Invasive plants are expanding their ranges due to climate change, creating new challenges for invasive species management. Early detection and rapid response could address some nascent invasions, but limited resources make it impossible to monitor for every range-shifting species. Here, we aimed to create a more focused watch list by evaluating the impacts of 87 plant species projected to shift into northern New England (the states of Maine, New Hampshire, and/or Vermont). We used the Environmental Impact Classification for Alien Taxa (EICAT) protocol to evaluate all ecological impacts reported in the scientific literature, scoring ecological impacts from 1 (minimal concern) to 4 (major) depending on the level of reported impact. For each species, we also recorded any reported impacts on socioeconomic systems (agriculture, human health, or economics) as “present.” We found 24 range-shifting species with impacts on ecological communities, of which 22 have reported impacts in ecosystems common to northern New England. Almost all of these species also had impacts on socioeconomic systems and were available for purchase at ornamental plant retailers or online. Thus, these species can be considered high risk to northern New England with climate change based on their large negative impacts and potential to arrive quickly with deliberate human introduction. Our study demonstrates the use of impact assessments for creating targeted priority lists for invasive species monitoring and management.

Risk assessments of biological invasions rarely account for native species performance and community features, but the assessment presented here could provide additional insights for management aimed at decreasing vulnerability or increasing resistance of a plant community to invasions. To gather information on the drivers of native plant communities' vulnerability and resistance to invasion, we conducted a literature search and meta-analysis. Using the data we collected, we compared native and invasive plant performance between sites with high and low levels of invasion. We then investigated conditions under which native performance increased, decreased, or did not change with respect to invasive plants. We analyzed data from 214 publications summing to 506 observations. There were six main drivers of vulnerability to invasion: disturbance, decrease in resources, increase in resources, lack of biotic resistance, lack of natural enemies, and differences in propagule availability between native and invasive species. The two mechanisms of vulnerability to invasion associated with a strong decline in native plant performance were propagule availability and lack of biotic resistance. Native plants marginally benefited from enemy release and from decreases in resources, while invasive plants strongly benefited from both increased resources and lack of enemies. Fluctuation of resources, decreases and increases, were strongly associated with higher invasive performance, while native plants varied in their responses. These differences were particularly strong in instances of decreasing water or nutrients and of increasing light and nutrients. We found overall neutral to positive responses of native plant communities to disturbance, but natives were outperformed by invasive species when disturbance was caused by human activities. We identified ecosystem features associated with both vulnerability and resistance to invasion, then used our results to inform management aimed at protecting the native community.

Many restoration projects rely on invasive plant removal to restore ecosystems. However, success of restoration efforts relying on invasive removal can be jeopardized, because in addition to displacing native plants, invasives can also dramatically impact soils. Many studies have documented invasives' effects on soil chemistry and microbiota. While European beachgrass [Ammophila arenaria (L.) Link] is a worldwide invasive problem in coastal dunes outside northern Europe, little attention has been paid to effects of this species on soil chemistry following invasion, even though it establishes persistent, dense monocultures. In our study, we evaluated effects of A. arenaria invasion on soil chemistry of coastal dunes at Point Reyes National Seashore (PRNS); persistence of effects following removal by mechanical or herbicide treatment (legacy effects); and effects of treatment independent of invasion. Dune restoration efforts at PRNS have met with mixed success, especially in herbicide-treated backdunes, where decomposition of dead A. arenaria has been greatly delayed. Based on results, invasion impacted 74% of 19 variables assessed, although there was a significant interaction in many cases with successional status (earlier vs. later). Almost 60% of invasion effects persisted after restoration, with legacy effects prevalent in herbicide-treated backdunes where sand deposition from adjacent beaches could not mitigate effects as it could in herbicide-treated foredunes. Mechanical removal—or inversion of invaded surface soils with less-contaminated subsoils—resulted in fewer legacy effects, but more treatment effects, primarily in backdunes. Soil chemistry may decelerate decomposition of A. arenaria due to the limited nitrogen (N) available to enable microbial breakdown of the high carbon(C):N (70.8:1) material, but microbial factors probably play a more important role. Success of restoration at PRNS may not be fully realized until legacy effects are resolved through additional actions such as inoculation with healthy microbiomes or necromass reduction through controlled burning.

Knotweed species in the genus Reynoutria are native to eastern Asia but have become noxious weeds in Europe and North America. In the United States, invasive populations of Japanese knotweed (Reynoutria japonica Houtt.), giant knotweed [Reynoutria sachalinensis (F. Schmidt) Nakai], and their interspecific hybrid known as Bohemian knotweed (R. × bohemica Chrtek & Chrtková) continue to expand their ranges. Although these plants are among the most invasive terrestrial species, there are relatively few molecular tools for identifying the parental species, the F₁ hybrid, or subsequent hybrids or introgressed individuals. We studied Reynoutria populations in Wisconsin, a state where all three taxa grow, to determine whether molecular data would be useful for distinguishing species and identifying hybrids. We obtained DNA sequence data from the plastid matK gene and the nuclear LEAFY gene and compared these to previously published sequences. Data from the uniparentally inherited matK region included haplotypes attributable to R. japonica and R. sachalinensis. Nuclear data indicated that R. sachalinensis plants are most similar to native plants in Japan, with two Wisconsin accessions exhibiting a monomorphic genotype for the LEAFY gene. Three Wisconsin accessions of R. japonica were each characterized by having three distinct kinds of LEAFY sequence. Most plants in our study were found to possess two or three phylogenetically distinct copies of the LEAFY gene, with the copies being most closely related to R. japonica and R. sachalinensis, respectively, and these were inferred to be interspecific hybrids. Altogether, five kinds of interspecific hybrids were identified, reflecting various combinations of LEAFY sequence types from the parental species. The widespread existence of hybrid plants in Wisconsin, many of which are morphologically identifiable as R. japonica, indicates a cryptic genetic diversity that should be examined more broadly in North America using molecular tools.

Garlic mustard [Alliaria petiolata (M. Bieb.) Cavara & Grande] is a biennial invasive plant commonly found in the northeastern and midwestern United States. Although it is not recommended to apply herbicides after flowering, land managers frequently desire to conduct management during this timing. We applied glyphosate and triclopyr (3% v/v and 1% v/v using 31.8% and 39.8% acid equivalent formulations, respectively) POST to established, second-year A. petiolata populations at three locations when petals were dehiscing and evaluated control, seed production, and seed viability. POST glyphosate applications at this timing provided 100% control of A. petiolata by 4 wk after treatment at all locations, whereas triclopyr efficacy was variable, providing 38% to 62% control. Seed production was only reduced at one location, with similar results regardless of treatment. Percent seed viability was also reduced, and when combined with reductions in seed production, resulted in a 71% to 99% reduction in number of viable seeds produced per plant regardless of treatment. While applications did not eliminate viable seed production, our findings indicate that glyphosate and triclopyr applied while petals are dehiscing is a viable alternative to cutting or hand pulling at this timing, as it substantially decreased viable A. petiolata seed production.

Medusahead [Taeniatherum caput-medusae (L.) Nevski] is an invasive annual grass spreading into rangelands throughout the western United States. We tested cattle (Bos taurus L.) utilization of T. caput-medusae following treatment with glyphosate in two forms of its salt (potassium salt and isopropylamine salt) at three different rates of application; low (236 g ae ha^-1), medium (394 g ae ha^-1), and high rates (788 g ae ha^-1) in eastern Washington. The herbicide was applied on April 26, 2016. A second location, northern Utah, was treated with glyphosate in the form of its isopropylamine salt at the high rate. The herbicide was applied on June 5, 2019. Cattle were allowed to start grazing T. caput-medusae 15 d after glyphosate treatment and had unlimited access to the glyphosate-treated plots for more than 85 d. The greatest utilization of T. caput-medusae occurred at the highest glyphosate application rate (P < 0.05), in Washington, with no difference between forms of glyphosate salt. Cattle also consumed T. caput-medusae at the Utah site (P < 0.05). Glyphosate treatment preserved the water-soluble carbohydrate content of T. caput-medusae at levels greater than the nontreated controls (P < 0.05) at both locations. The glyphosate treatment assisted in the increased utilization of T. caput-medusae by cattle and is a viable option for the reduction of T. caput-medusae while increasing the forage value of the weed.

Exotic conifers are rapidly spreading in many regions of New Zealand, as well as in many other countries, with detrimental impacts on both natural ecosystems and some productive sector environments. Herbicides, in particular the active ingredient triclopyr, are an important tool to manage invasive conifers, yet there is a paucity of information that quantifies the amount of herbicide required to kill trees of different sizes when applied as a basal bark treatment. Two sequential experiments were conducted to define the amount of triclopyr required to kill individual invasive lodgepole pine (Pinus contorta Douglas ex Loudon), trees of different sizes when applied in a methylated seed oil to bark (either the whole stem or base of the tree) and to determine which tree size variates (height, diameter at breast height [DBH], crown diameter [CD]) or derived attributes (crown area, crown volume index) best characterized this dose–response relationship. The outcomes of the dose–response research were compared with field operations where triclopyr was applied to the bark of trees from an aerial platform. Applying the herbicide to the whole stem, as opposed to the base of the tree only, significantly increased treatment efficacy. The tree size variates DBH, CD, crown area, and crown volume index all provided good fits to the tree mortality data, with >91% prediction accuracy. Of these variates, CD provided the most practical measure of tree size for ease of in-field calculation of dose by an operator. Herbicide rates used in field operations were seven to eight times higher than lethal doses calculated from experimental data. Our results highlight the potential for substantial reductions in herbicide rates for exotic conifer control, especially if dose–response data are combined with remotely sensed quantitative measurements of canopy area or volume using new precision technologies such as unmanned aerial vehicles.

Nonnative invasive plant species are a major cause of ecosystem degradation and impairment of ecosystem service benefits in the United States. Forested riparian areas provide many ecosystem service benefits and are vital to maintaining water quality of streams and rivers. These systems are also vulnerable to natural disturbances and invasion by nonnative plants. We assessed whether planting native trees on disturbed riparian sites may increase biotic resistance to invasive plant establishment in central Vermont in the northeastern United States. The density (stems per square meter) of invasive stems was higher in non-planted sites (x = 4.1 stems m^-2) compared with planted sites (x = 1.3 stems m^-2). More than 90% of the invasive plants were Japanese knotweed [Fallopia japonica (Houtt.) Ronse Decr.; syn. Polygonum cuspidatum Siebold & Zucc.]. There were no significant differences in total stem density of native vegetation between planted and non-planted sites. Other measured response variables such as native tree regeneration, species diversity, soil properties, and soil function showed no significant differences or trends in the paired riparian study sites. The results of this case study indicate that tree planting in disturbed riparian forest areas may assist conservation efforts by minimizing the risk of invasive plant colonization.