Ecological restoration of shrub–steppe communities in the western United States is often hampered by invasion of exotic annual grasses during the process. An important question is how to create restored communities that can better resist reinvasion by these weeds. One hypothesis is that communities comprised of species that are functionally similar to the invader will best resist invasion, while an alternative hypothesis is that structurally more complex and diverse communities will result in more effective competitive exclusion. In this field experiment, we examined the effects of restored community structure on the invasion success of three annual grass weeds (downy brome, jointed goatgrass, and cereal rye). We created replicated community plots that varied in species composition, structural complexity and density, then seeded in annual grass weeds and measured their biomass and seed production the following year, and their cover after 1 and 3 yr. Annual grass weeds were not strongly suppressed by any of the restored communities, indicating that it was difficult for native species to completely capture available resources and exclude annual grass weeds in the first years after planting. Perennial grass monocultures, particularly of the early seral grass bottlebrush squirreltail, were the most highly invaded communities, while structurally complex and diverse mixtures of shrubs (big sagebrush, rubber rabbitbrush), perennial grasses (bluebunch wheatgrass and bottlebrush squirreltail) and forbs (Lewis flax, Utah sweetvetch, hairy golden aster, gooseberryleaf globemallow) were more resistant to invasion. These results suggest that restoration of sagebrush steppe communities resistant to annual grass invasion benefits from higher species diversity; significant reduction of weed propagule pressure prior to restoration may be required.

Four rates of aminopyralid (30, 60, 90, and 120 g ae ha−1 [0.4, 0.9, 1.3, and 1.8 oz ae acre−1]) were compared for their ability to reduce abundance of nonnative dicot species and favor native species in an invaded Cascade Mountain meadow near Trout Lake, WA. Treatments were applied in two replicated studies (June 2009 and 2010), and foliar cover and species richness were monitored for two years. First-year control of nonnative dicots from application of 30 g ae ha−1 of aminopyralid (69%) was greater than that of native dicots (29%); whereas, significant control of both species groups occurred at the higher rates. By the second year after treatment, absolute differences in cover between treated and non-treated plots averaged −17% and −21% for native and nonnative dicots, respectively, and +1% and +27% for native and nonnative monocots, respectively. First-year control of Canada thistle and oxeye daisy was greater after treatment in 2009 (88% and 90%, respectively) than after treatment in 2010 (56% and 55%, respectively), probably because lower spring temperatures in 2010 limited vegetation development and plant susceptibility to aminopyralid. Cover of Kentucky bluegrass and sheep fescue averaged 20% and 6% greater, respectively, in treated plots than in non-treated plots. Application of 30 g ae ha−1 of aminopyralid had no detectable effect on second-year richness of native and nonnative species relative to non-treated plots; however, higher rates caused 24% to 43% reductions in richness of each species group. Research results suggest that application of aminopyralid at 30 g ae ha−1 has the potential to reduce abundance of nonnative dicot species in similar meadow communities of the Pacific Northwest with little or no negative impacts to abundance and richness of native species. As a potential strategy to limit the subsequent spread of Kentucky bluegrass, a grass herbicide, such as fluazifop or sethoxydim, could be added to the treatment.

Preventing the establishment of saltcedar in new areas requires early detection and rapid response. However, it is unclear when saltcedar develops perennating tissue and which treatments are most efficacious for young plants. The effectiveness of mowing, herbicide, and fire treatments, alone and in combination, was evaluated on saltcedar plants grown from seed to 4, 8, and 12 wk age in 2011 and 6 and 12 wk age in 2012. Plants were clipped to 2 cm height or remained intact. Plants were then exposed to no treatment (control), herbicide application (0.12 mg ae imazapyr), or treated with fire for 30 or 60 s. Six weeks after treatment, plant survival and tallest living shoot height were recorded and roots were dried and weighed for biomass comparison. Saltcedar survival increased with greater plant age. No 4-wk-old plants survived herbicide or fire treatments, whereas 6-wk-old plants were eliminated by fire. Clipping alone did not control plants of any age but clipping before fire was the most effective control for older plants. Herbicide alone did not kill 8- and 12-wk-old plants during the study period, but reduced plant vigor suggests that these applications may be effective in the long-term. Fire alone for 60 s was the most effective single treatment for 12-wk-old plants. Root biomass was reduced for all treatments relative to untreated plants with the lowest biomass typically associated with fire treatments. Resprouts were shortest for combined clipping and herbicide and clipping and fire treatments. Results indicate that saltcedar grown from seed can develop viable belowground reproductive tissues between 6 and 8 wk after germination. Multiple intensive control practices may be required to kill saltcedar plants ≥8 wk of age, whereas younger plants can be controlled by single, less-intensive treatments such as fire.

Prevention is an integral component of many management strategies for aquatic invasive species, yet this represents a formidable task when the landscapes to be managed include multiple invasive species, thousands of waterbodies, and limited resources to implement action. Species distributional modeling can facilitate prevention efforts by identifying locations that are most vulnerable to future invasion based on the likelihood of introduction and environmental suitability for establishment. We used a classification tree approach to predict the vulnerability of lakes in Washington State (United States) to three noxious invasive plants: Eurasian watermilfoil (Myriophyllum spicatum), Brazilian egeria (Egeria densa), and curlyleaf pondweed (Potamogeton crispus). Overall, the distribution models predicted that approximately one-fifth (54 out of 319 study lakes) of lakes were at risk of being invaded by at least one aquatic invasive plant, and many of these predicted vulnerable lakes currently support high native plant diversity and endemism. Highly vulnerable lakes are concentrated in western Washington in areas with the highest human population densities, and in eastern Washington along the Columbia Basin Irrigation Project and the Okanogan River Basin that boast hundreds of lakes subject to recreational use. Overall, invasion potential for the three species was highly predictable as a function of lake attributes describing human accessibility (e.g., public boat launch, urban land use) and physical–chemical conditions (e.g., lake area, elevation, productivity, total phosphorous). By identifying highly vulnerable lake ecosystems, our study offers a strategy for prioritizing on-the-ground management action and informing the most efficient allocation of resources to minimize future plant invasions in vast freshwater networks.

The common edible fig is a subcanopy tree that has invaded many of the remnant riparian forests of California's Central Valley. Fig is unusual in its ability to invade low-light, low-disturbance, native-plant–dominated environments. Dendrochronology combined with regression and spatial analyses allowed us to empirically quantify the expansion rate and spatial pattern of the fig invasion into the native plant community at Caswell Memorial State Park (Ripon, CA) over a 70-year invasion period. Fig uses a combination of short-distance dispersal, which results in constant, linear expansion at source population sites and long-distance dispersal, which eventually leads to high recruitment of satellite populations in ideal environments. Although fig initially experienced a long lag in its invasion rate, at the time of this study, it was expanding at an exponential rate at the landscape scale in Caswell. We identified a number of characteristics intrinsic to the fig population (shade suppression, pollinator presence, highly specialized reproduction, and propagule pressure) as well as extrinsic characteristics of the receiving environment (hydrologic alteration from the construction of a dam, safe sites for juvenile recruitment, and target effects from environmental heterogeneity) that may have influenced the rate and pattern of fig invasion. The Central Valley riparian forests have been reduced to less than 6% of their original area, and invasive fig is a significant threat to the remaining fragments of this important vegetation community. We include suggestions for fig eradication based on knowledge gained in this study.

Propagule pressure significantly contributes to and limits the potential success of a biological invasion, especially during transport, introduction, and establishment. Events such as multiple introductions of foreign parent material and gene flow among them can increase genetic diversity in founding populations, often leading to greater invasion success. We applied the tools and theory of population genetics to better understand the dynamics of successful biological invasion. The focal species, cogongrass, is a perennial invasive grass species significantly affecting the Gulf Coast and southeastern region of the United States. The literature indicates separate, allopatric introductions of material from East Asia (Philippines and Japan) into the U.S. states of Mississippi and Alabama. Molecular analysis of samples from those two states utilized amplified fragment length polymorphism (AFLP) markers on 388 individuals from 21 localities. We hypothesized that previously isolated lineages of cogongrass are present and crossing in the Southeast. We observed genetic variation within localities (0.013 ≤ heterozygosity (He) ≤ 0.051, mean = 0.028 ± 0.001) with significant and substantial population structure (FST = 0.534, P < 0.001). Population structure analyses detected two genetically defined and statistically supported populations, which appear to have experienced some admixture. The geographic distribution of those populations was consistent with the two-introduction scenario reported previously. These results are also consistent with contact in the invasive range of previously isolated lineages from the native range.

Aquarium release is a vector for introducing nonnative species that threatens the ecological integrity of aquatic systems. Following coastal invasions by released aquarium strains of Caulerpa taxifolia, aquarists began using the macroalgal genus Chaetomorpha. Use of Chaetomorpha now exceeds 50% of U.S. aquarium hobbyists we surveyed. Aquarium strains of this macroalgal genus possess broad environmental tolerances, demonstrate high nutrient uptake and growth rates, and reproduce by fragmentation. Although these characteristics make Chaetomorpha a desirable aquarium inhabitant, they may also promote invasive tendencies if the alga is introduced into a natural ecosystem. We sought to proactively mitigate this potential invasion risk by testing algal disposal techniques that serve as responsible alternatives to releasing viable individuals. We tested methods used by aquarium hobbyists—boiling, microwaving, freezing, desiccation, and exposure to freshwater. We determined the minimum durations that these techniques must be used in order to induce mortality in three aquarium purchases of Chaetomorpha. We found that boiling for at least 1 min, microwaving for at least 15 s, or freezing for at least 24 h were sufficient to induce 100% mortality in 1-cm-long fragments and clumps up to 1.5 g. Desiccation required more than 24 h when exposed to air and 6 d for samples kept in closed containers. Freshwater exposure was effective at 6 d. These results indicate that disposal of excess or unwanted Chaetomorpha via garbage (if destined for a landfill) or indoor plumbing (e.g., sinks and toilets) represent safe alternatives to release. Disposal of algal tissue, shipping water, or tank water containing small algal fragments down stormwater drains, however, could introduce this hardy species into favorable conditions that could result in detrimental biological invasions.

The objective of this article is to identify growth patterns of Japanese knotweed propagules distributed by high-water events. Along four river systems, we collected and measured Japanese knotweed propagules that had been distributed by flooding approximately 1 yr earlier. Results indicate that the size of the emergent shoot may be determined by the extent of underground growth late in the growing season, although initially it is linked to the size of the propagule. Our results show that 70% of new plants originated from rhizome fragments, and 30% from stems. This proportion is similar to regeneration rates shown in laboratory studies. We suggest that the best way to prevent the spread of Japanese knotweed along rivers is to focus control efforts on those stands most susceptible to erosion and propagule dispersal. We also suggest that an early detection and rapid response management approach can be effectively utilized to eradicate these propagules, and effectively suppress the spread of Japanese knotweed. Our data-collection method also provides evidence that control of newly distributed propagules can be effectively accomplished without the use of herbicides or heavy mechanical tools.

The sterile hybrid, giant miscanthus, has emerged as a promising cellulosic bioenergy crop because of its rapid growth rate, high biomass yields, and tolerance to poor growing conditions; these are traits that are desirable for cultivation, but also have caused concern for their contribution to invasiveness. New seed-bearing lines of giant miscanthus would decrease establishment costs for growers, yet this previously unresearched propagule source increases fears of escape from cultivation. To evaluate the consequences of seed escape, we compared seedling establishment among seven habitats: no-till agricultural field, agricultural field edge, forest understory, forest edge, riparian, pasture and roadside; these were replicated in Virginia (Blacksburg and Virginia Beach) and Georgia (Tifton), USA. We use a novel head-to-head comparison of giant miscanthus against five invasive and three noninvasive species, thus generating relative comparisons. Overall seed germination was low, with no single species achieving germination rates >37%, in all habitats and geographies. However, habitats with available bare ground and low resident plant competition, such as the agricultural field and forest understory, were more invasible by all species. Giant miscanthus seeds emerged in the roadside and forest edge habitats at all sites. Early in the growing season, we observed significantly more seedlings of giant miscanthus than the invasive and noninvasive species in the agricultural field. Interestingly, overall seedling mortality of giant miscanthus was 99.9%, with only a single 4 cm (1.58 in) tall giant miscanthus seedling surviving at the conclusion of the 6-mo study. The ability to make relative comparisons, by using multiple control species, was necessary for our conclusions in which both giant miscanthus and the noninvasive control species survival (≤1%) contrasted with that of our well-documented invasive species (≤10%). Considering the low overall emergence, increased propagule pressure may be necessary to increase the possibility of giant miscanthus escape. Knowledge gained from our results may help prepare for widespread commercialization, while helping to identify susceptible habitats to seedling establishment and aiding in the development of management protocols.

Mile-a-minute weed or devil's tearthumb (Polygonum perfoliatum, syn. = Persicaria perfoliata) is an invasive annual vine in the Mid-Atlantic and Northeastern United States that reproduces solely through seeds. Our study aimed to identify how mile-a-minute seed viability is affected by time of year and the maturity of the fruit surrounding the achene. Full-sized immature (green) and mature (blue) fruits were collected from five field sites every 2 wk over a 3 mo period, and seed viability was assessed using a triphenyl tetrazolium chloride (TZ) assay. At the onset of seed production in mid-August, 35% of seeds from immature fruits were viable. This percentage increased steadily, peaking at 84% in late September before declining at some sites around the time of the first frost. In contrast nearly all seeds with mature fruits (96%) were viable at all collection dates. Thus land managers who apply physical or chemical control methods for mile-a-minute weed should do so before the onset of any seed production and not simply before fruit maturation. If it is necessary to apply control methods after fruit set, it should be done as early in the season as possible.

Miscanthus sinensis is a perennial grass native to Asia, but since its introduction to the United States in the late 19th century, it has become both a major ornamental crop and invasive species. Previous studies of the ecology of M. sinensis in both its introduced and native ranges have suggested that it may be occupying a novel ecological niche in the introduced range. Miscanthus sinensis and its daughter species, Miscanthus × giganteus, are under evaluation as bioenergy crops; therefore, characterization of the ecology and environmental niche of M. sinensis is essential to mitigate the risk of fostering future invasion in the United States. In July 2011, we surveyed 18 naturalized M. sinensis populations spanning the U.S. distribution, covering a 6° latitudinal gradient from North Carolina to Massachusetts. Miscanthus sinensis populations ranged in size from 3 to 181,763 m2 with densities between 0.0012 and 2.2 individuals m−2, and strongly favored highly disturbed and unmanaged habitats such as roadsides and forest edges. Population size and individual plant morphology (i.e., tiller height, basal diameter, and tiller number) were not affected by soil characteristics and nutrient availability, though increased tree canopy cover was associated with reduced population size (P < 0.0001). Plant size and vigor were not significantly affected by low light availability, which supports previous suggestions of shade tolerance of M. sinensis. In summary, M. sinensis can tolerate a broad range of climatic conditions, light availability, and nutrient availability in the eastern United States, suggesting risk of further invasion beyond its current distribution in the United States.

Although much is known about the physiological capabilities of reed canarygrass (RCG) and the consequences of invasion, less is known about the roles that wetland type and surrounding disturbances play in facilitating the spread of RCG in predominantly forested landscapes. Therefore, the goals of our study were to test if (1) certain wetland types in the Northern Great Lakes region were more susceptible to RCG invasion, (2) certain disturbances facilitated RCG, and (3) the level of road development and the presence or absence of a ditch bordering each observed road influences the frequency of adjacent RCG populations. We randomly selected 28 wetlands within the Keweenaw Bay Indian Community reservation in the Upper Peninsula of Michigan. At each wetland, we collected plant community and environmental data and catalogued disturbances. In all, 287 plant species were identified. Cluster analysis revealed 16 distinct vegetation communities, which were distributed among three broader wetland community types: (1) nonforested graminoid, (2) Sphagnum peatlands, and (3) forested wetlands. Occurrence of RCG was most common in the nonforested graminoid communities and was also positively correlated to disturbance. The most frequent disturbances were roads, off-road vehicle trails, and logging activity. Additionally, paved and graded roads, and roads with ditches, were more likely to have RCG alongside them than unpaved dirt roads. Our data suggest that RCG occurrence is controlled by interactions between wetland types and disturbance.

Buffelgrass (Pennisetum ciliare) is a fire-prone, African bunchgrass spreading rapidly across the southern Arizona desert. This article introduces a model that simulates buffelgrass spread over a gridded landscape over time to evaluate strategies to control this invasive species. Weed-carrying capacity, treatment costs, and damages vary across grid cells. Damage from buffelgrass depends on its density and proximity to valued resources. Damages include negative effects on native species (through spatial competition) and increased fire risk to land and buildings. We evaluate recommended “rule of thumb” control strategies in terms of their ability to prevent weed establishment in newly infested areas and to reduce damage indices over time. Two such strategies—potential damage weighting and consecutive year treatment—used in combination, provided significant improvements in long-term control over no control and over a strategy of minimizing current damages in each year. Results suggest specific recommendations for deploying rapid-response teams to prevent establishment in new areas. The long-run population size and spatial distribution of buffelgrass is sensitive to the priority given to protecting different resources. Land managers with different priorities may pursue quite different control strategies, posing a challenge for coordinating control across jurisdictions.

The United States created national parks to conserve indigenous species, ecological processes, and cultural resources unimpaired for future generations. Curtailing impacts of exotic species is important to meeting this mission. This synthesis identified 56 studies reported in 60 publications that evaluated effects of exotic plant treatments on National Park Service lands. Studies encompassed 35 parks in 20 states and one U.S. territory and included 157 exotic plant species. Eighty-seven percent of studies reported that at least one treatment reduced focal exotic species. Of 30 studies evaluating response of native vegetation, 53% reported that natives increased, 40% reported neutral responses, and 7% reported that natives decreased. For at least some of the neutral cases, neutrality was consistent with management objectives. In other cases, insufficient time may have elapsed to thoroughly characterize responses, or restoration might be needed. Nonfocal exotic species increased in 44% of the 16 studies evaluating them, but the other 56% of studies reported no increase. Results suggest that: (1) a range of exotic species spanning annual forbs to trees have been effectively treated; (2) developing effective treatments often required extensive experimentation and balancing nontarget impacts; (3) presence of multiple exotic species complicated treatment efforts, highlighting importance of preventing invasions; and (4) placing treatment objectives and outcomes in context, such as pretreatment condition of native vegetation, is important to evaluating effectiveness. Attaining the goal in national parks of conserving native species and ecological processes minimally influenced by exotic species will likely require comprehensive management strategies inclusive of treatment interactions with focal exotic species, other potential invaders, and native species.

The miconia (Miconia calvescens) invasion of the East Maui Watershed (EMW) started from a single introduction over 40 yr ago, establishing a nascent patch network spread across 20,000 ha. In 2012, an accelerated intervention strategy was implemented utilizing the Herbicide Ballistic Technology (HBT) platform in a Hughes 500D helicopter to reduce target densities of seven nascent patches in the EMW. In a 14-mo period, a total of 48 interventions eliminated 4,029 miconia targets, with an estimated 33% increase in operations and 168% increase in recorded targets relative to the adjusted means from 2005 to 2011 data (prior to HBT adoption). This sequence of interventions covered a total net area of 1,138 ha, creating a field mosaic of overlapping search coverage (saturation) for each patch (four to eight interventions per patch). Target density reduction for each patch fit exponential decay functions (R2 > 0.88, P < 0.05), with a majority of the target interventions spatially assigned to the highest saturation fields. The progressive decay in target density led to concomitant reductions in search efficiency (min ha−1) and herbicide use rate (grams ae ha−1) in subsequent interventions. Mean detection efficacy (± SE) between overlapping interventions (n = 41) was 0.62 ± 0.03, matching closely with the probability of detection for a random search operation and verifying imperfect (albeit precise) detection. The HBT platform increases the value of aerial surveillance operations with 98% efficacy in target elimination. Applying coverage saturation with an accelerated intervention schedule to known patch locations is an adaptive process for compensating imperfect detection and building intelligence with spatial and temporal relevance to the next operation.

Restoration of abandoned agricultural lands to create resilient ecosystems in arid and semi-arid ecosystems typically requires seeding or transplanting native species, improving plant–soil–water relations, and controlling invasive species. We asked if improving water relations via irrigation or surface mulch would result in negative tradeoffs between native species establishment and invasive species competition. We examined the effects of sprinkler irrigation and straw mulch on native seed mixtures planted in two consecutive years in an abandoned agricultural field in a cold desert shrubland in southwestern Nevada, USA. Restoration effects differed among years because of contingency effects of growing season conditions. Precipitation was low during the first year and seeded plant density and biomass increased in response to irrigation. Precipitation was relatively high during the second year, seeded plant densities and biomass were generally high, and irrigation had inconsistent effects. Mulch increased native plant cover in the absence of irrigation during the dry year. Invasive plant biomass and cover also were influenced by year, but irrigation increased invasive plants regardless of precipitation. Positive effects of irrigation on seeded plant density, cover, and biomass outweighed negative tradeoffs of increases in invasive plants. In ecosystems with highly variable precipitation, the most effective restoration strategies will most likely be adaptive ones, requiring determination of timing and amount of irrigation based on precipitation, native plant establishment, and invasive species composition and abundance.

One of Australia's most serious weeds, Chrysanthemoides monilifera subsp. rotundata (bitou bush) was recently found for the first time in Western Australia as a well established population in Kwinana, a major port and industrial area south of Perth, the State's capital. This population is remote from other bitou bush infestations in Australia and had escaped detection despite extensive surveys in the same State for the other subspecies that is present in Australia, Chrysanthemoides monilifera subsp. monilifera (boneseed). The main reasons it went undetected are thought to be the tightly controlled access to this area because of mineral processing and port activities, the unusual invasion route via a heavy industrial area and the morphological similarity to a native species when it is not flowering. Two surveys defined the core population of 1038 plants that are spread along the coast over a 25-ha semi-circle with about a 500-m (1640 ft) diameter. Subsequent surveys of first a 500 m buffer zone and later a 1-km (0.621 mi) buffer found four additional plants, indicating that there is considerable potential for dispersal. We concluded that the survey has not delimited the distribution because of the potential and evidence for long distance dispersal. Cooperation by the various land managers has led to all plants being killed, as an initial step to management of this species. Other steps to be undertaken include an awareness campaign in the area that would need to be surveyed for delimitation of the spatial distribution and seed bank assessment to measure potential dispersal both in space and through time. It remains to be determined what is the best strategic response: eradication or containment.