The Eurasian perennial mustard, hoary cress, Cardaria draba (Brassicaceae), is an invasive weed in western North America that can displace native plants and other desirable forage species in pastures and rangelands. This study investigated the competitive ability of 11 grasses representing nine species in the genera Festuca and Poa from either North America (NA) or Europe (EU) against hoary cress. Hoary cress was grown alone and with four different grass densities under controlled conditions with ample water and nutrient supply, typical for riparian and disturbed habitats, in which hoary cress infestations are commonly found. Of the five Festuca grasses tested, only Schedonorus phoenix (= F. arundinacea) decreased hoary cress biomass. Four of the six Poa grasses tested decreased hoary cress biomass and were ranked by decreasing competitive effects as follows: P. annua > P. compressa (EU) > P. pratensis (EU) > P. compressa (NA). The most competitive grass species also experienced higher intraspecific competition. Within both grass genera, Eurasian species were more competitive against hoary cress than their North American congeners, which suggests that hoary cress establishment may be facilitated by lower biotic resistance in the invaded range. Regardless of origin, sod-forming Poa species were competitive at low densities, while Festuca species only had significant effects at high densities if at all. Based on our results we recommend the use of Poa species for restoration of riparian and disturbed sites following the control of hoary cress infestations to restrict recolonization.

Knowledge of photosynthetic capacity is crucial for fully understanding a species’ invasive potential and for the development of appropriate control strategies. Although growth and reproductive data are available for the invasive swallowwort vines Vincetoxicum nigrum and V. rossicum, photosynthetic data are wanting. These herbaceous, perennial congeners were introduced from separate European ranges during the late 19th century and became invasive during the following century in the northeastern United States and southeastern Canada. Vincetoxicum nigrum has been observed growing mainly in high light environments, whereas V. rossicum occurs across a wide range of light environments, suggesting niche divergence and that different management strategies might be needed for the two species. In this work, we investigated whether the differing habitat associations of these species is reflected in their photosynthetic capacities and leaf morphology. Photosynthetic parameters and specific leaf mass were determined across a range of light environments represented by four field habitats (common garden, forest edge, old field, and forest understory) and two greenhouse environments (high and low light). In the high-light common garden habitat, V. nigrum achieved 37% higher maximum photosynthetic rates than V. rossicum, but photosynthetic performance of the two species was the same in the forest edge habitat. Additionally, species’ performance was virtually identical in high light, low light, and transitions between high and low light regimes in the greenhouse. Specific leaf mass of V. nigrum was 17% higher in the common garden and 19% higher in the greenhouse compared with V. rossicum. Both invasive Vincetoxicum spp. appear capable of growing within a broad range of light environments and their management should be similar regardless of light environment. Other explanations are required to explain the scarcity of V. nigrum in low light natural areas.

The interactive effects of herbivory, exotic species, and other human-mediated changes to the biosphere are reducing species diversity and altering ecosystem services globally. In this study, we tested whether facilitation could be used as a management technique to restore a degraded northeast Wisconsin forest understory facing high white-tailed deer (Odocoileus virginianus) browse pressure and high abundance of the exotic herb garlic mustard [Alliaria petiolata (Bieb.) Cavara & Grande]. Specifically, we attempted to facilitate native understory restoration by planting or seeding native herbs into three different matrix densities of the native, browse-tolerant grass Virginia wildrye (Elymus virginicus L.), which were either protected from (fenced), or accessible to, deer browsing. Deer had minimal impacts on E. virginicus but significantly reduced the cover, survival, and flower production of white snakeroot [Ageratina altissima (L.) King & H.E. Robins.] transplants, largely independent of the density of E. virginicus plantings in open-access plots. In contrast, the richness and abundance of native-seeded species were not affected by deer access but were reduced by increasing E. virginicus densities. Alliaria petiolata cover was significantly higher in plots accessible to deer but declined significantly with increasing E. virginicus planting density in both open-access and fenced plots. These results were largely corroborated by results from an associated observational study, with the exception that natural E. virginicus stands supported slightly higher native-species richness than did adjacent areas lacking E. virginicus. Thus, although we found little support that establishing E. virginicus facilitated browse-susceptible native understory herbs during our short-term experimental study, restored E. virginicus successfully established, thereby increasing native species cover and significantly reducing the cover of the exotic A. petiolata. We suggest the planting of browse-tolerant native species, such as E. virginicus, as a viable restoration technique in heavily browsed, A. petiolata–invaded woodlands.

Ornamental jewelweed (Impatiens glandulifera Royle) is an alien invasive plant in Europe. This annual plant often grows in riparian habitats where herbicides are prohibited. Several studies have reported the negative effect on ecosystem and ecosystem services by this species. However, limited research is published on control measures and the aim of our study was to explore use of hot water and cutting to control I. glandulifera. A lab experiment showed that the lethal water temperature for seed was between 45 and 50 C. In a pot experiment with seeds in soil, emergence of I. glandulifera was reduced by 78% and 93% compared with the untreated control with volumes of hot water (80 C) of 7.2 and 14.5 L m−2, respectively. When treatments were conducted on relatively tall plants (almost 60 cm) in late June, hot water gave significantly better control than cutting. Compared with an untreated control, I. glandulifera cover was reduced by 97% and 79% after hot water and cutting, respectively. Application of hot water to smaller (<40 cm) and less developed plants (BBCH 12–13) in early June and cutting of plants with visible flower buds (mid-July) led to no significant difference in cover. Compared with an untreated control, I. glandulifera cover was reduced by 99% (cut below first node) and 91% (hot water and cut above first node). When relatively tall plants (almost 60 cm) were treated, hot water use was high (31.1 L m−2) and required twice as many work hours (4.8 min m−2) as cutting (2.4 min m−2). When smaller plants (<40 cm) were targeted, work hours and hot water use were reduced to 2.1 min m−2 and 13.7 L m−2, respectively.

Chinese privet (Ligustrum sinense Lour.) is a deciduous to evergreen shrub with an expansive nonnative global range. Control costs are often high, so land managers must carefully consider whether the plant’s potential negative effects warrant active management. To help facilitate this decision-making process, we reviewed and synthesized the literature on the potential ecological effects of L. sinense invasion. We also identified research gaps in need of further study. We found ample evidence of negative relationships between L. sinense invasion and native plant communities. While observational studies are not able to confirm whether L. sinense is driving these relationships, experimental evidence suggests that there is a cause–effect relationship. Of particular concern is the possibility that L. sinense could suppress forest regeneration and cause areas to transition from forest to L. sinense–dominated shrublands. Although this outcome would obviously impact a wide variety of wildlife species, empirical evidence of negative effects of L. sinense on wildlife are limited, and some species may actually benefit from the additional cover and foraging opportunities that L. sinense can provide. Further research on the potential effects of L. sinense invasion on large-scale forest structure and wildlife populations is needed. In areas where L. sinense invasion is a concern, evidence suggests early detection and management can mitigate control costs.

Callery pear (Pyrus calleryana Decne.) is rapidly spreading in the United States, gaining attention in the last two decades as a serious invasive pest. Recommended control methods include foliar, basal bark, cut stump, and hack-and-squirt application of herbicides, but there are few published studies with replicated data on efficacy. Four readily available herbicidal active ingredients and a combination of two active ingredients were tested for control efficacy against P. calleryana in old-field areas and loblolly pine (Pinus taeda L.) understory. Basal bark applications (triclopyr, triclopyr + aminopyralid), foliar applications (glyphosate, imazapyr), and a soil application (hexazinone) effectively killed P. calleryana with the exception of hexazinone at one site, where rainfall may not have been optimal. Foliar application of glyphosate provided the most consistent control. Our results demonstrate efficacy of registered herbicide formulations for P. calleryana control in two geographic locations and two habitat types. The need for development of integrated pest management programs for P. calleryana is discussed.

Multiple introductions of an exotic species can facilitate invasion success by allowing for a wider range of expressed trait values in the adventive range. Schinus terebinthifolius (Brazilian peppertree) is an invasive shrub that was introduced into Florida in two separate introductions and has subsequently hybridized, resulting in three distinct lineages (eastern, western, and hybrid). To determine whether allocation of aboveground biomass differed by introduction history, we destructively sampled 257 stems from each of six populations with differing introduction histories. The proportion of aboveground biomass allocated to fruit, wood, and leaves differed among the three populations. To determine whether the relationship between stem size and several dependent variables that measure plant performance (total dry weight, wood dry weight, number of fruits, fruit dry weight, leaf dry weight, and number of leaves) differed quantitatively by introduction history, we performed analyses of covariance. Slopes of these relationships (dependent variable vs. stem size) varied by lineage. Hybrid populations had the steepest slopes for one set of dependent variables (total dry weight, wood dry weight, and leaf dry weight), while western populations had the steepest slopes for a different set of dependent variables (number of fruits, fruit dry weight, and number of leaves). The parameterized regression equations for each dependent variable and lineage were used to nondestructively estimate different kinds of production by individuals that are part of long-term longitudinal studies to understand the demographic consequences of these different biomass allocation strategies for the performance of S. terebinthifolius individuals across the invaded range in Florida.

Bushkiller, an aggressive perennial vine native to Southeast Asia, has invaded several sites in Alabama, North Carolina, Texas, Louisiana, and Mississippi. Bushkiller has only recently been discovered in North Carolina. The potential economic and environmental consequences associated with established exotic invasive perennial vines and the lack of published control measures for bushkiller prompted research to be conducted at North Carolina State University that may be used in an early-detection rapid-response program. Field and greenhouse studies were conducted to determine bushkiller response to selected foliar-applied herbicides. Field study 1 evaluated efficacy of glyphosate, triclopyr, triclopyr plus 2,4-D, triclopyr plus aminopyralid, and triclopyr plus glyphosate applied postemergence to bushkiller. No control was evident from any treatment at 10 mo after application. In a separate experiment, aminocyclopyrachlor, imazapyr, metsulfuron, sulfometuron, and sulfometuron plus metsulfuron were applied postemergence to bushkiller. Control with aminocyclopyrachlor, imazapyr, sulfometuron, and sulfometuron plus metsulfuron was 88 to 99% at 10 mo after application. Each treatment was also applied to bushkiller in a greenhouse trial. Aminocyclopyrachlor and triclopyr-containing treatments generally resulted in the greatest control, lowest dry weights, and shortest vine lengths among the treatments. These results indicate that several herbicides may be employed initially in an early-detection, rapid-response program for bushkiller. Additional research is needed to determine how effective these herbicides would be in multiple-season treatments that may be required at well established bushkiller infestation sites.

Orange hawkweed is a perennial European plant that has colonized roadsides and grasslands in south-central and southeast Alaska. This plant is forming near-monotypic stands, reducing plant diversity, and decreasing pasture productivity. A replicated greenhouse study was conducted in 2006 and repeated in 2007 to determine the efficacy of six herbicides (aminopyralid, clopyralid, picloram, picloram + chlorsulfuron, picloram + metsulfuron, and triclopyr) for orange hawkweed control. Based on results of the greenhouse trials, replicated field studies were conducted at two sites each year in 2007 and 2008 with three rates each of aminopyralid and clopyralid to determine efficacy of orange hawkweed control and impacts on nontarget native vegetation. In the field, only aminopyralid at 105 g ae ha−1 (0.1 lb ae ac−1) and clopyralid at 420 g ae ha−1 controlled orange hawkweed consistently, with peak injury observed 1 yr after treatment. Control with clopyralid was slightly less than that provided by aminopyralid at all observation times, except at Homer, AK, in 2007, where there was a near-monotypic stand of orange hawkweed, and clopyralid did not remove all orange hawkweed plants. Aminopyralid controlled clover (Trifolium spp.), seacoast angelica (Angelica lucida), arctic daisy (Chrysanthemum arcticum), common hempnettle (Galeopsis tetrahit), and willow (Salix spp.) in the treated areas. Other plant species, such as grasses and some annual forbs, recovered or increased following control of the hawkweed. Clopyralid had less impact on nontarget species with most recovering the year after treatment. In a pasture system, where grasses are preferred to forbs and shrubs, aminopyralid has an advantage because it controls a broader array of forbs compared with clopyralid. In natural areas, where the desire to retain biodiversity and the aesthetics of multiple forb species mixed with grasses and willows is preferred, clopyralid will leave greater species diversity than aminopyralid.

Cuban club-rush is an invasive aquatic weed that is spreading northward in the southeastern United States. It is reported for the first time from Mississippi and from significantly farther northward in Alabama than was previously known. Cuban club-rush dissemination and rapid population growth are attributed to two types of reproduction: corky floating achenes and asexual reproduction by fragmentation. An illustration of Cuban club-rush and photos of its habit and habitat are provided.

Giant reed (Arundo donax L.) has recently shown great potential as a feedstock for the bioenergy industry. However, before A. donax can be grown commercially, due to its invasive nature, management strategies must be developed to reduce the risk of unintended spread. This research was conducted in northeastern Oregon (USA) during two growing seasons. Nine control strategies were evaluated in a field that previously had A. donax as a crop. The control strategies included mechanical practices (stem cutting and rhizome digging), physical practices (covering with an opaque tarp), chemical practices (glyphosate applications at different rates and timings), and a combination of these practices. Spring samplings of A. donax regrowth in the season following treatments indicated that stem cutting in the spring without follow-up control practices provided no control. Covering plants with a tarp after cutting them (either with or without a glyphosate treatment after cutting) resulted in 96% control. Application of glyphosate alone also resulted in excellent control, although timing of application was an important factor for maximizing efficacy. The best results were found when the maximum dose (10.2 L ai ha−1) was split among two or three applications (>99% of control) compared with the maximum dose applied once (75% to 94%). Control was lower (73% to 89%) for two of the strategies that included mechanical practices, stem cutting + glyphosate and rhizome digging, in comparison to other strategies involving tarps and/or glyphosate applications (88% to 100%). Results indicated that it is very difficult to eradicate volunteer A. donax in 1 yr, but very good control can be achieved with several of the strategies tested.

Competitive interactions between native plants and nonnative, invasive plant species have been extensively studied; however, within degraded landscapes, the effect of interspecific interactions among invasive plants is less explored. We investigated a competitive interaction between two sympatric, invasive mustard species that have similar life history strategies and growth forms: garlic mustard and damesrocket. Greenhouse experiments using a full range of reciprocal density ratios were conducted to investigate interspecific competition. Garlic mustard had a negative effect on the final biomass, number of leaves, and relative growth rate in height of damesrocket. Survival of damesrocket was not negatively affected by interspecific competition with garlic mustard; however, garlic mustard showed higher mortality because of intraspecific competition. These results indicated that although garlic mustard has been observed to be the dominant species in this landscape, it may not completely outcompete damesrocket in all situations. Studies of invasive species in competition are important in degraded landscapes because this is the common situation in many natural areas.

Two of the five species of European blackberry (Rubus fruticosus L. aggregate) along the West Coast of the United States are considered invasive. They are also similar in appearance. Biological control of invasive blackberry by Phragmidium violaceum, causal agent of a rust disease, had been under consideration when rust-diseased blackberry was discovered in Oregon in 2005. An investigation was initiated to determine whether this disease would be an important factor affecting population density of these blackberries. Surveys were made over a 5-yr period at more than 30 field sites in the Willamette Valley and along the Pacific coast of Oregon. Diseased and nondiseased blackberry specimens were collected for artificial greenhouse inoculations and for identification. The two blackberry species, Rubus armeniacus and R. praecox, were identified as the most invasive. They were readily distinguished morphologically on the basis of inflorescence and flower characteristics and to a certain extent by differences in primocane leaf and leaflet shape. Artificial greenhouse inoculation studies revealed that R. praecox was susceptible to the rust disease and that R. armeniacus was not. These results were confirmed during a field survey. Results of this investigation revealed that the rust disease will not be effective for biological control of R. armeniacus and other approaches to management of this particular species will be required.

Yellow starthistle represents one of the most spectacular examples of biological invasion in the western United States. However, the mechanisms leading to its success have not been clearly elucidated. Although its success has been attributed to superior competitive ability, few competition studies have been performed with yellow starthistle to test this assertion. Yellow starthistle and wild oat (a dominant component of California annual grasslands) were grown in monocultures and mixtures to assess the strength of competitive interactions between them. For either species, intraspecific competition exerted a greater influence over mean plant weight than did interspecific competition. A companion study revealed temporal separation in the phenology of these plants, explaining the weak role of interspecific competition. Additional measurements of growth and soil moisture dynamics in large 270-cm-tall by 50-cm-diam polyvinyl chloride columns also showed a lack of interspecific competition and confirmed that water use patterns differed between these species, indicating niche partitioning. Wild oat reduced soil moisture to 5% but only to a depth of approximately 150 cm. Yellow starthistle depleted soil moisture to less than 5% throughout the column to a depth of at least 270 cm. These patterns were present when wild oat and yellow starthistle were grown individually or together in the columns, indicating that yellow starthistle had a greater impact on soil moisture and to greater depths. Yellow starthistle's invasion of grasslands in California does not appear to be due to superior competitive ability, but may be due to its ability to access deeper soil moisture. These results support the empty niche hypothesis that implies that invasive species are successful in new habitats because they access resources not available to resident species.

Invasive weed management in wildland areas is often challenging due to the need to protect and preserve the integrity of natural ecosystems. Russian knapweed is an aggressive, deep-rooted, creeping perennial forb that was first identified as a problematic species in Dinosaur National Monument in 1977. From 2002 to 2005, extensive nonnative plant inventories were conducted in portions of the monument. Results were used to develop and implement an aggressive weed management program beginning in 2005. Emphasis was placed on reducing Russian knapweed infestations around Josie's Ranch in the Cub Creek Watershed. Several other species were targeted as well. In 2010, a reinventory was conducted in selected areas to evaluate how implemented management strategies affected the overall distribution and abundance of targeted species on the landscape. Comparisons between inventories indicate that management strategies were successful in reducing the total infested area of Russian knapweed by 79%. Treatments used for other targeted species also appear to have been effective in reducing their overall distribution and abundance on the landscape. In addition, the reinventory identified several new species with the potential to become problematic in the area. Although this case study documents the substantial progress that has been made at Dinosaur National Monument toward obtaining specific weed management objectives, it more importantly illustrates the process and benefits of an adaptive approach in sustaining long-term invasive plant species management efforts.

Invasive saltcedar species have replaced native riparian trees on numerous river systems throughout the western United States, raising concerns about how this habitat conversion may affect wildlife. For periods ranging from 2 to 11 yr, I used live-trapping to monitor small mammal populations in paired saltcedar and native riparian woodlands at four sites in western Nevada and eastern California. Heteromyid rodents, such as Merriam's and Ord's kangaroo rats, were more likely to occur in saltcedar habitats, but other rodent species, particularly the montane vole and western harvest mouse, occurred more often in native habitats, and this balanced species richness in habitat comparisons. The most common species at all sites, the deer mouse, did not show any consistent differences in abundance or in mean body mass between the two habitat types. However, the ratio of captured male to female deer mice was higher in saltcedar than native habitats at two sites. Deer mice as well as Ord's kangaroo rats also had higher rates of being recaptured following initial capture in native habitats, which may have been due to fewer transient individuals occurring in these habitats. By contrast, Merriam's kangaroo rats may have been more transient in native habitats because they were more likely to be recaptured in saltcedar. Individuals of two species, pinyon mouse and white-tailed antelope ground squirrel, had greater mean body mass in native habitats than they did in saltcedar, implying that they may have maintained superior condition in native habitats.

Meadow knapweed is a fertile European hybrid between black and brown knapweed that has expanded its North American distribution to 27 of the United States and four provinces in Canada. Two experiments were conducted on meadow knapweed in northwestern Washington to determine (1) whether meadow knapweed is similarly sensitive to several herbicides commonly used to control related knapweeds, (2) if herbicide application timing plays a role in control of meadow knapweed, and (3) whether mowing before, after, or instead of herbicide treatment can aid in meadow knapweed control. In the first experiment, herbicides were applied to meadow knapweed at rosette or bolting stages of growth, and again with the same herbicides in the autumn. In the summer following treatment, clopyralid alone or with 2,4-D, dicamba + 2,4-D, and triclopyr ester + 2,4-D ester provided 81 to 100% meadow knapweed control; the only other treatment providing similar control was glyphosate + ammonium sulfate applied at bolting and in autumn in the 2004 to 2005 trial. In the second experiment, combinations of mowing and dicamba + 2,4-D were applied at rosette or early flowering stages of growth. In the 2002 to 2003 trial, control when dicamba + 2,4-D was used exceeded 90%, except when meadow knapweed was mowed at rosette and sprayed at early flowering (78% control). Mowing twice the previous year had only a slight effect on meadow knapweed (10% control). Grass biomass exceeded meadow knapweed biomass in all herbicide-treated plots. In the 2004 to 2005 trial, meadow knapweed control and grass biomass was maximized when plots were mowed at rosette and treated with dicamba + 2,4-D at early flowering or when treated twice with these herbicides; these were the only treatments where grass biomass exceeded meadow knapweed biomass.

English ivy is an exotic liana that invades forests by forming dense monocultures on forest floors and by climbing trees. In this study, we evaluated the effectiveness of high-intensity–short-duration goat browsing to control English ivy in the Willamette Valley of Oregon. Species composition and percent cover were determined in August 2006 before the first browsing treatment and before and after treatments in 2006 and 2007. A final assessment was made in July 2008. Browsing effects were evaluated by comparison of multiple means statistical methods. English ivy cover declined significantly in samples browsed once or twice compared with untreated samples. The difference between levels of browsing was also significant, indicating that repetition of browsing for a second year is effective. Species composition change was minimal, with the appearance of sword fern and removal of Himalayan blackberry from 2006 to 2008.

Although several invasive species have induced changes to the fire regime of invaded communities, potential intraspecific shifts in fire-related traits that might enhance the invasion success of these species have never been addressed. We assumed that traits conferring persistence and competitiveness in postfire conditions to downy brome, a quintessential invasive species of the Great Basin (North America), might be under selection in areas with recurrent fires. Therefore, we hypothesized that populations from frequently burned regions of the Great Basin would have (1) greater tolerance to fire at seed level, (2) higher relative seedling performance in postfire environments, and (3) greater flammability than unburned Central European populations that evolved without fire. Seeds were collected from three introduced populations from frequently burned regions in North America and three introduced populations of rarely or never burned sites from Central Europe. We performed (1) germination experiments with seeds subjected to the effect of different fire components (heat shocks, smoke, flame, ash), (2) pot experiments analyzing the effect of postfire conditions on the early growth of the seedlings, and (3) a series of flammability tests on dry biomass of plants reared in a common garden. All seeds tolerated the low-temperature treatments (40 to 100 C), but were destroyed at high heat shocks (140 and 160 C). Only the 100 C heat treatment caused a difference in reaction of seeds from different continents, as the European seeds were less tolerant to this heat shock. We found significantly increased seedling height and biomass after 4 wk of growth under postfire conditions in American populations, but not in European ones. American populations had enhanced flammability in three out of five measured parameters compared to European populations. In summary, these intraspecific differences in fire-related traits might contribute to the persistence and perhaps invasiveness of the frequently burned North American downy brome populations.

Wild parsnip is an invasive species with a global distribution in temperate climates. Parsnips are native to Eurasia and have been cultivated for more than five centuries. It is unclear whether the global invasion of this species is a consequence of escape from cultivation or the accidental introduction of a Eurasian wild subspecies. In this study, we used nuclear ribosomal DNA internal transcribed spacer (ITS) and chloroplast DNA (cpDNA) markers to evaluate the genetic structure of wild parsnip in its native range (Europe) and in three distinct geographic regions where it is considered invasive: eastern North America, western North America, and New Zealand. We also compared wild and cultivated parsnips to determine whether they are genetically distinct. From 112 individuals, we recovered 14 ITS and 27 cpDNA haplotypes. One ITS haplotype was widespread; few haplotypes were rare singletons. In contrast, at least two lineages of cpDNA haplotypes were recovered, with several novel haplotypes restricted to Europe. Cultivated parsnips were not genetically distinct from wild parsnips, and numerous wild parsnip populations shared haplotypes with cultivars. High genetic diversity was recovered in all three regions, suggesting multiple introductions.