Japanese stiltgrass is a nonnative invasive grass occupying a range of habitats in the eastern United States. Conventional management recommendations include hand-removal, mowing, or a nonselective herbicide application in autumn prior to flowering. However, no study has directly compared the ecological impacts of long-term management strategies on Japanese stiltgrass populations or recruitment and establishment of native flora. An experiment was initiated in 2002 and continued for three growing seasons in mixed pine-hardwood forests in central North Carolina. Conventional treatments included hand-removal, mowing, or an application of glyphosate (1.1 kg ai/ha) once in autumn, and selective removal by hand or fenoxaprop-P (0.19 kg ai/ha) season-long as needed. All treatments were compared to nontreated plots. Percent vegetation cover by species was recorded twice annually. Data were aggregated into five classes; Japanese stiltgrass, other exotic plants, native forbs, native monocots, and native woody plants. The soil seed bank of all species was estimated annually by extracting soil cores and documenting seedling emergence. All Japanese stiltgrass management treatments significantly reduced Japanese stiltgrass cover and seed bank over time compared to no management. However, recruitment and reestablishment of native plants and overall species richness were greater with selective Japanese stiltgrass management treatments including both hand-removal and fenoxaprop-P. Relative cover of other exotic plants decreased 2% to 49% after 3 yr with all Japanese stiltgrass management treatments except season-long hand-removal, which increased relative cover of other exotic plants 51%.

Aphthona spp. flea beetles have established and reduced the density of leafy spurge in much of the western United States. One way to measure the long-term impact and effectiveness of a weed control program is by monitoring the changes in the seedbank over time. The change in leafy spurge stem density and seed in the seedbank were evaluated 5 yr after Aphthona spp. were released to control this weed in the Little Missouri National Grasslands in southwestern North Dakota. Leafy spurge density and seed in both loamy overflow and loamy ecological sites decreased, whereas desirable (high-seral) forbs increased 5 yr after the biological control agents were released. Leafy spurge topgrowth was reduced from an average of over 200 stems/m2 to less than 8 stems/m2 in the most densely infested sites, and leafy spurge seed was reduced from an average of 68% of the seedbank to only 14% in both ecological sites. High-seral forb seed increased by over 300% in the seedbank, which indicated the floristic quality of the sites, was returning to a preinfestation state. Species with the largest increase included western rock jasmine and fringed sage, which increased at least three-fold in both sites. Less desirable low-seral forbs and grasses accounted for about 30% of the seedbank. In a greenhouse study, native grass production was reduced nearly 50% when grown in soil from Aphthona spp. release sites compared to nonrelease sites. Switchgrass production was reduced to a greater extent (66%) than green needlegrass, little bluestem, or western wheatgrass. The cause and extent of reduced native grass production in sites where Aphthona spp. were released has yet to be determined. The decrease in leafy spurge topgrowth and seed in the soil seedbank as desirable species seed increased, should lead to the long-term recovery of the plant community.

Nine sites of cogongrass were included in a study of genotypic diversity and spread dynamics at the point of introduction and its adjacent areas in the southern United States. Clones evaluated with two primer pairs yielded a total of 137 amplified fragment length polymorphism (AFLP) loci of which 102 (74.4%) were polymorphic. Genetic diversity was measured as the percentage of polymorphic, Shannon's information index, Nei's gene diversity, and panmictic heterozygosity. Nei's gene diversity (HS) across all nine sites was estimated to be 0.11 and within site gene diversity ranged from 0.06 to 0.16. Bayesian estimate of gene diversity and Shannon's information index were higher (0.17 and 0.17, respectively). The samples from the point of introduction (Pi) had the lowest genetic diversity for all types of estimates. Within site variance accounted for 56% of the total variation and among site variance 44% (P < 0.05). Differentiation among sites was assessed using F ST. The greatest difference was found between the Pi and the others. No relationship was found between genetic and geographic distances. Principal component analysis as well as cluster analysis separated individuals into three main clusters. The Pi formed a separate subcluster. Gene flow (Nm), inferred from Φ-statistics describing the genetic differentiation between pairs of sites ranged from 0.6 to 5.55. The lack of significant relationship between gene flow and geographic distance as well as genetic and geographic distances suggests that the invasion dynamics of cogongrass into the southern United States is primarily through anthropogenic activities and to the lesser extent through natural forces.

The ability of arbuscular mycorrhizal fungi (AMF) to influence the performance of nonnative invasive plants in their introduced range has received increasing attention. The dependence of the invasive nonnative vine pale swallow-wort on AMF was studied in three greenhouse experiments. The aims of the present work were to (1) determine AMF colonization levels of field-collected pale swallow-wort plants and several co-occurring native and nonnative plant species, (2) evaluate the growth response of pale swallow-wort to different components of the soil microbial community from an infested site, and (3) determine the growth response of pale swallow-wort when grown with a nonlocal AMF species. AMF root colonization was greater in pale swallow-wort (85, 98, and 50% arbuscules, hyphae, and vesicles, respectively) than in leek (72, 80, and 25%), a species that has been frequently used as a predictor of AMF density in soil. Root colonization of pale swallow-wort in the field was also greater than root colonization of common milkweed, a native herbaceous species often co-occurring in the same habitats, as well as two other herbaceous species, Canada goldenrod and blueweed. Survival of pale swallow-wort plants was significantly greater in soil collected underneath dense monospecific stands of pale swallow-wort in a Henderson Harbor, NY, field site than in sterilized soil. After 12 wk, plants grown in sterilized soil had a 33% survival rate, whereas all plants grown in the unamended soil, with an intact microbial community, were alive. Moreover, plants grown in the unamended soil were 130% taller, had 50% more leaves, and had 83% greater total biomass compared with plants grown in sterile soil. Plants grown in soil containing a Glomus intraradices isolate collected in Troy, AL, were 50% shorter and had 15% lower total biomass than plants grown in the unamended New York field soil. These pale swallow-wort seedlings also had a high mycorrhizal dependency of 93%. Plants grown in a sterilized soil that was reamended with an AMF-free microbial wash had significantly lower belowground and total biomass than plants grown in the unamended soil with the resident AMF community. There was a trend of decreasing height and biomass for plants grown in sterile soil relative to the unamended controls treatment. Plants grown in sterilized soil had significantly (28%) greater total biomass than plants reamended with the AMF-free microbial wash. These findings suggest that AMF occurring in invaded habitats have beneficial effects on pale swallow-wort survival and growth.

The invasive biotype of common reed has taken over wetlands and can spread into roadsides, turf, and ornamental sites. Additional control options are needed for managing this biotype in noncrop areas and other sites. In container trials, fosamine, glyphosate, and imazapyr all provided excellent control of common reed. Triclopyr suppressed common reed, with increasing rates improving the growth reduction. Chlorflurenol, primisulfuron, and sulfosulfuron did not suppress common reed growth. In field trials, glyphosate and imazapyr were more effective than fosamine, providing 82 and 93% control, respectively, the following April after either June or September applications. The optimum window of application for glyphosate may therefore include both summer and fall applications, wider than the commonly accepted window of only fall treatments. Fosamine provided greater suppression of common reed when applied in September (68% control) compared to June applications (43% control).

Prescribed burning can control invasive annual grasses that threaten the biological and economic value of California grasslands. Susceptibility of grass seed to burning can depend on burn timing, exposure time, and type of exposure (direct flame heat or convective heat); thus, these factors can influence the success of a prescribed burning program. To further investigate these factors, laboratory simulations were conducted on barb goatgrass, medusahead, and ripgut brome at several stages of seed maturity, as determined by percent moisture of the inflorescences. Seeds were exposed either to direct flame using a Bunsen burner or to heated air in a muffle furnace. Flame treatments were conducted at one temperature (∼400 C) and several exposure times (0 to 14 s), depending on the species. Furnace treatments included four temperatures (150, 200, 250, and 300 C) and seven exposure times (0, 10, 20, 30, 40, 60, or 80 s). Seed germination was analyzed for each temperature series to determine the LD50 and LD90 in seconds of exposure time. Susceptibility to furnace treatments, which simulated heat exposure of seeds on the soil surface, was not statistically different within a range of seed moisture levels for all three species. The LD50 values at 250 C (typical soil temperature with grassland fire) ranged from 28 to 49 s, which far exceeds the time of exposure during a typical grassland fire. Susceptibility to flame showed a similar lack of change over maturation of medusahead and barb goatgrass seeds, with LD90 values ranging between 4.8 and 7.4 s for all seed moisture levels. In contrast, ripgut brome seeds exposed to flame showed increasing susceptibility with reduced seed moisture content. The LD90 values for exposure were less than one second for seed moisture levels at or below 10%, compared to 3.7 s for seeds at 55 to 60%. Although flame susceptibility increased for ripgut brome, seeds at all maturation stages were more sensitive than medusahead and barb goatgrass. Additionally, the LD90 values for all three species are attainable under field conditions. Thus, burn prescriptions for these three species are not constrained by maturation stage, but should occur prior to seed drop and when fuel loading is high. This will maximize exposure time of seeds to direct flame.

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.

Screening tools that effectively predict which nonnative species are likely to become invasive are necessary because of the disproportionate ecological and economic costs associated with invaders. We tested the effectiveness of the Australian Weed Risk Assessment system (WRA) in distinguishing plant species that are major invaders, minor invaders, and noninvaders in Florida. The test included 158 annuals and perennials in six growth forms from 52 families in 27 orders. The WRA with a secondary screen met all hypothesized accuracy levels: it correctly rejected 92% of test species that have been documented to be invasive in Florida and correctly accepted 73% of the noninvaders. The incorrect rejection of noninvaders was 8% with the remaining 19% of noninvaders falling into the “evaluate further” outcome. Only 10% of the 158 species required further evaluation. Invaders of natural areas and agricultural systems were identified with equal accuracy. Receiver operating characteristic analysis demonstrated high separation of invaders from noninvaders. The degree to which the WRA is precautionary may be adjusted by altering the cutoff scores that define the “accept, evaluate further,” and “reject” outcomes. This approach could be adopted in Florida as a screening mechanism to reduce importation of new invaders.

Pale swallow-wort is a nonnative vine, invading natural areas across much of the northeastern United States and southeastern Canada. Control of this clump-forming herbaceous perennial has been difficult. We conducted a 2-yr study (2005–2006) in a heavily infested site in Chaumont, NY to assess the response of swallow-wort to triclopyr applied once as a foliar treatment (1.9 kg ae/ha) (2005 only) alone or in combination with clipping 4 wk later, followed by a single clipping in 2006. We also evaluated the response of swallow-wort to one or two clippings during each of the 2 yr. Two yr after treatments began, swallow-wort cover was lower in plots treated with triclopyr (20 ± 5%) compared with plots subjected to clipping-only (56 ± 6%) or unmanaged controls (76 ± 6%). Stem densities were also lower in triclopyr-treated plots (25 ± 5 stems/m2) than in clipping-only (188 ± 9 stems/m2) and control (178 ± 10 stems/m2) plots across three different sample dates. Seedling densities were lower in triclopyr-treated plots (160 ± 50 seedlings/m2) relative to clipping-only (1,120 ± 180 seedlings/m2) and control (960 ± 50 seedlings/m2) plots after the 2005 growing season. The cover of other plant species was negatively correlated with swallow-wort cover and was higher in triclopyr-treated plots (75 ± 3%) than in clipping-only (5 ± 1%) and control (7 ± 4%) plots in 2006. Across both years, swallow-wort in control and clipped plots produced follicles, but not in triclopyr-treated plots. Regardless of clipping frequency, clipping in June or July was not effective in reducing swallow-wort stem density, cover, or follicle production. Although a single application of triclopyr provided considerable suppression of swallow-wort after two growing seasons, application of triclopyr in subsequent years is likely required to achieve long-term control.

Previous studies have characterized the development of the biological control insects, Galerucella calmariensis and Galerucella pusilla on purple loosestrife and on nontarget Lythraceae species, including two species native to Minnesota, winged loosestrife, and swamp loosestrife. The impact of Galerucella spp. on these plants, when grown in outdoor mesocosms that more closely mimics ecological host range, has not been reported. The first objective of this study was to compare the growth and seed capsule production of purple loosestrife, winged loosestrife, and swamp loosestrife, with and without exposure to Galerucella spp. With purple loosestrife, larval feeding on apical and lateral shoot buds resulted in fewer seed capsules, and reduced aboveground biomass and plant height compared to control plants. No measured plant growth or reproductive parameters were reduced as a result of beetle feeding on swamp loosestrife. Presence of Galerucella spp. on winged loosestrife resulted in a reduction of seed capsules in one of 2 yr of study. A second objective of our study was to compare the phenology of the three Lythraceae species in relation to that of Galerucella spp. In the northern United States, flowering and seed development in swamp loosestrife occurred a month later than in purple or winged loosestrife. The delayed flowering of swamp loosestrife resulted in avoidance of shoot meristem feeding damage caused by the first generation of beetles. Laboratory studies might have overestimated the host range of Galerucella spp. on swamp loosestrife with the finding of asynchronous flowering times of purple and swamp loosestrife. Our mesocosm studies confirm that previous laboratory host range testing did accurately predict the ecological host range of winged loosestrife.

The invasive grass Bromus tectorum (cheatgrass, downy brome) has extensive ecological breadth in its introduced range, and is increasing in abundance at high elevations. As a predominantly selfing species with high potential population growth rates, it is a likely candidate for having general-purpose genotypes that facilitate its invasion into high elevations. We evaluated evidence for general-purpose genotypes by examining phenotypic and molecular variation in B. tectorum from low- and high-elevation sites in the Rocky Mountains. Common garden experiments revealed that plants from high elevations germinated faster, but no other phenological, morphological, or physiological differences were found between elevations. Leaf length was longer for low-elevation sites in a common garden using field-collected seeds, but this was not found in a common garden using greenhouse-grown seeds, suggesting maternal effects. For most traits, there was significant phenotypic variation among sites and individuals. Using microsatellites, we genotyped individuals from low- and high-elevation sites. We found no indication of genetic differentiation at these loci between high and low elevations, but there was significant variation within and among sites. No single genotype was common across sites. The success of B. tectorum is not due to a few general-purpose genotypes, but rather many genotypes associated with many different phenotypes. Understanding the mechanisms behind the success of invaders enhances our knowledge of basic evolutionary and ecological processes and is crucial to our ability to manage ongoing, and prevent future, invasions, especially into new habitats.

Limited data exist on the affect of downy brome invasion on biogeochemical cycling. Biogeochemical cycling was quantified in a winterfat community in northeastern, CA that was separated into three invasion classes: noninvaded (NI), invaded for 3 yr (I3), and 10 yr (I10) by downy brome. On each plot, all aboveground vegetation was harvested and separated by species, dried, weighed, and tissue nutrients quantified. In addition, soil samples were collected from 0- to 30-, 30- to 60-, and 60- to 100-cm depths and various nutrient pools quantified. Aboveground biomass g/m2 was significantly greater, with downy brome averaging over 90% of the plant mass on the I10 plots (280 g) compared to the NI plots (148 g). In comparison to the NI plots, vegetation fluxes (g/m2/yr) of carbon (C) were significantly greater, and fluxes of Ca, Fe, and Cu were significantly less on I10 plots. Soils occupied for 10 yr by downy brome have significantly greater total N and organic C, and greater availability of Fe, Mn, Cu, ortho-P, Ca, and K compared to NI soil. For the I10 plots, available soil N (dominantly NO3 −) was greatest in the 60- to 100-cm-depth increment, whereas for the other plots, N availability was greatest in the 0- to 30-cm-depth increment. Net N soil mineralization potential was near 0 on the I10 plots at all depth increments. These data suggest that invasion by downy brome facilitates elevated nutrient availability, possibly increases system leakiness of N, and fosters differential plant nutrient cycling relative to a native noninvaded community. Elevated nutrient availability promulgated by downy brome invasion might increase its competitive stature. Long-term occupation of environments by downy brome might affect the vertical distribution of nutrients, which can alter soil evolution and plant successional patterns.