invasion. The mechanism of resistance by turf is not known, but likely involves either more intense competition or higher levels of herbivory associated with turf. The mechanism, as well as the logistics and cost of turf establishment in logged areas, require further exploration. [ABSTRACT FROM AUTHOR]