The suitability of a target weed for classical biological control should be considered early in the process of plant invasion. Concerns have been raised about the recent arrival of Mikania micrantha Kunth in south Florida and its potential to spread and invade natural and managed ecosystems. This weed is native to the neotropics, and has been introduced into many Asian countries and, more recently, into Australia. In Asia, M. micrantha is particularly problematic in plantation crops, but also threatens natural areas and disturbed ecosystems. Several aspects of the biology and ecology of M. micrantha are discussed in this review to evaluate its suitability as a target of biological control in Florida. Based on an ecological niche model, the climate in southern and central Florida is highly favorable for the invasion of this weed. Previous biological control programs provide valuable information for Florida scientists about the availability of natural enemies and potential areas for future foreign surveys. Genetic comparison of the Florida population and native range populations, in addition to climate matching, should be considered when selecting areas for foreign exploration. Finally, the authors used a scoring system and risk-benefit–cost analysis to evaluate M. micrantha as a potential target for biological control in Florida. We suggest that similar feasibility evaluations should be conducted not only for well-established problematic weeds but also for new invaders with a known history of invasiveness.

Rural landscapes are increasingly diverse and heterogeneous, involving a mix of small and large parcels, amenity and agricultural properties, and resident and absentee owners. Managing invasive plants in landscapes with changing ownership requires understanding the views and practices of different landowners. We surveyed landowners in two rural valleys with 26% absentee ownership and a large number of small parcels in Missoula County, Montana. Landowners indicated a high level of awareness and concern about weeds; more than 80% agreed that weeds are a problem in their valley. Seventy-eight percent of landowners managed weeds, but only 63% were effective at weed management. Absentee owners were far less likely to manage weeds on their properties and less likely to utilize herbicides, as compared with resident landowners. Landowners reported that seeds coming from adjacent properties were the most significant barrier to effective weed control. Many landowners manage weeds to be a good neighbor and believe that cooperation between neighbors is critical to weed management.

The interactions between native and exotic species occur on a continuum from facilitative to competitive. A growing thrust in invasive species science is differentiating where particular native species occur along this continuum, with practical implications for identifying species that might reduce the invasibility of ecosystems. We used a greenhouse experiment to develop a competitive hierarchy of 27 native species with red brome, an invasive annual grass in the arid lands of the southwestern United States, and a field study to assess in situ responses of brome to native perennial species in the Mojave Desert. Native species most competitive with brome in the competition experiment included the annuals Esteve's pincushion and western fiddleneck and the perennials eastern Mojave buckwheat, sweetbush, and brittlebush, which reduced brome biomass to 49 to 70% of its grown-alone amount. There was no clear difference in competitive abilities with brome between annual and perennial natives, and competiveness was not strongly correlated (r = 0.15) with the biomass of the native species. In the field, sweetbush and brittlebush supported among the least cover of brome, suggesting congruence of the strong early competitive abilities of these species with in situ patterns of brome distribution. At the other extreme, brome attained its highest average cover (19%) below littleleaf ratany, significantly greater than all but 3 of the 16 species evaluated. Cover by brome was only weakly related (r = 0.19) to the area of the perennial canopy, suggesting that factors other than the sizes of perennial plants were linked to differences in brome cover among species. Results suggest that (1) interactions with brome differ substantially among native species, (2) these interactions are not as closely linked to biomass production as in more temperate regions, and (3) there is potential for identifying native species that can reduce invasion of desert ecosystems.

Japanese stiltgrass, an annual grass species native to eastern Asia, has become a serious invasive-plant problem in the eastern United States. We compared the efficacy of herbicides and nonchemical options found effective for controlling stiltgrass in earlier studies, with organic herbicides and herbicides used at reduced rates in a wooded floodplain along the lower Connecticut River. We compared the effect of 2 yr of conventional and alternative treatments on cover of other nonnative and native species. Four blocks of 18 plots (3 by 4 m [9.8 by 13.1 ft]) were established in May 2008. Treatments included directed heating with a propane torch (June, July), hand-pulling (July), mowing with a string trimmer (July, August), foliar applications of household vinegar [5% acetic acid] (June, July) and the herbicides imazapic (June), pelargonic acid (June, July), and pelargonic acid plus pendimethalin (June). The following herbicides were applied at labeled doses and at one-fourth labeled doses: fenoxaprop-p-ethyl (July), glufosinate (August), and glyphosate (August). Stiltgrass cover and height were evaluated periodically, and plant samples were collected in autumn of 2008 and 2009 to determine the number of viable seeds produced. Final evaluations were conducted in June 2010 after 2 yr of treatment. Stiltgrass cover averaged 88% on untreated plots in fall. All treatments reduced stiltgrass cover and seed production. The least-effective treatments were hand-pulling, pelargonic acid, and vinegar in July. Direct heating, mowing, and vinegar in June reduced seed production by more than 90%. All treatments containing imazapic, pelargonic acid plus pendimethalin, fenoxaprop-p-ethyl, glufosinate, and glyphosate completely prevented stiltgrass seed production in the second year of treatment. Effective control of stiltgrass can be achieved during a 2-yr period with a variety of herbicides, including herbicides at one-fourth of the labeled dose, and through nonchemical treatments.

Prescribed grazing is an effective tool for controlling some rangeland weeds. Forage quality of eight nonnative rangeland weeds common to northern Idaho was determined. Five collection sites were established for each weed species: rush skeletonweed, meadow hawkweed, houndstongue, sulfur cinquefoil, yellow starthistle, Dalmatian toadflax, hoary cress, and tansy ragwort. Plants were collected at rosette, bolt, flower, and seed set stages; dried and separated into leaves, stems, flowers, or seed; and analyzed separately for crude protein (CP), neutral detergent fiber (NDF), and mineral content (ash). As expected, plants became more fibrous as they matured because stems increased in NDF value throughout the season and the leaf : stem ratio of most weeds we examined decreased throughout the season. In general, the weeds we examined expressed only moderate fiber levels, beginning the season with NDF values near 34% in the rosette stage and becoming near 52% NDF in the seed set stage. CP values were near 25% in the rosette stage for houndstongue, rush skeletonweed, and hoary cress whereas other weeds we examined had about 15% CP in the rosette. As the season advanced, all plants lost protein content and ended the season with CP values from 5 to 8% at seed set. Ash values declined for all species as the season progressed. Our results indicate that these weeds have forage values similar to many native plants and could be reasonable forage resources for livestock. We did not examine the content of secondary compounds in these weeds that may render them unpalatable and limit their forage value.

Broadleaf herbicides are commonly used to suppress exotic weeds with the intent of releasing native species from negative impacts of invasion. However, weed control measures can also have unintended consequences that should be considered along with intended effects. We conducted a controlled field experiment within bunchgrass communities of western Montana to examine if broadcast application of the broadleaf herbicide, picloram, may mitigate impacts of the exotic forb, spotted knapweed, on the dominant native grass, bluebunch wheatgrass, and forb, arrowleaf balsamroot. Local-scale relationships between native species and spotted knapweed cover served as a baseline for evaluating treatment effects at differing spotted knapweed invasion levels. To examine secondary invasion, we also measured treatment effects on the exotic grass, downy brome, relative to initial levels of spotted knapweed cover. Picloram application suppressed spotted knapweed cover by 70 to 80%. Treatment appeared to release cover and seed production of bluebunch wheatgrass, causing increases that varied positively with initial spotted knapweed cover. Bluebunch wheatgrass measures were elevated by as much as fourfold in treated vs. control plots, exceeding baseline levels in noninvaded plots. For arrowleaf balsamroot, negative effects of treatment prevailed, particularly where initial spotted knapweed cover was low. Arrowleaf balsamroot cover and fecundity variables were reduced by as much as 60% in treated vs. control plots, to levels typifying baseline conditions in highly invaded plots. In addition, treatment released downy brome, with cover increases from 2- to 20-fold. A controlled experiment selectively removing spotted knapweed showed similar release of downy brome. Our results show that picloram effects can depend on initial levels of weed invasion and may include substantial side effects, particularly when broadcast applications are used. Integrated approaches that include seeding of desirable species may be needed to enhance plant community resistance to secondary invaders and reinvasion by the target weed.

Japanese knotweed (JK) is one of the most aggressive invasive plants known in the U.K., where its biology has been well-studied. It was introduced into Canada around 1900, but only recently has it become a serious concern in the province of British Columbia (BC). Climatic conditions, including annual degree days and mean-annual minimum temperatures at knotweed sites in British Columbia were modeled in BioSIM, using weather normals and long-term daily weather data, and compared to published thresholds (degree day = 2,505 DD, minimum temperature = −30.2 C, base temperature 0 C). The degree-day threshold was more limiting to JK in British Columbia than mean-minimum temperature (12.3% of province habitat was suitable for JK based on degree days compared with 26% for mean-minimum temperature). A new annual-precipitation threshold of 735 mm/year based on 95% of known knotweed sites in BC was identified. The best-fit logistic regression model included degree days and annual precipitation and predicted knotweed presence/absence with over 97% efficiency. Existing knotweed sites occupy just over half of the suitable habitat in BC, indicating there are still significant areas to be invaded. The limiting threshold for knotweed was reversed in Southern Ontario with between 35 to 53% of the habitat suitable based on minimum temperatures, whereas degree-day accumulations and annual precipitation were not limiting. Warmer temperatures from 2000 to 2008 resulted in an increase to 53% of the habitat in Southern Ontario being suitable for knotweed, compared to 35% when 1971 to 2000 weather normals were used. Different climatic thresholds among provinces might result in selection for different invasive knotweed genotypes. This could influence the success of biological control agents because of differential host suitability of knotweed genotypes. Habitat suitability maps generated will enable better targeting of knotweed surveys based on the risk of knotweed establishment.

Giant reed is an invasive weed throughout the southern half of the United States, with the densest stands growing along the coastal rivers of southern California and the Rio Grande in Texas. The objective of this study was to use aerial photography to map giant reed infestations and to estimate infested areas along the Texas–Mexico portion of the Rio Grande. Aerial color-infrared photographs were taken along the Rio Grande between Brownsville and El Paso, TX, in June and July 2002. Based on the aerial photographs and ground surveys, the portion of the river from San Ygnacio to Lajitas, which has a river length of 898 km (558 mi), was found to be infested with giant reed. To estimate infested areas along both sides of the river, 65 (13.5%) of the 480 aerial photographs taken between Lajitas and San Ygnacio were randomly selected. The aerial photographs were digitized, rectified to Google Earth imagery, and then classified using maximum-likelihood classification techniques. The infested areas on both sides of the river, as well as water area and river length, from each photographic image were determined. Based on the estimates from the 65 aerial photos, the ratio of giant reed area to water area and the ratio of giant reed area to river length were calculated. The total giant reed area along the Rio Grande between Lajitas and San Ygnacio was estimated to be 5,981 ha (14,779 ac) with 3,714 ha or 62% on the U.S. side and 2,267 ha or 38% on the Mexican side. This study provides the first accurate estimates of giant reed infestations along the Texas–Mexico portion of the Rio Grande and will be useful for both land owners and government agencies for the estimation of water usage and economic loss and for the management and control of giant reed.

Aminopyralid efficacy on Canada thistle (Cirsium arvense) and potential to injure native species was evaluated in a restored prairie at the Glacial Ridge Preserve managed by The Nature Conservancy in Polk County, MN. Canada thistle stem density was reduced from 17 to 0.1 stems m−2 10 mo after treatment (MAT) with aminopyralid applied in the fall at 120 g ha−1. Aminopyralid also altered the composition of both Canada thistle–infested and native plant communities. Aminopyralid controlled Canada thistle and removed or reduced several undesirable forb species from the restored prairie communities, such as absinth wormwood (Artemisia absinthium) and perennial sowthistle (Sonchus arvensis). A number of high seral forbs were also reduced or removed by aminopyralid, including maximilian sunflower (Helianthus maximiliani) and purple prairie clover (Dalea purpurea). Foliar cover of high seral forbs in the native plant community was reduced from 12.2 to 7% 22 MAT. The cover of high seral grass species, such as big bluestem (Andropogon gerardii) and Indiangrass (Sorghastrum nutans) increased after aminopyralid application in both the Canada thistle–infested and native plant communities and averaged 41.4% cover compared with only 19.4% before removal of Canada thistle. Species richness, evenness, and diversity were reduced after aminopyralid application in both Canada thistle–infested and native plant communities. However, the benefits of Canada thistle control, removal of undesirable species, and the increase in native grass cover should lead to an overall improvement in the long-term stability and composition of the restored prairie plant community, which likely outweigh the short-term effects of a Canada thistle control program.

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%.

The success of Japanese stiltgrass as an invader may be due to its ability to respond to stochastic events (e.g., by sexual reproduction via chasmogamous [CH] flowers) and to maintain a beneficial genetic make-up (e.g., by self-fertilizing via cleistogamous [CL] flowers) when conditions are stable. This paper evaluates the importance of Japanese stiltgrass seed type (chasmogamous seeds, cleistogamous seeds, and seeds originating from forest-interior [F-I] plants) in terms of seed mass, viability, and germination across variable moisture regimes (three regions in West Virginia) and at two light levels (roadside and forest interior). Seeds from nine populations were sampled in three site types in 2005 and 2008 and stored at 5 C until testing in April 2009. Seeds were tested for viability using a dye test. Seeds were germinated under both constant and fluctuating day/night temperatures. Additional samples of CH and CL seeds collected in 2008 were tested for viability again in September 2010 for a measure of seed longevity. CL and F-I seeds were smaller in mass than CH seeds. Seeds from the drier sites were smaller in mass than seeds from the more mesic sites. CL seeds, followed by F-I seeds, were less viable than CH seeds in 2005 and 2008. CL and F-I seeds had lower germination rates than CH seeds for each site type in 2005, but germination rates of the seed types did not differ in 2008. Differences in seed longevity for 2008 seeds were lower for CL compared to CH seeds, but only in the drier sites. Japanese stiltgrass' longer-lived and larger CH seeds from the roadsides may ensure population survival over the long term. Younger CL and F-I seeds differ less from CH seeds in terms of germination than older seeds, which may help Japanese stiltgrass to maintain populations under relatively stable conditions in the short term.

In eastern deciduous forests of North America, invasive shrubs are increasing in richness and abundance at the expense of native species across taxa. Invasive shrubs create an understory that is more dense than both recent and historical preinvasion conditions. Interest in invasive shrub removal to restore native habitat is growing, but our understanding of natural regeneration following treatment of a diverse invasive shrub community is lagging. Using an invasive shrub removal experiment, we provide insight into the effect of repeated removal of a suite of 18 invasive shrub species dominated by border privet (Ligustrum obtusifolium Siebold & Zucc.). In 2009, invasive shrubs were removed from five 20-m-diameter treatment plots, each with a paired control plot. Seven years later, we find an increase in plant diversity, native understory species abundance, and overstory tree species regeneration for individuals under a meter in height. For plants 1 to 4 m in height, the removal treatment has a positive effect on understory woody species, but there has been no change in regenerating overstory trees. A lack of overstory tree regeneration to greater heights is not surprising, given the time frame and the closed-canopy conditions. However, other factors, such as white-tailed deer (Odocoileus virginianus Zimmermann) browse, could be serving as an impediment to taller tree regeneration in the forest understory. An ambient sampling approach in unmanaged, invaded, and uninvaded forest has been used in other studies to estimate the potential impacts of invasive shrub species to native plant communities. However, in this study the ambient sampling approach underestimated the impacts of invasive shrubs compared to their experimental removal. Overall, invasive shrub removal increased plant diversity and allowed passive natural regeneration of native plants that exceeded native cover in the unmanaged, ambient forest under minimal invasive shrub abundance.

Russian-olive is a small tree or large multistemmed shrub that was introduced to Canada and the United States from Eurasia in the early 1900s. It was provisioned in large numbers during the last century to prairie farmers as a shelterbelt plant and remains a popular and widely available ornamental. Now invasive within some riparian ecosystems in the western United States, Russian-olive has been declared noxious in the states of Colorado and New Mexico. With traits including high shade tolerance and a symbiotic association with nitrogen-fixing bacteria, Russian-olive has the potential to dominate riparian vegetation and thus radically transform riparian ecosystems. Especially alarming is its capacity to influence nutrient dynamics within aquatic food webs. Our objective is to draw attention to Russian-olive as a potential threat to riparian ecosystems within Canada, especially in the southwest, where invasion is becoming commonplace. We review what is known about its biology and about the threats it poses to native organisms and ecosystems, and we summarize management and control efforts that are currently underway. We conclude by proposing a research agenda aimed at clarifying whether and how Russian-olive poses a threat to riparian ecosystems within western Canada.

Two factors that can degrade native plant community composition and structure, and hinder restoration efforts, are invasive species and chronic overbrowsing by ungulates such as white-tailed deer. Beginning in 2007, the effectiveness, costs, and impacts of Japanese barberry control treatments and herbivory on nonnative and native plant communities was examined at eight study areas over 4 to 5 yr. Prescribed burning and mechanical mowing by wood shredder or brush saw were utilized as initial treatments to reduce the aboveground portion of established barberry and were equally effective. Without a follow-up treatment, barberry had recovered to 56 to 81% of pretreatment levels 50 to 62 mo after initial treatment. Follow-up treatments in mid-summer to kill new sprouts included directed heating and foliar herbicide applications. Relative to untreated controls, follow-up treatments lowered barberry cover 50 to 62 mo after initial treatment by at least 72%. Although all follow-up treatments were equally effective, the labor cost of directed heating was four times higher than for herbicide applications. Follow-up treatment type (directed heating vs. herbicide) had minimal impact on species other than barberry. White-tailed deer herbivory had a larger impact on other species than did barberry control treatments. Native grass and fern cover was higher outside of exclosures. Areas inside exclosures had higher cover of Oriental bittersweet and multiflora rose, but not Japanese barberry. Thus, recovery of native communities will require more than simply removing the dominant invasive species where deer densities are high. Excellent reduction of Japanese barberry cover can be achieved using either directed heating or herbicides as follow-up treatments in a two-step process, but other invasive plants may become a problem when barberry is removed if deer populations are low.

Medusahead is one of the most problematic rangeland weeds in the western United States. In previous studies, prescribed burning has been used successfully to control medusahead in some situations, but burning has failed in other circumstances. In this study, trials were conducted using the same protocol at four locations in central to northern California to evaluate plant community response to two consecutive years of summer burning and to determine the conditions resulting in successful medusahead control. During 2002 through 2003 large-scale experiments were established at two low-elevation, warm-winter sites (Fresno and Yolo counties) and two higher elevation, cool-winter sites (Siskiyou and Modoc counties). Plant species cover was estimated using point-intercept transects, and biomass samples were taken in each plot. After 2 yr of burning, medusahead cover was reduced by 99, 96, and 93% for Fresno, Yolo, and Siskiyou counties, respectively, compared to unburned control plots. Other annual grasses were also reduced, but less severely, and broadleaf species increased at all three sites. In contrast, 2 yr of burning resulted in a 55% increase in medusahead at the coolest winter site in Modoc County. In the second season after the final burn, medusahead cover remained low in burned plots at Fresno and Yolo counties (1 and 12% of cover in unburned controls, respectively), but at the Siskiyou site medusahead recovered to 45% relative to untreated controls. The success of prescribed burning was correlated with biomass of annual grasses, excluding medusahead, preceding a burn treatment. It is hypothesized that greater production of combustible forage resulted in increased fire intensity and greater seed mortality in exposed inflorescences. These results demonstrate that burning can be an effective control strategy for medusahead in low elevation, warm-winter areas characterized by high annual grass biomass production, but may not be successful in semiarid cool winter areas.

The ecological effects of Chinese tallowtree are well documented in the southeastern United States, yet this known invasive plant continues to be planted extensively in California's Central Valley, where it has recently naturalized in several locations. Climate modeling suggests that most of California's lowland riparian habitat is susceptible to invasion by Chinese tallowtree; however, no field tests are available to corroborate this result for California or to identify local environmental constraints that might limit potential habitats. We used observational and experimental methods to evaluate invasion potential of Chinese tallowtree in riparian habitats in California's Central Valley. High invasion potential, indicated by an intersection of the maxima of dispersal probability, germination, and survivorship of seedlings, occurred at low elevations immediately next to perennial waters. The main factor limiting Chinese tallowtree invasion potential in more elevated habitats appears to be lack of seedling drought tolerance. These findings suggest that California's riparian habitats are vulnerable to invasion by Chinese tallowtree, especially downstream of current naturalized populations where water or bird dispersal will deposit seeds in environments ideal for germination and growth.

Common practices for invasive species control and management include physical, chemical, and biological approaches. The first two approaches have clear limitations and may lead to unintended (negative) consequences, unless carefully planned and implemented. For example, physical removal rarely completely eradicates the targeted invasive species and can cause disturbances that facilitate new invasions by nonnative species from nearby habitats. Chemical treatments can harm native, and especially rare, species through unanticipated side effects. Biological methods may be classified as biocontrol and the ecological approach. Similar to physical and chemical methods, biocontrol also has limitations and sometimes leads to unintended consequences. Therefore, a relatively safer and more practical choice may be the ecological approach, which has two major components: (1) restoration of native species and (2) biomass manipulation of the restored community, such as selective grazing or prescribed burning (to achieve and maintain viable population sizes). Restoration requires well-planned and implemented planting designs that consider alpha-, beta-, and gamma-diversity and the abundance of native and invasive component species at local, landscape, and regional levels. Given the extensive destruction or degradation of natural habitats around the world, restoration could be most effective for enhancing ecosystem resilience and resistance to biotic invasions. At the same time, ecosystems in human-dominated landscapes, especially those newly restored, require close monitoring and careful intervention (e.g., through biomass manipulation), especially when successional trajectories are not moving as intended. Biomass management frequently uses prescribed burning, grazing, harvesting, and thinning to maintain overall ecosystem health and sustainability. Thus, the resulting optimal, balanced, and relatively stable ecological conditions could more effectively limit the spread and establishment of invasive species. Here we review the literature (especially within the last decade) on ecological approaches that involve biodiversity, biomass, and productivity, three key community/ecosystem variables that reciprocally influence one another. We focus on the common and most feasible ecological practices that can aid in resisting new invasions and/or suppressing the dominance of existing invasive species. We contend that, because of the strong influences from neighboring areas (i.e., as exotic species pools), local restoration and management efforts in the future need to consider the regional context and projected climate changes.

The intent of this study was to determine if composting is an effective means of managing water hyacinth while producing a quality horticultural compost product. Preliminary tests for the study included germination and seed mortality tests. Germination tests found that water hyacinth seeds germinated on filter paper media soaked in distilled water while placed in petri dishes held at a constant temperature of 27 C for 14 d. Seed mortality test results found that seeds of water hyacinth were rendered inviable at temperatures equal to or above 57 C. The study successfully developed a large-scale composting system that used water hyacinth as a primary feedstock. Eleven compost piles were derived from 10,000 kg of water hyacinth, 9,000 kg of food waste, 11,300 kg of poultry litter, and 17,200 kg of wood chips. Results indicated that the composting process reached and sustained sufficiently high enough temperatures to inactivate and fully decompose seeds and other propagules of water hyacinth. Therefore, water hyacinth can be composted without the potential danger of it spreading. Compost quality tests found that the compost produced was within acceptable to ideal ranges of accepted industry quality standards, though there was a learning curve by student workers in the preparation of the piles using the large equipment.

Lepidium latifolium (perennial pepperweed) is recognized as a threat to wetland habitats throughout much of the western United States, but its role in tidal marshes has not been explored. Over three seasons in three regions of San Francisco Estuary (Suisun, San Pablo, and South San Francisco bays), we characterized locations in tidal marshes where monotypic stands of L. latifolium are replacing the native Sarcocornia pacifica (pickleweed). Soils within L. latifolium stands had significantly lower moisture, salinity, organic matter, and carbon : nitrogen (C : N) and higher pH than in adjacent S. pacifica stands at similar elevation and distance from channels. In addition, L. latifolium canopies were 2 to 3 times taller, thus increasing light reaching soils, and tended to support different insect/spider assemblages (sampled only at the Suisun site). Patterns were generally consistent across the three sites, although less pronounced for some measures at the South Bay site. Overall, these data suggest that L. latifolium invasion of tidal marshes is leading to modification of both structural and functional properties, several of which might further facilitate spread of the invader; however, additional study is needed to determine cause vs. effect for several soil characteristics.

Habitat suitability and disturbance can shape the early stages of biological invasions in important ways. Much that we know about habitat suitability and invasion originates from point-in-time studies, which characterize invasive plant abundance and associated site characteristics. In our study, we tested the influence of habitat suitability by creating small-scale invasions in a range of environments. Seeds of the invasive annual grass Japanese stiltgrass [Microstegium vimineum (Trin.) A. Camus] were planted into six environments in a deciduous forest in central Pennsylvania, and patch growth was monitored for 4 yr. Each of the 30 sites included a subplot subjected to litter disturbance at time of planting. This litter disturbance led to increased seedling recruitment only in the first 2 yr. Although patches were generally larger in wetland and roadside habitats, site influence was highly variable. Environmental variables (soil moisture, ammonium–N, pH, and plant species richness) measured in each plot were better predictors of population success than broad habitat categories. We conclude that risk assessment for species such as M. vimineum should focus not on habitat types but on areas likely to experience the physical changes that release M. vimineum populations.