Plant species that receive significant human introduction effort and assistance generally are the most problematic invaders. Despite this, invasive ornamental species in urban settings have received relatively little attention if not invading natural areas. Here we consider Centranthus ruber in an urban setting in South Africa as a case study and explore when emerging invaders are able to cross the urban–wildland interface and what hinders early eradication in urban environments. Centranthus ruber was introduced into Cape Town, South Africa, more than a century ago as a garden ornamental, but until recently was not considered invasive. We determine the current and potential future distribution in South Africa, evaluate current management activities, and provide recommendations for control and legislation. By August 2013, we had found 64 populations, of which 31 were casual, 27 naturalized, and 6 invasive. This increased to more than 530 identified populations by the end of 2015, due to both spread and increased awareness. Centranthus ruber can invade near-pristine areas, with one population found in natural vegetation in the Table Mountain National Park. However, with only one slowly spreading population, the threat might be limited. We found no difference in plant mortality between chemical and mechanical clearing, but with mechanical clearing stimulating the soil seedbank, we recommend chemical methods. Using a species distribution model, we found large parts of South Africa, including the southwestern Cape where we conducted our surveys, to be climatically suitable for this species. Consequently, the category 1b regional listing in NEM:BA in the Western Cape is justified, but a listing in other parts of the country also might be appropriate. Based on our findings, we suggest that the extirpation of C. ruber in South Africa is possible, but without buy-in from residents in urban environments, reinvasion will render this impossible. This study stresses the importance of managing and legislating emerging invaders at the urban–wildland interface and the monitoring of common ornamental species with invasive traits.

Buffelgrass [Pennisetum ciliare (L.) Link] is a drought-tolerant invasive grass that is a threat to native biodiversity in the drylands of the Americas and Australia. Despite efforts from land managers to control P. ciliare, management approaches tend to have mixed success, treatment results can be poorly communicated among entities, and there are few long-term controlled studies. In this literature review, we synthesize data from both peer-reviewed and “gray” literature on the efficacy of management techniques to control P. ciliare and the secondary impacts to native plant communities. Our search resulted in 42 unique sources containing a total of 229 studies that we categorized into 10 treatment types, which included herbicide, seeding, manual removal, fire, grazing, biocontrol, fire + additional treatments, manual removal + additional treatments, herbicide + additional treatments, and herbicide + manual removal. We found that treatments that used multiple techniques in tandem along with follow-up treatments were the most effective at controlling P. ciliare. Fewer than one-third of the studies reported impacts of management on native species, and the most commonly studied treatment (herbicide, N = 130) showed detrimental impacts on native plant communities. However, the average time between treatment and outcome measurement was only 15 mo; we suggest the need for more long-term studies of treatment efficacy and secondary impacts of treatment on the ecosystem. Finally, we conducted a second literature review on P. ciliare biology and traits for mechanisms that allows P. ciliare to alter the invaded environment to facilitate a competitive advantage over native species. We found evidence of self-reinforcing feedbacks of invasion being generated by P. ciliare through its interactions with water availability, nutrient cycling, and disturbance regimes. We developed a conceptual model of P. ciliare based on these feedback loops and offer management considerations based on its invasion dynamics and biology.

Integrated weed management strategies (IWM) are being advocated and employed to control invasive plants species. In this study, we compared three management strategies (biological control alone [BC], BC with fire [BC + F], and BC with mowing [BC + M]) to determine if physical controls reduce seed production by Scotch broom and interfere with the action of the biological control agent—the Scotch broom seed weevil. We measured seed production and seed predation by the weevil at both pod and plant scale, and seed bank density over two field seasons. We found no difference in the number of seeds per pod among management strategies. However, combining management strategies (BC + M and BC + F) resulted in significant reductions in pods per plant, mature seeds per plant, and seed bank density relative to biological control alone. We did not find differences among management strategies in number of weevils per pod or proportion of seeds predated by the weevil at either pod or whole-plant scale. However, combining management strategies (BC + M and BC + F) resulted in a significant reduction in healthy mature seeds per plant relative to biological control alone. Although both integrated strategies outperformed biological control alone in reducing seed production and the seed bank, with no statistical difference between them, we propose that short-rotation prescribed fire could prove to be a more effective strategy for long-term management of Scotch broom due to its potential for slightly greater depletion of the seed bank.

Beach vitex is a salt-tolerant, perennial, invasive shrub that has naturalized in coastal areas of the southeastern United States. Since its introduction in the 1980s, this Pacific Rim native has invaded many fragile beach-dune ecosystems along the Mid-Atlantic, Southern Atlantic, and Gulf of Mexico. Large-scale monocultures of beach vitex supplant native species through rapid vegetative reproduction and seed production. Fruits are capable of water-based dispersal, allowing for potential rapid range expansion in coastal areas. Ecosystem damage resulting from exclusion of native plant species by beach vitex and fears associated with potential negative effects on sea turtle nesting have served to promote the control and survey efforts presently underway in coastal areas of the Carolinas, Virginia, and Maryland.

Riparian plant invasions can result in near-monocultures along stream and river systems, prompting management agencies to target invasive species for removal as an ecological restoration strategy. Riparian plant invaders can alter resource conditions in the benthos and drive bottom-up shifts in aquatic biota. However, the influence of management activities on the structure and function of aquatic communities is not well understood. We investigated how removal of a riparian invader, Lonicera maackii (Amur honeysuckle), influenced aquatic macroinvertebrate community functional and taxonomic diversity in a headwater stream. We hypothesized that removal of L. maackii from invaded riparia would result in (H1) increased aquatic macroinvertebrate abundance, density, and diversity; (H2) a taxonomic and functional shift in community composition; and, in particular, (H3) increased functional diversity. Aquatic macroinvertebrates were sampled monthly from autumn 2010 to winter 2013 in headwater stream riffles with a dense riparian L. maackii invasion and those where L. maackii had been experimentally removed. We found macroinvertebrate density was significantly higher in the L. maackii removal reach (P<0.05) and that macroinvertebrate community structure and functional trait presence was distinct between stream reaches and across seasons (P<0.05). The removal reach exhibited greater functional richness during spring and summer and had more unique functionally relevant taxa (20% and 85%) compared with the L. maackii reach (5% and 75%) during summer and autumn seasons. Our results suggest bottom-up processes link restoration activities in the riparian corridor and aquatic biota through alterations of functional composition in the benthic community.

Tamarisk (a.k.a. saltcedar, Tamarix spp.) is an invasive plant species that occurs throughout western riparian and wetland ecosystems. It is implicated in alterations of ecosystem structure and function and is the subject of many local control projects, including removal using heavy equipment. We evaluated short-term vegetation responses to mechanical Tamarix spp. removal at sites ranging from 2 to 5 yr post-treatment along the Virgin River in Nevada, USA. Treatments resulted in lower density and cover (but not eradication) of Tamarix spp., increased cover of the native shrub Pluchea sericia (arrow weed), decreased density and cover of all woody species combined, increased density of both native annual forbs and the nonnative annual Salsola tragus (prickly Russian-thistle), and lower density of nonnative annual grasses. The treated plots had lower mean woody species richness, but greater herbaceous species richness and diversity. Among herbaceous species, native taxa increased in richness whereas nonnative species increased in both species richness and diversity. Thus, efforts to remove Tamarix spp. at the Virgin River reduced vegetative cover contributing to fuel loads and probability of fire, and resulted in positive effects for native plant diversity, with mixed effects on other nonnative species. However, absolute abundances of native species and species diversity were very low, suggesting that targets of restoring vegetation to pre-invasion conditions were not met. Longer evaluation periods are needed to adequately evaluate how short-term post-treatment patterns translate to long-term patterns of plant community dynamics.

The alien invasive vines black and pale swallowwort are currently spreading across eastern North America, invading parklands, old fields, restored forest sites, and other natural areas. These plants spread by wind-borne seed and can form dense stands where they become established. Although their current geographic ranges overlap, there is little known overlap locally. Preliminary observations and anecdotal information have associated black swallowwort with low-pH inceptisols and pale swallowwort with high-pH alfisols. We conducted a common garden field experiment repeated over two years and a growth chamber germination experiment to assess whether seedling emergence and performance of these two swallowworts are affected by soil type and/or soil pH. Soil pH was artificially modified. In the common garden experiment, plants of both species grown on the Onondaga County soil type (an alfisol) produced a smaller root mass than on the Orange County soil (an inceptisol). Also, in one of two years more seedlings emerged and plants produced more follicles on the Onondaga County soil. Soil pH did not affect seedling emergence, although plants grown on low pH soils had a smaller root dry mass compared with plants grown on higher pH soils. Soil pH effects on stem length, stem dry mass, and follicle production were inconsistent among years. Species differences were also evident, with more pale swallowwort seedlings emerging than black swallowwort seedlings, whereas black swallowwort plants mostly had greater biomass and fecundity than pale swallowwort plants. In the growth chamber experiment, final percentage seed germination was greater on the Onondaga County soil than on the Orange County soil. The germination speed index as well as the probability to reach 50% germination for black swallowwort was higher on the Onondaga County soil than the Orange County soil, but only at lower pH levels. The germination speed index of pale swallowwort on the Orange County soil was higher than black swallowwort at low, but not high, pH levels. In contrast, black swallowwort had a higher probability of reaching 50% germination than pale swallowwort on the Orange County soil at higher pH levels. Contrary to our expectations, interactions between the two swallowwort species with their associated soil type or with their presumably preferred soil pH were weak, contradictory, or non-existent in both experiments. This suggests that these two species can colonize and grow well in a relatively wide range of soil pH conditions. From a management perspective, our results suggest that the current range and local overlap of these two species will continue to increase and that early detection rapid response (EDRR) programs should be established in susceptible regions not yet colonized by these two invasive vines.

On the west coast of North America and in Australia, there have been parallel cases of sequential invasion and replacement of the shoreline plant American sea-rocket by European sea-rocket. A similar pattern has also occurred in New Zealand. For 30 to 40 yr, from its first recording in 1921, American sea-rocket spread throughout the eastern coastlines of the North and South Islands of New Zealand. European sea-rocket has so far been collected only on the North Island. From its first collection in 1937, European sea-rocket spread to the northern extremity of the island by 1973, and by 2010, it had reached the southernmost limit. In the region where both species have occurred in the past, American sea-rocket is now rarely found. This appears to be another example of congeneric species displacement.

Prediction of invasive species distributions from survey data is widely recognized as a significant component of forest management and conservation planning. Leucaena leucocephala is the most aggressive invasive shrub and tree in the Hengchun peninsula in southern Taiwan. We analyzed geo-referenced data to identify potential variables of invasion and to predict likelihood of further invasion using boosted regression trees. Our results, which classified 92% of the cells correctly with regard to species presence and absence, indicated probability of invasion is correlated with climatic conditions (temperature and precipitation), landscape features (altitude; slope ratio and aspect; percentages of natural or secondary forest, agriculture land, developed area, and water bodies; and distances to the nearest forest edge and river), and anthropogenic factors (length of forest edge, and distances to the nearest road and agriculture land). The most influential variables are average annual temperature, altitude, precipitation, and slope. Continued range expansion by L. leucocephala is most likely to proceed (1) from the eastern and western portions toward the central portion of Hengchun township and (2) throughout the southern and toward the eastern portions of Manjhou township. Our model should provide useful information to aid forest managers in the development of long term monitoring and control strategies for L. leucocephala, in the early detection and eradication of newly established invasions, and also a framework for the integration and analysis of new presence and absence field data as they become available.

Several states in the Midwestern United States are using risk assessment to determine the invasiveness of introduced plant species, and each assessment process is different. This may lead to differences in results for the same species between states, creating concern about credibility by those using the assessments. In this study, risk assessments for six Midwestern states were compared, examining format, content, and assessment committee membership. Case studies were conducted for four species for which at least five of the six states in the study completed a risk assessment; results were compared in the context of general differences in assessment content and those specific to each species. Furthermore, 14 species for which only four of the six states completed assessments were briefly examined for outcome differences only, and possible reasons for these inconsistencies. Overall, differences in assessments did not result in incompatible conclusions for the species compared, suggesting that unique assessments in each state can provide consistent and credible results. We propose that these Midwestern states share species resources with each other to further improve consistency between the assessments.

Because of its ability to transform ecosystems by increasing the prevalence of fire, the invasive annual red brome is a priority exotic species for management in arid lands of the southwestern United States. By sampling red brome presence and 97 environmental (climatic, topographic, and soil) and native vegetation (e.g., perennial species richness) variables on 126 sites, we assessed biophysical correlates with red brome distribution on a 755,000-ha (1.9 million ac) Mojave Desert landscape. Brome occupied 55 of 126 (44%) 0.09-ha plots. The simplest models (i.e., those containing the fewest or most easily obtained variables) in multivariate (classification trees and nonparametric multiplicative regression) and univariate (χ2) models often portrayed red brome distribution as well, or nearly as well, as more complicated models containing more variables harder to obtain. The models varied, however, in their abilities for describing brome presence compared with absence. For example, a simple classification tree using only elevation, soil great group, parent material, and vegetation type improved estimates of brome presence for 55% of sites, absences for 87%, and overall for 73% of sites compared with a naïve model containing the observed frequency of brome in the data. Conversely, a more complicated model, including soil boron and sulfur, performed better for presences (96%) than for absences (73%; 83% overall). Results also showed variable support for two general postulates in invasive species science. Red brome distribution was not correlated with soil N, which is inconsistent with the supposition that nutrient-rich soils are more prone to invasion. Brome was correlated with native perennial species richness to support the postulate that exotic species abundance is correlated with species-rich habitats, but the correlation was weak (r = 0.38) and similar in strength to correlations with many other environmental variables. On this relatively low-elevation landscape, the areas currently most invaded by red brome include the higher elevations (> 777 m [2,549 ft]), limestone–sandstone soils, and burrobush and mixed perennial communities. Areas least inhabited by brome are the lowest elevations (< 491 m), gypsum soils, and creosotebush and saltbush communities.

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.

Successful invasive plant eradication is rare, because the methods used target the adult stage, not taking into account the development capacity of a large seedbank. Heating by microwave was considered, because it offers a means to quickly reach the temperature required for loss of seed viability and inhibition of germination. Previous results were not encouraging, because homogeneous and deep-wave penetration was not achieved, and the various parameters that can affect treatment effectiveness were incompletely addressed. This study aimed to determine, under experimental conditions, the best microwave treatment to inhibit invasive species seed germination in terms of power (2, 4, 6 kW) and duration (2, 4, 8 min) of treatments and depending on soil moisture (10%, 13%, 20%, 30%) and seed burial depth (2, 12 cm). Three invasive species were tested: Bohemian knotweed, giant goldenrod, and jimsonweed. The most effective treatments required relatively high power and duration (2kW8min, 4kW4min, 6kW2min, and 6kW4min; 4kW8min and 6kW8min were not tested for technical reasons), and their effectiveness diminished with increasing soil moisture with germination percentage between 0% and 2% for the lowest soil moisture, 0% and 56% for intermediate soil moisture, and 27% and 68% in control treatments. For the highest soil moisture, only 2kW8min and 4kW4min reduced germination percentage between 2% and 19%. Occasionally, germination of seeds located at the 12-cm depth was more strongly affected. Giant goldenrod seeds were the most sensitive, probably due to their small size. Results are promising and justify further experiments before developing a field microwave device to treat large volumes of soil infested by invasive seed efficiently and with reasonable energy requirements. Other types of soil, in terms of texture and organic matter content, should be tested in future experiments, because these factors influence soil water content and, consequently, microwave heating.

The introduced exotic vines pale and black swallowwort rapidly have become invasive throughout regions of the northeastern United States and adjoining areas of Canada. Preliminary studies have reported that the species are allelopathic, possibly contributing to their competitive ability and invasiveness. Results from our laboratory assays indicated that swallowwort root exudates caused significant root length reductions (e.g., 40% for butterfly milkweed and 20% for large crabgrass) and reduced germination (e.g., 25% for lettuce) of indicator species. Additional bioassays with dried swallowwort tissues demonstrated that tissue leachates caused varied responses in indicators, with both significant stimulatory and inhibitory effects. In particular, significant congeneric interactions were noted between the two swallowwort species. Evidence from this study of swallowwort tissue phytotoxicity has important implications for developing effective management and habitat restoration strategies for the two invasive species.

Scotch broom is an invasive leguminous shrub in California and other Pacific Northwest states, as well as New Zealand and Australia. It is highly competitive in forest and shrub communities and can significantly impact reestablishment of conifer forests. The objective of this experiment was to evaluate mechanical methods (Weed Wrench, lopping), several herbicides, and herbicide application techniques for control of Scotch broom in a premontane site in California. Three herbicides were evaluated (glyphosate, imazapyr, and triclopyr ester) for canopy reduction using foliar, drizzle, and basal bark treatments. All treatments were made in both fall and late spring. In addition, we conducted a cost analysis of the various herbicide treatments and application methods. Results indicate that both mechanical treatments were effective, but their optimum timing depended on soil moisture conditions. In addition, there were no significant differences among herbicides at both timings, among all rates, and for any application method. All herbicides provided effective control of Scotch broom. However, the cost analysis demonstrated that the drizzle application method with glyphosate was the most cost-effective treatment, due to low herbicide cost and reduced labor requirements. These results provide several options for Scotch broom control and give land managers considerable flexibility with timing, herbicide, and application technique in their management programs.

Understanding competition for soil nitrate between common shrub-steppe, potential reclamation species, and common invasive species is necessary to identify mechanisms associated with ecosystem invasion and can assist with developing weed management scenarios. We designed a field experiment to evaluate the differential competitive effects of the invasive annual grass downy brome, the invasive biennial forb dyer's woad, and the reclamation shrub prostrate kochia, on nitrate acquisition of the perennial grass crested wheatgrass, the native forb western yarrow, and the native shrub big sagebrush. Individual plants were grown in two-plant neighborhoods, and a K15NO3 tracer was injected into the soil between plants and recovered from leaf material after 5 to 11 d. We also evaluated neighbor effects on shoot and root growth, leaf carbon : nitrogen ratio, and leaf nitrogen concentrations to better understand how these traits are associated with differences in nitrate acquisition and nitrogen allocation among the three growth forms. Nitrate acquisitions by crested wheatgrass and western yarrow were significantly lower when competing with downy brome than with dyer's woad and prostrate kochia; however, competitors had similar, negative effects on nitrate acquisition by big sagebrush. Nitrate acquisition ratios between competing neighbors revealed that: (1) the grasses always acquired more nitrate than neighbors of a different growth form, (2) western yarrow was equally competitive with dyer's woad and prostrate kochia, and (3) all neighbors acquired more nitrate than big sagebrush. More successful competition for nitrate in the grasses was associated with greater specific root length. Compared to species of the same respective growth form, the two invasive weeds (downy brome and dyer's woad) and prostrate kochia always had significantly lower leaf carbon : nitrogen ratio, and greater leaf nitrogen concentration, which have been broadly correlated with leaf lifespan and nutrient use efficiency, and indicate differing strategies to persist in the semiarid shrub-steppe ecosystems.

The European vines pale swallowwort and black swallowwort are invading various habitats in northeastern North America. It is unclear how these plants might respond to potential biological control agents, as they experience little herbivore damage in North America, or longer durations of mowing given the reported lack of efficacy of mechanical control. We evaluated the effect of six seasons of artificial defoliation (50 or 100% defoliation once or twice per season) and clipping (once, twice, or four times at 8 cm above the soil level) on the survival, growth, and reproduction of mature plants of the two species grown in a common garden field experiment. No plants died from damage after 6 yr. Black swallowwort produced more aboveground biomass, whereas pale swallowwort produced more root biomass and root crown buds, compared with its congener species. For most damage treatments, root biomass and the number of crown buds and stems increased over time, whereas aboveground biomass and viable seeds per plant generally did not change. Substantial overlap in plant size and seed production occurred among damage treatments and species. The most severe defoliation treatment did not substantially limit growth and reproduction compared with undamaged plants. While two clippings per season sometimes prevented seed production, four clippings per season was the only type of damage that consistently prevented plant growth and eliminated seed production. Pale and black swallowwort display a high tolerance to aboveground tissue loss in high-light environments without plant competition. The annual increase in plant size calls into question the potential efficacy of a defoliating insect against field populations of swallowworts, and it seems likely the only benefits of a long-term mowing regime will be to eliminate seed production.

Invasion of closed canopy forests by shade-tolerant alien plants has the potential to modify species composition, stand structure, ecosystem function, and long-term forest development patterns. Ligustrum sinense is a shade-tolerant alien shrub that has invaded bottomland forests throughout the southeastern United States. This species has received comparatively little attention in the literature despite its potential to drastically alter invaded sites. The overarching goal of our study was to document the relationships between Ligustrum sinense invasion and woody plant biodiversity and development patterns in an intact southeastern U.S. bottomland forest. The forest was dominated by Quercus nigra and Liquidambar styraciflua. Ligustrum sinense ranked fifth in basal area contribution, occurred in 97% of our plots, and represented 95% of all understory stems. Spearman's rho for dominance (based on basal area of stems > 5 cm diameter at breast height [dbh]) of L. sinense and woody plant species richness for each plot revealed a significant negative relationship (rs = −0.69, P < 0.01). A similar relationship was revealed between L. sinense density and woody plant species diversity (rs = −0.78, P < 0.01) and evenness (rs = −0.82, P < 0.01). Spearman's rho for L. sinense density and native understory stem density (individuals ≥ 1 m height, < 5 cm dbh) also revealed a significant negative association (rs = −0.48, P < 0.01). Under the current disturbance regime and without active management, we projected the forest would shift to support a stronger component of L. sinense and that structure would transition from tree to shrub dominance for sites within the forest.

The relationship between plant community composition and invasibility has been studied extensively but seldom in the context of ecosystem restoration. Experimental riparian restoration plots differing in species composition and density were established and evaluated for susceptibility to invasion by giant reed, a common riparian invader in California, and natural recruitment by riparian species over time. Plots were planted in 2002 with cuttings of common threesquare (a sedge), seepwillow (a shrub), and Goodding's willow (a tree) at two densities in monoculture and all possible mixture combinations. Giant reed rhizomes were introduced into half of the plots in the spring of 2003, while the remaining plots were allowed to undergo natural recruitment for an additional year. In late winter 2004, giant reed rhizomes were planted in the remaining plots. Both planting groups were followed for one growing season to evaluate giant reed establishment, survival, and growth. Community composition affected giant reed performance, particularly in 2003 before natural recruitment occurred. In that year, plots containing seepwillow + willow had the lowest giant reed shoot production, growth, and survival. All plots containing seepwillow were resistant to colonization by natural recruitment in 2004, but none of the planting treatments affected giant reed success in that year. Giant reed was more successful overall in 2004 despite deeper shade and drier soils. This pattern could be attributed to larger initial rhizome size in 2004, which allowed giant reed to overcome environmental stress during establishment. Planting density did not impact giant reed or natural recruitment independently, but may affect environmental parameters and warrants further study as a potential contributor to restoration success. Our results indicate that choice of species composition in restoration might impact giant reed invasion success initially, but community resistance might not be sustainable and maintenance-free over time.

Aminopyralid efficacy on Canada thistle and changes in species composition in both Canada thistle–infested and native plant communities were evaluated in Theodore Roosevelt National Park, ND. Aminopyralid at 120 g ae/ha was applied in September 2004 to one-half of 30 native or 30 Canada thistle–infested sites. Canada thistle density 10 mo after treatment (MAT) averaged 2 stems/m2 compared to 31 stems/m2 in the control and 22 MAT averaged 16 stems/m2 compared to 42 stems/m2. Although Canada thistle was reduced in treated sites, species richness was similar to the untreated control. Species evenness increased in treated subplots compared to the control as foliar cover of low- and high-seral forbs and high-seral monocot species increased. Species richness was reduced in native plant communities treated with aminopyralid from approximately 12 to 9 species but evenness was not affected. Plant species diversity was similar in Canada thistle–infested treated and nontreated subplots but was reduced by aminopyralid in native subplots. Despite the decrease in Canada thistle and increased foliar cover of native perennial plants, composition and structure 22 MAT in the Canada thistle–infested plant community did not resemble native plant communities. Native plant communities were composed of many perennial monocots and high-seral forbs, which had increased but were in lower relative abundance in the treated Canada thistle–infested subplots.