Fire is often used in northern grasslands to control invasive grass species but has unknown effects on Tamarix spp., more recent invaders. Temperature (using an oven as a fire surrogate) and duration combinations that would be most lethal to Tamarix seeds and seedlings were determined. Tamarix seeds were sown in soil-lined dishes, water added to saturation, and seedlings grown for 1 to 5 d. Seeds were also placed in water-saturated or dry soil just before temperature exposure (79 to 204 C [175 to 400 F]) by duration (1 to 5 min) treatments. After treatment, soil water loss was measured by weight difference, and surviving seedlings were counted for 6 d. Tamarix seedling establishment and survival decreased with increasing temperature and duration. The 5-d-old seedlings were the most affected. No 5-d-old seedlings survived 1- and 2-min exposures to 204 C, whereas 1-d-old seedlings had greater than 25% survival. If soils were saturated, two to four times more seedlings established following seed exposure to 177 and 204 C. Longer durations at lower temperatures were required to reduce Tamarix survival. Increasing duration from 2 to 5 min at 121 C decreased 5-d-old seedling survival from more than 80% to less than 10% and eliminated those seedlings at 149 C. Five minutes at 149 C decreased dry-soil seed viability to about 15%, whereas germination on saturated soils remained high (∼75%). No seeds survived the exposure to 177 and 204 C. Soil moisture loss values associated with 90% mortality of 5- and 1-d-old seedlings were 1.7 and 2.2%, respectively. On saturated soils, 90% of seeds died with 2.5% water loss. Under suitable conditions, fire can decrease Tamarix seedling survival. Fire may be useful for controlling Tamarix seedlings in northern grasslands and should be considered for management of new invasions.

Nomenclature: Tamarix spp.: Chinese tamarisk, Tamarix chinensis Lour. TAACH; saltcedar, Tamarix ramosissima Ledeb. TAARA.

Management Implications: Tamarix can invade moist-soil grasslands in the Northern Great Plains, and it is necessary to treat seedlings before they become well-established. Fire is an important management tool used to control nonnative grasses in northern grasslands, but this practice may also minimize Tamarix spread from seed. Although temperatures and durations associated with prescribed grassland fire are reported to be minimal, under appropriate conditions controlled burns in northern grasslands can provide soil surface temperatures and exposure periods lethal to most Tamarix seeds and developing seedlings. Deposited seeds are more likely to survive elevated temperatures than newly emerging seedlings but because seeds germinate rapidly upon wetting, fire will more likely encounter seedlings. To effectively control new infestations, Tamarix seedlings should be treated before they develop belowground perennating buds that generate new shoots following dormancy or disturbance. Repeated burning regimes are less likely to increase nonnative invasions in grasslands than other habitats suitable for Tamarix establishment because these ecosystems have evolved with this disturbance. Fire is, therefore, a viable option for Tamarix seedling control in northern grasslands, but burn programs should not target this species until more information is available.

Temperate and boreal forests in Canada and the northeastern United States have been invaded by several exotic species, including European earthworms (family Lumbricidae) and garlic mustard. Earthworms and garlic mustard co-occur and are both known to adversely impact some native plant species. However, relatively little is known about potential interactions between these two invaders. In a series of growth chamber experiments, we determined the palatability of garlic mustard and six native herbaceous forest species (shooting star, columbine, wild geranium, sweet cicely, butterfly milkweed, and yellow jewelweed) to the common nightcrawler. We also assessed the ability of the common nightcrawler to bury and digest garlic mustard and wild geranium. When offered seeds from garlic mustard and a native plant species, the earthworms ingested more garlic mustard seeds than seeds from four of the six native species. In a mesocosm experiment, the common nightcrawlers apparently digested 72 and 27% of garlic mustard and wild geranium seeds, respectively, that were placed on the soil surface. No seeds were observed on the soil surface at the end of the experiment but the majority of recovered seeds for both species were found within the top 10 cm (3.94 in). More seeds were recovered in 0- to 10-cm and 31- to 40-cm sections for wild geranium than for garlic mustard. No difference in seed recovery was detected at the other depths. Garlic mustard seed is readily consumed by common nightcrawlers and appears to be preferred over some native plant species suggesting that common nightcrawlers may reduce the size of the garlic mustard seed bank.

Nomenclature: Common nightcrawler, Lumbricus terrestris L.; garlic mustard, Alliaria petiolata (M. Bieb.) Cavara & Grande ALAPE; columbine, Aquilegia canadensis L.; butterfly milkweed, Asclepias tuberosa L. ASCTU; common shooting star, Dodecatheon meadia L.; wild geranium, Geranium maculata L.; yellow jewelweed, Impatiens pallida Nutt; sweet cicely, Osmorhiza claytonia (Michx.) C.B. Clarke.

Interpretive Summary: The common nightcrawler is an introduced earthworm species that has been linked to reduced native plant diversity and to altered soil processes in invaded temperate and boreal forests in North America. The common nightcrawler can consume large quantities of seeds but the effect of this invasive species on the seed bank dynamics of native and invasive plants in its invaded range is unknown. In this study, we examined the palatability of garlic mustard, an invasive plant species, and six native species to the common nightcrawler. We also assessed the ability of the common nightcrawler to bury seeds below the soil surface. The earthworm preferred garlic mustard seeds over the seeds of four of the six native plant species and consumed or buried large quantities of garlic mustard seed. Although the net effect of the common nightcrawler on native plant species in invaded forests may be negative, our findings suggest that this earthworm has the potential to reduce the size of the garlic mustard soil seed bank.

Cutleaf teasel is an invasive, biennial plant that poses a significant threat to native species along roadsides in Missouri. Flowering plants, together with understory rosettes, often grow in dense patches. Detection of cutleaf teasel patches and accurate assessment of the infested area can enable targeted management along highways. Few studies have been conducted to identify specific species among a complex of vegetation composition along roadsides. In this study, hyperspectral images (63 bands in visible to near-infrared spectral region) with high spatial resolution (1 m) were analyzed to detect cutleaf teasel in two areas along a 6.44-km (4-mi) section of Interstate I-70 in mid Missouri. The identified classes included cutleaf teasel, bare soil, tree/shrub, grass/other broadleaf plants, and water. Classification of cutleaf teasel reached a user's accuracy of 82 to 84% and a producer's accuracy of 89% in the two sites. The conditional κ; value was around 0.9 in both sites. The image-classified cutleaf teasel map provides a practical mechanism for identifying locations and extents of cutleaf teasel infestation so that specific cutleaf teasel management techniques can be implemented.

Nomenclature: Cutleaf teasel, Dipsacus laciniatus L.

Interpretative Summary: Cutleaf teasel is an exotic weed that infests roadside environments in Missouri. As a growing biennial, the plant develops as a rosette during the first year and bolts during the second. Dense patches contain flowering plants with understory rosettes. The objective of this work was to develop approaches for detecting cutleaf teasel patches with accurate assessment in a complex of species along a roadside. Thus, management of cutleaf teasel could be located at specific sites. Two hyperspectral images (63 bands with 1-m spatial resolution) were analyzed to detect cutleaf teasel along the Interstate Highway I-70 in mid Missouri. Classification of cutleaf teasel reached a user's accuracy of 82 to 84% and a producer's accuracy of 89% at the two sites. The image-classified teasel map provides a practical mechanism for identifying the locations and extents of cutleaf teasel infestation so that specific management techniques can be implemented.

Invasive weed control within cleared, forested sites in the inland Northwest is complicated by the susceptibility of ponderosa pine to synthetic auxin herbicide injury, used to control broadleaf weeds. Herbicide injury may lead to decreased canopy volume and variable growth patterns of ponderosa pine, which is a commercially important tree species. Herbicide injury to ponderosa pine can be decreased with dormant-season applications, a timing suited to control many weeds that may occur within ponderosa pine sites. However, spring-timed herbicide applications are needed to control other weeds, such as meadow hawkweed, and that application timing coincides with active ponderosa pine growth. In this study, we determined the level of injury to ponderosa pine resulting from spring-timed aminopyralid, clopyralid, and picloram applications beneath ponderosa pine canopies. Herbicide injury to leader and lateral candles and needle elongation was evaluated 1 and 12 mo after treatment (MAT). Low rates of aminopyralid alone (0.05 kg ae ha⁻¹ [3 fl oz ac⁻¹]) and aminopyralid clopyralid (0.05 0.10 kg ae ha⁻¹) resulted in herbicide injury ratings that did not differ from untreated trees. The high rate of aminopyralid (0.12 kg ae ha⁻¹) resulted in leader candle injury on 75% of treated trees, 5% of which were necrotic at 12 MAT. Herbicide injury was observed on 30% of lateral candles. In comparison, picloram (0.28 kg ae ha⁻¹) treatments resulted in necrosis or mortality of leader and lateral candles on 65% and 40% of trees, respectively, at 12 MAT. Results suggest that use of low rates of aminopyralid alone or in combination with low rates of clopyralid minimizes the risk of nontarget injury to ponderosa pine (> 5 yr old) while controlling hawkweed with a spring application.

Nomenclature: Aminopyralid; clopyralid; picloram; meadow hawkweed, Hieracium caespitosum Dumort; ponderosa pine; Pinus ponderosa Lawson & C. Lawson.

Interpretive Summary: Broadleaf weeds under ponderosa pine canopies are often controlled in the fall when trees are dormant but the weeds are susceptible to treatment. Meadow hawkweed, however, is not susceptible to fall herbicide treatments. Managing broadleaf weeds in the spring risks injury to ponderosa pine, thus herbicide application rates were tested to determine whether injury could be minimized. Spring applications of low rates of aminopyralid (0.05 kg ae ha⁻¹) with or without low rates of clopyralid (0.10 kg ae ha⁻¹) did minimize the risk of injury to ponderosa pine when applied below the canopy. Our results suggest that use of high aminopyralid label rates (0.12 kg ae ha⁻¹) or picloram (0.28 kg ae ha⁻¹) should be avoided when targeting invasive weeds in the spring because of likely negative effects to ponderosa pine. In particular, the results of this study should improve meadow hawkweed management decisions in the Pacific Northwest, which requires spring-timed applications but are effective at low use rates.

The use of benthic barriers alone or in combination with other control methods could initiate the eradication of pioneer populations of Eurasian watermilfoil and facilitate maintenance of acceptable population levels in water bodies where the weed is widely established. We evaluated the effects of duration of geotextile fabric panel placement on small Eurasian watermilfoil population control and nontarget plant abundance. In 2006, benthic barriers were placed over Eurasian watermilfoil infestations and removed at intervals of 4, 8, 10, and 12 wk. The 4-wk duration reduced Eurasian watermilfoil biomass 75%, and all other duration treatments reduced Eurasian watermilfoil biomass 100%. The 4-wk treatment had no effect on native plant biomass, whereas other treatments reduced native plant biomass by 79 to 93%. At the conclusion of the 12-wk study, Eurasian watermilfoil biomass had increased in the 4-wk treatment but did not reestablish within treatment plots of longer duration. Native plant biomass had increased to 21% of the untreated control in the 8-wk barrier treatment. Results suggest the 8-wk duration is sufficient for removal of Eurasian watermilfoil while allowing regrowth of native aquatic plants. A walk-in growth chamber experiment was established to evaluate the effect of sediment accumulation on the benthic barrier. Eurasian watermilfoil fragments grown on sediment depths of 0 to 3 cm (0 to 1.2 in) did not differ for shoot or root biomass. At sediment depths of 4 and 5 cm, Eurasian watermilfoil root and shoot biomass increased when compared with the control, suggesting benthic barrier maintenance should include sediment removal when sediment reaches a depth of 4 cm.

Nomenclature: Eurasian watermilfoil, Myriophyllum spicatum L.

Interpretive Summary: Exotic weeds have invaded and impaired aquatic ecosystems in western North America. Restoring native aquatic habitats degraded by exotic plants should decrease invader abundance and also result in a return to preinvasion levels of desirable vegetative communities and native diversity. Eurasian watermilfoil is a submersed, aquatic, Eurasian perennial plant that that has invaded waterways throughout the United States and Canada, forming dense mats of vegetation on the water's surface, which interfere with water-based recreational activities, inhibit water flow, and impair critical fish and wildlife habitat. Bottom modification treatments, including the use of portable panels of synthetic weed fabric placed on the bottom of ponds and lakes can be an effective control measure for widely established populations of Eurasian watermilfoil and can be used to remove small populations. Although the effectiveness of bottom barriers for the control of nuisance aquatic plants in confined areas has been shown in previous studies, evaluation of optimum coverage time, maintenance requirements, and nontarget plant community response is lacking. We recorded the weight of Eurasian watermilfoil and native plants in 40 plots on the bottom of Round Lake in 2006 and Chatcolet Lake in 2007 near Plummer, ID. We also recorded shoot weight, root weight and length, and sediment depth during an aquarium-based study conducted at the University of Idaho in Moscow, ID, in 2007. The results of this study suggest that the 8-wk barrier placement is sufficient for removal of Eurasian watermilfoil while allowing regrowth of native aquatic plants. Our results also suggest that barriers should be cleaned of sediment when deposition reaches a depth of 4 cm to prevent weed establishment.

Early detection of an invading nonindigenous plant species (NIS) may be critical for efficient and effective management. Adaptive survey sampling methods may provide unbiased sampling for best estimates of distribution of rare and spatially clustered populations of plants in the early stages of invasion. However, there are few examples of these methods being used for nonnative plant surveys in which travelling distances away from an initial or source patch, or away from a road or trail, can be time consuming due to the topography and vegetation. Nor is there guidance as to which of the many adaptive methods would be most appropriate as a basis for invasive plant mapping and subsequent management. Here we used an empirical complete census of four invader species in early to middle stages of invasion in a management area to assess the effectiveness and efficiency of three nonadaptive methods, four adaptive cluster methods, and four adaptive web sampling methods that all originated from transects. The adaptive methods generally sampled more NIS-occupied cells and patches than standard transect approaches. Sampling along roads only was time-efficient and effective, but only for species with restricted distribution along the roads. When populations were more patchy and dispersed over the landscape the adaptive cluster starting at the road generally proved to be the most time-efficient and effective NIS detection method.

Management Implications: It is often not possible or cost-effective to conduct a complete inventory of potentially invasive plant species in large management areas, particularly at the early stages of invasion when populations may be infrequent and dispersed on the landscape. Detection at the early stages of invasion may be crucial for effective and cost-effective management. Thus managers must have survey methods that are effective and efficient for estimating the distribution of invading species. To accomplish different survey goals, which may include finding early invading populations, locating many different invasive plant species, finding the most populations of a single species, or collecting information to characterize species distributions, knowing which survey technique to use is critical. We tested three standard and eight adaptive survey methods on a virtual landscape populated with four empirically censused invasive plant species: Canada thistle, Dalmatian toadflax, smooth brome, and common St. Johnswort. The species exhibited somewhat different growth forms, reproductive patterns, and seed dispersal distances and were in different stages of invasion. Random transects with adaptive cluster sampling generally performed best when the survey goal was to find the largest number of populations in the shortest amount of time for species that were well established and occupied areas away from the road. If the species was in the early stages of invasion and only occupied roadside habitat, surveying along roads performed best. When the survey goal was to accurately assess the proportion of the landscape infested by each species, stratified random targeted transects without adaptive sampling performed best for all species. However, managers should be aware that adaptive sampling methods overestimate infested area. This study indicates that adaptive sampling methods can improve nonindigenous species patch detection for management, but regardless of the sampling method, detection remains relative low (maximum of 33% of patches) with typical management constraints and therefore seriously challenges the concept of early detection and rapid response.

Conservation and land management decisions often are based primarily on natural science, but could be more successful if human influences were effectively integrated into decision making. This is especially true for efforts to manage invasive plants, whose arrival is usually the product of deliberate human introduction. Risk-assessment models that predict the probability that a nonnative plant will naturalize or invade are useful tools for managing invasive plants. However, decisions based on such models could affect stakeholders differently. Careful assessment of risk-analysis methodologies should consider the importance of stakeholder participation. We surveyed the perceptions of four stakeholder groups (conservation professionals, master gardeners, professional horticulturists, and woodland landowners) in Iowa about invasive plants, general management approaches, and risk-assessment models. We also examined whether or not a stakeholder's nature relatedness plays a role in shaping his or her responses. Stakeholder perceptions varied less than expected across all four groups. Eighty-seven percent of respondents agreed invasive plants are a problem, and 88.4% agreed that we have a responsibility to manage them to protect natural areas. Support for the use of risk-assessment models also was high, with 78.7% of respondents agreeing that their use has potential to prevent plant invasions. Nature relatedness scores for all groups were correlated with respondent perspectives on invasive plants. Respondents believed biologically significant error rates (errors that might introduce a new invasive plant) should not exceed 5 to 10%. Respondents were more tolerant of horticulturally limiting errors (errors that restrict sale/use of a plant that would not have become invasive), reporting rates of 10 to 20% as acceptable. Researchers developing risk-assessment models might wish to aim for error rates within these bounds. General agreement among these stakeholder groups suggests potential support for future risk-management efforts related to invasive plants.

Management Implications: Many conservation professionals and land managers have spent countless hours containing or eradicating invasive plants encroaching on natural areas. Given the costs and effort associated with their control, prohibiting the introduction of new nonnative plants that are likely to become invasive would be very beneficial. Risk-assessment models are statistical tools that can be used to screen new plant introductions for invasiveness, but implementing these models comes with challenges. Because most new plant introductions are deliberately initiated by humans, stakeholders' needs must be taken into consideration if these pre-emptive management efforts are to be successful. We identified and surveyed four stakeholder groups (conservation professionals, master gardeners, professional horticulturists, and woodland landowners) in Iowa, who are important voices in decision-making for invasive plants, about their perspectives on general management approaches, and risk-assessment models. We also examined whether or not nature relatedness (a person's sense of connection to the natural world) plays a role in shaping these perspectives. We found these stakeholder groups had relatively minor differences of opinion. Stakeholders agreed that invasive plants were a problem that we have a responsibility to manage, and were open to the idea of passing state laws or mandates to achieve that goal. This was true even of professional horticulturists and master gardeners, who would potentially incur more costs than benefits from such regulations. Stakeholders also displayed consistently high levels of nature relatedness, and concern these groups have about invasive plants might be influenced by their identification with nature. Overall, our findings suggest that risk analysis to limit introduction of potentially invasive plants is l

Japanese stiltgrass is one of the most aggressive, rapidly spreading invasive plants in the eastern United States. Management guidelines state that mowing can help manage Japanese stiltgrass but that mowing is most effective when done late in the season after the plants begin to flower and before they set seed. In this study, I tested the effectiveness of mowing at three different times between mid-June and early September in 2009 and 2010, as well as mowing twice in 1 yr and for two consecutive years. The effectiveness of mowing Japanese stiltgrass was determined by measuring percentage of cover, biomass, seed production, and the number of stems in the summer following mowing. All mowing treatments significantly reduced percentage of cover, biomass, seed production, and the number of Japanese stiltgrass stems the following year. In 2009, all of the mowing treatments significantly reduced biomass, percentage of cover, and seed production. The latest mow, at the end of August, resulted in a slightly greater reduction of cleistogamous seeds. In 2010, the earliest mowing treatment, in mid-June, did not reduce cover and biomass as much as the other mowing treatments. Overall, these results suggest that mowing can be an effective control method for Japanese stiltgrass and that mowing any time after June should be effectively equivalent, although later mowing may provide some marginal advantage.

Nomenclature: Japanese stiltgrass, Microstegium vimineum (Trin.) A. Camus var. imberbe (Nees) Honda.

Management Implications: Mowing is used to control many invasive plant species. For some species, such as Japanese stiltgrass, the timing of mowing within the season is considered important for effective control. The accepted mowing method for Japanese stiltgrass is to mow late in the summer after the plants begin to flower. Mowing earlier in the summer is believed to allow plants to recover and set seed, and is generally considered an ineffective control strategy. However, the evidence to support the importance of late-season mowing is limited and has not been rigorously tested. I tested three timings of mowing (from June to September), and the effect of mowing twice in a year, on Japanese stiltgrass invasions. The earliest mowing treatment (June 18) was less effective than the other timings, but all other times after July 1 significantly reduced cover, biomass, seed production, and the next year's recruitment of Japanese stiltgrass. Some seed production occurred in all mowing treatments, even plots that were mowed after plants began to flower in September. Much of this seed production was from obligately selfing (cleistogamous) flowers that are produced in the axillary nodes of the grass. These flowers are not apparent without close observation and can be produced even under poor growing conditions. The results of this study suggest that Japanese stiltgrass can be controlled reasonably well by mowing in July or later, and that mowing does not have to be restricted to late summer once plants have begun to flower. Some seed production is still likely, however, and mowing needs to be repeated over several years to reduce the seed bank.

Invasive species researchers often ask: Why do some species invade certain habitats while others do not? Ecological theories predict that taxonomically related species may invade similar habitats, but some related species exhibit contrasting invasion patterns. Brassica nigra, Brassica tournefortii, and Hirschfeldia incana are dominant, closely related nonnative species that have overlapping, but dissimilar, distributions. Brassica tournefortii is rapidly spreading in warm deserts of the southwestern United States, whereas B. nigra and H. incana are primarily limited to semiarid and mesic regions. We compared traits of B. tournefortii that might confer invasiveness in deserts with those of related species that have not invaded desert ecosystems. Brassica tournefortii, B. nigra and H. incana were compared in controlled experiments conducted outdoors in a mesic site (Riverside, CA) and a desert site (Blue Diamond, NV), and in greenhouses, over 3 yr. Desert and mesic B. tournefortii populations were also compared to determine whether locally adapted ecotypes contribute to desert invasion. Experimental variables included common garden sites and soil water availability. Response variables included emergence, growth, phenology, and reproduction. There was no evidence for B. tournefortii ecotypes, but B. tournefortii had a more rapid phenology than B. nigra or H. incana. Brassica tournefortii was less affected by site and water availability than B. nigra and H. incana, but was smaller and less fecund regardless of experimental conditions. Rapid phenology allows B. tournefortii to reproduce consistently under variable, stressful conditions such as those found in Southwestern deserts. Although more successful in milder, mesic ecosystems, B. nigra and H. incana may be limited by their ability to reproduce under desert conditions. Rapid phenology and drought response partition invasion patterns of nonnative mustards along a gradient of aridity in the southwestern United States, and may serve as a predictive trait for other potential invaders of arid and highly variable ecosystems.

Nomenclature: Black mustard, Brassica nigra (L.) Koch; Sahara mustard, Brassica tournefortii Gouan; shortpod mustard, Hirschfeldia incana (L.) Lagr.-Foss.

Interpretive Summary: Sahara mustard (Brassica tournefortii), black mustard (Brassica nigra), and shortpod mustard (Hirschfeldia incana) are common invasive mustards throughout parts of the western United States. In particular, all three species are found in southern California, but they have different distributions. Brassica tournefortii is primarily an invader of warm desert ecosystems, black mustard is most common in more mesic regions, and shortpod mustard is most common in the arid Mediterranean-climate regions between the coast and warm deserts. We hypothesized that B. tournefortii's adaptations to arid North Africa might predispose it to successfully invade North America's arid, warm-desert ecos

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.

Nomenclature: Saltcedar, Tamarix ramosissima Ledeb.

Management Implications: Invasion of riverside and other riparian vegetation by saltcedar species changes the appearance and structure of riparian habitats in the arid western United States conspicuously. Because riparian environments occupy a small fraction of these arid lands yet support an inordinately high number of animal species, there is considerable concern regarding effects of saltcedar invasion on wildlife. Here, I present results of small mammal monitoring using annual live-trapping for up to 11 years at four sites in the western Great Basin in both saltcedar-invaded and native riparian habitats. Although the total number of species found during the entire monitoring period was equal or greater in native habitat than in saltcedar at all sites, the number of small mammal species sampled did not differ statistically between native and saltcedar habitats. A general effect of saltcedar invasion was an increase in rodents in the family Heteromyidae. This is consistent with well-known adaptations of heteromyid rodents to open, arid environments and conversion of riparian areas to more open, desert-like habitats at many sites invaded by saltcedar. By contrast, rodent species, such as montane voles and western harvest mice, which are typically associated with the greater cover and more mesic conditions provided by native riparian vegetation tended to be uncommon or absent in saltcedar-converted habitat. Such species may benefit at sites where native vegetation can be successfully restored following saltcedar removal.

Some plants being considered as bioenergy crops share traits with invasive species and have histories of spreading outside of their native ranges, highlighting the importance of evaluating the invasive potential before the establishment of large-scale plantings. The Asian grass Miscanthus × giganteus is currently being planted as a bioenergy crop in the north central region of the United States. Our goal was to understand the demographic rates and vegetative spread of this species in unmanaged arable lands in Illinois to compare with those of large-statured invasive grasses (LSIGs). We collected data from 13 M. × giganteus plantings in Illinois, ranging in age from 1 to 7 yr, recording tiller number, plant spatial extent, spikelet production, and plant survival over 4 yr. Additionally, to understand recruitment potential, we conducted a greenhouse germination experiment, and, to estimate establishment from rhizome fragments, field trials were performed. Miscanthus × giganteus demographic rates were age dependent. Spikelet production was high, with 1- and 4-yr plants producing an annual average of more than 10,000 and 180,000 spikelets plant⁻¹, respectively; however, data from our germination trial suggested that none of these spikelets had the potential to yield seedlings. On average, plants expanded vegetatively 0.15 m yr⁻¹. Tiller density within the center of a clone decreased with age, possibly leading to a “dead center” found among some LSIGs. Rhizome establishment increased with weight, ranging from 0 to 42%. Survival was low, 24%, for first-year plants but quickly climbed to an asymptote of 98% survival for 4-yr-old plants. Our results suggest that efforts should be made to eradicate plants that escape biomass production fields within a year of establishment, before the onset of high survival. Future work is needed to determine what types of natural and anthropogenic disturbances can fragment rhizomes, leading to regeneration.

Nomenclature: Miscanthus × giganteus Greef et Deu ex Hodkinson et Renvoize, Giant miscanthus.

Management Implications: As demand for alternative fuels continues to grow, biofuels from herbaceous perennial biomass crops will be an important component of the U.S. energy portfolio. Before growing such crops across wide areas, it is important to understand both the risks and benefits of such an action. Because of similarities between many bioenergy crops and plant invaders, and the onset of new projects to scale-up biomass production rapidly in the Midwest, it is important to understand the potential behavior of candidate species for bioenergy production outside of cultivation. We quantified the age-specific demographic rates of the Illinois clone variety of Miscanthus × giganteus, a leading bioenergy crop in the Midwest. All M. × giganteus demographic rates were strongly dependent on plant age. Spikelet production was low in first-year plants and rose to more than 180,000 spikelets per plant by the fourth year, but none yielded seedlings. Vegetative expansion of M. × giganteus was moderate compared with other large grasses; 4-yr-old plants were observed to have an average vegetative creep radius of 0.15 m yr⁻¹. Survival of first-year plants was low, 24%, but quickly rose to 97% by the fourth year. The lack of viable seed production and slow vegetative expansion in M. × giganteus observed in this study suggest that the sterile clonal cultivar for this bioenergy crop has low invasive potential in Illinois, assuming that the tough rhizomes are not fragmented by disturbance. However, several similarities between