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Herbicides are the primary method used to control exotic, invasive plants. This study evaluated restoration efforts applied to grasslands dominated by an invasive plant, sulfur cinquefoil, 6 yr after treatments. Of the five herbicides we evaluated, picloram continued to provide the best control of sulfur cinquefoil over 6 yr. We found the timing of picloram applications to be important to the native forb community. Plots with picloram applied in the fall had greater native forb cover. However, without the addition of native perennial grass seeds, the sites became dominated by exotic grasses. Seeding resulted in a 20% decrease in exotic grass cover. Successful establishment of native perennial grasses was not apparent until 6 yr after seeding. Our study found integrating herbicide application and the addition of native grass seed to be an effective grassland restoration strategy, at least in the case where livestock are excluded.
Nomenclature: Picloram, sulfur cinquefoil, Potentilla recta L.
If the objective is simply to reduce abundance of an invasive plant for temporary control, one application of the proper herbicide may suffice. This study found that a one-time application of picloram effectively reduced sulfur cinquefoil for 6 yr.
Although herbicide applications were successful at reducing sulfur cinquefoil abundance, they were was not successful at reducing overall exotic plant cover, as other exotics species, primarily annual grasses generally replaced sulfur cinquefoil.
Simply seeding native perennial grasses into plots was effective at reducing sulfur cinquefoil abundance, and herbicide application combined with native grasses seeding provided best control of sulfur cinquefoil while at the same time increasing native species abundance.
Seeding success appeared poor in the first 1 to 3 yr because of the slow growth and small size of native perennial grass species in the years immediately following seeding. Therefore, longer-term monitoring is needed to evaluate success of seeding efforts.
A species' successful invasion into a new site depends on its ability to persist in the local environment. An experiment was conducted to examine the response of giant reed to intermittent periods of shading for 2 yr. Results indicate that giant reed persisted when exposed to significant shading (i.e., 90% reduction of full sun) and that shading also caused changes in a number of plant characteristics, such as stem height, internode length, leaf nitrogen, leaf chlorophyll content, specific leaf area, total leaf area per plant, and leaf life span. Estimates of leaf photosynthetic rates did not differ across shade levels. Giant reed's ability to persist and grow under intermittent low-light conditions implies that plants would be poised to take advantage of sun flecks and disturbances that create gaps within the resident plant community.
Nomenclature: Giant reed, Arundo donax L. ABKDO.
Management Implications: Giant reed (Arundo donax L.) occurs throughout the southern half of the United States, from California to Maryland. It is considered an invasive plant in some parts of this range but not others. To understand how giant reed successfully invades new habitats, experiments were performed to determine the effect of shading on several aspects of its growth. Giant reed tolerated significant shading (i.e., 90% reduction of full sun) and that shading also caused changes in a number of plant characteristics, such as stem height, internode length, leaf nitrogen, leaf chlorophyll content, specific leaf weight, total leaf area per plant, and leaf life span. Giant reed's ability to persist and grow under intermittent, low-light conditions implies that the plants would be poised to take advantage of sun flecks and disturbances that create gaps within the resident plant community.
Garlic mustard is an invasive, exotic herb that is now widespread in North America. Recent research has shown that garlic mustard exudes biochemical compounds that inhibit the growth of entomopathogenic fungi. We investigated how the removal of garlic mustard would affect the abundance of entomopathogenic fungi in forest soils in eastern New York. Using a standard bioassay, we compared the abundance of entomopathogenic fungi in soil with and without garlic mustard both before and 45 d after garlic mustard had been experimentally removed. In soil from which garlic mustard had been experimentally removed 45 d earlier, the abundance of entomopathogenic fungi was restored to levels found in soil with no history of garlic mustard. These results suggest it is possible to increase the abundance of entomopathogenic fungi in the soil in a short time by eradicating garlic mustard plants from an invaded area. Recolonization by entomopathogenic fungi could be beneficial to humans if it increases the mortality of arthropods that are vectors of infectious disease, such as blacklegged ticks, but harmful if it increases the mortality of arthropods that provide valuable ecosystem services, such as bees and ants.
Management Implications: Garlic mustard is known to have an inhibitory effect on arbuscular mycorrhizal fungi in the soil. However, less is known about the herb's effects on underground entomopathogenic fungi, or more specifically, about how the soil microbiota responds to the removal of the plant. We investigated how long it takes for the soil to recover natural levels of entomopathogenic fungi once garlic mustard is removed. In this study, we sampled soil for entomopathogenic fungi in areas invaded by garlic mustard and areas free of garlic mustard. We then removed garlic mustard plants from forest plots and sampled the soil again 45 d later. The abundance of entomopathogenic fungi in all areas that had garlic mustard removed from them increased during the 1.5-mo period and reached even greater levels compared with areas which had no history of garlic mustard. Soil disturbance alone did not have an effect on the abundance of entomopathogenic fungi. The ability of entomopathogens to recover shortly after garlic mustard removal can benefit humans because the number of disease-carrying arthropod vectors (e.g., blacklegged ticks) may decline as a result. However, it can also be harmful to us because the number of arthropods that provide valuable ecosystem services (e.g., ants, bees) may also diminish. We conclude that garlic mustard removal might be one of the ways to increase the abundance of entomopathogenic fungi in the soil during a short time and, therefore, a way to restore a natural mechanism for arthropod population control.
Widely-held, untested assumptions in many prairies are that high-intensity fires conducted during droughts will decrease native herbaceous species richness and lead to rapid invasions by alien species. We compared native and exotic herbaceous species richness and aboveground herbaceous biomass one year following the application of high-intensity growing-season fires in Texas coastal prairie. Fires were conducted in June 2008, when precipitation was 96% below the long-term monthly average, at the end of a severe 5-mo drought, resulting in high fire intensities within treatment units. Native forb species richness was greater in burned than unburned areas. In contrast, species richness of native grasses, exotic forbs, and the frequency of King Ranch (KR) bluestem [Bothriochloa ischaemum (L.) Keng.] did not significantly differ between burned and unburned treatments. The potential to use prescribed extreme fire to maintain native herbaceous species richness while not increasing KR bluestem provides preliminary evidence that growing season fires conducted during droughts can be a viable management strategy in coastal prairies.
Nomenclature: King Ranch bluestem, Bothriochloa ischaemum (L.) Keng.
Management Implications: King Ranch (KR) bluestem is an invasive C4 grass introduced from Europe and Asia that has led to declines in the richness of plant, avian, and arthropod species in prairie ecosystems. Managers in Texas Gulf coastal prairies are concerned that using fire to manage woody plant encroachment will trigger rapid invasion by KR bluestem and decrease native herbaceous species richness. The objective of this study was to compare the rapid responses of native and exotic herbaceous species in burned and unburned treatments in a huisache–mesquite-invaded coastal tallgrass prairie. In this study, prescribed fires were conducted in the growing season with special permission during a county-mandated burn ban initiated by a severe, prolonged drought that caused considerable grass curing and the potential for high fire intensities (referred to as prescribed extreme fire). Our results counter previously unsubstantiated assumptions in coastal tallgrass prairies and show that prescribed extreme fire increased native forb species richness while maintaining the richness of native grasses, and not increasing King Ranch bluestem significantly. Because other research has shown burning in the dormant season is more likely to facilitate rapid invasion by KR bluestem, prescribed extreme fires in the growing season might be more effective at maintaining herbaceous species richness in coastal prairies prone to KR bluestem invasion.
Prickly nightshades are troublesome weeds of natural habitats, pastures, feedlots, right-of-ways, and croplands. Native and nonnative invasive weedy species of prickly nightshades were compared to determine growth, development, and morphological differences. Six (Solanum bahamense, Solanum capsicoides, Solanum carolinense, Solanum dimidiatum, Solanum donianum, and Solanum pumilum) of the 18 species of prickly nightshades studied are native to the US. Two species, Solanum citrullifolium and Solanum rostratum, are annuals; the others are perennials or are short lived perennials or annuals in northern extremes of their range in North America. Tables were developed from new and existing data to differentiate vegetative and reproductive characteristics among 18 species of prickly nightshade found in the southeastern US. In greenhouse experiments, average plant height ranged from 24 and 26 cm (9.45 and 10.24 inch) for S. carolinense and Solanum jamaicense, respectively, to 100 and 105 cm for Solanum tampicense and Solanum sisymbriifolium, respectively at 10 wk after emergence (WAE). By 10 WAE, the average number of leaves per plant ranged from < 10 for S. carolinense and Solanum torvum to > 40 leaves/plant for S. rostratum and S. dimidiatum. Average number of nodes/plant main stem ranged from 11, 12, and 14 nodes in S. jamaicense, S. torvum, and S. carolinense, respectively, to 54 nodes in S. rostratum. Average plant dry weights were collected at 10 WAE and were greatest for Solanum mammosum and (> 17 g/plant) (0.6001 oz/plant) and least for S. carolinense (1 g/plant). Based on these data, nightshade growth rate and dry weight were variable among some species and variability may be a result of phenology and life cycles, annual or perennial. Plants of S. rostratum, an annual, were relatively tall and produced high number of nodes and leaves and had the shortest period from emergence to flower among the prickly nightshades evaluated.
Water hyacinth is among the most widespread invasive plants worldwide; however, its effects on waterbirds are largely undocumented. We monitored site use by waterbirds at Lake Chapala, the largest lake in Mexico and recently designated Ramsar site, to evaluate the potential influence of water hyacinth cover on species composition and aggregate measures of the waterbird community, including waterbird density, species richness, and Simpson's index of diversity. We examined the response of waterbirds to changes in percent water hyacinth cover at 22 independent sites around the lake during six study seasons from May 2006 to February 2008. We found little evidence to suggest that percent water hyacinth cover affected aggregate community measures; however, multivariate analysis of relative species composition suggested that water hyacinth cover corresponded with seasonal species composition (Canonical Correspondence r = 0.66, P = 0.007) when seasonal site cover averaged 17.7 ± 4.67% (winter 2007). Several migratory species were not observed during this season, which could suggest that some small-bodied migratory species avoided Lake Chapala during the winter of high water hyacinth cover. We suspect that observed changes in the waterbird community are in response to species-specific tolerances for water hyacinth and indirect abiotic and biotic effects of its presence (e.g., invertebrate and fish composition).
Nomenclature: Glyphosate, water hyacinth, Eichhornia crassipes (Mart.) Solms.
Management Implications: Water hyacinth is among the most widespread aquatic invasive plants in the world. Yet despite its global recognition, our understanding of how nonnative ecosystems and animal communities, specifically waterbirds, respond to the establishment of this species is relatively weak, which hinders our ability to choose the best potential management strategy. The waterbird community comprises herbivores as well as secondary and tertiary consumers; therefore, changes in community composition and abundance can greatly affect food web structure and energy flows within an affected ecosystem. In addition to their ecological role, waterbirds are the source of cultural and economic values in many regions. A shift in waterbird community composition resulting from water hyacinth establishment has the potential to enhance or reduce socioeconomic values associated with freshwater ecosystems. By understanding how waterbirds respond to water hyacinth, field practitioners (e.g., researchers, managers, and decision-makers) will be better informed when developing research and management priorities. Our results suggest that water hyacinth cover had little effect on aggregate measures of the waterbird community at Lake Chapala, including waterbird density, species richness, and diversity (Simpson's index of diversity), but that some small-bodied, migratory species might avoid sites and/or entire ecosystems with water hyacinth. Although the decision to control water hyacinth and other invasive plants is surely based on a wide assortment of socioeconomic objectives and desired benefits (i.e., protection of municipal water supply), we believe that the ecological response to nonnative plants and potential control methods should be evaluated. Our results suggest a benign relationship between water hyacinth and waterbirds, with the potential exception of some small-bodied, migratory species. Understanding the waterbird response to water hyacinth provides critical pieces in the ecological and socioeconomic puzzle of invasive plant science and management and can help managers evaluate the benefits and costs of control more holistically.
The Asian grass Miscanthus sinensis (Poaceae) is being considered for use as a bioenergy crop in the U.S. Corn Belt. Originally introduced to the United States for ornamental plantings, it escaped, forming invasive populations. The concern is that naturalized M. sinensis populations have evolved shade tolerance. We tested the hypothesis that seedlings from within the invasive U.S. range of M. sinensis would display traits associated with shade tolerance, namely increased area for light capture and phenotypic plasticity, compared with seedlings from the native Japanese populations. In a common garden experiment, seedlings of 80 half-sib maternal lines were grown from the native range (Japan) and 60 half-sib maternal lines from the invasive range (U.S.) under four light levels. Seedling leaf area, leaf size, growth, and biomass allocation were measured on the resulting seedlings after 12 wk. Seedlings from both regions responded strongly to the light gradient. High light conditions resulted in seedlings with greater leaf area, larger leaves, and a shift to greater belowground biomass investment, compared with shaded seedlings. Japanese seedlings produced more biomass and total leaf area than U.S. seedlings across all light levels. Generally, U.S. and Japanese seedlings allocated a similar amount of biomass to foliage and equal leaf area per leaf mass. Subtle differences in light response by region were observed for total leaf area, mass, growth, and leaf size. U.S. seedlings had slightly higher plasticity for total mass and leaf area but lower plasticity for measures of biomass allocation and leaf traits compared with Japanese seedlings. Our results do not provide general support for the hypothesis of increased M. sinensis shade tolerance within its introduced U.S. range compared with native Japanese populations.
Management Implications: Eulaliagrass (Miscanthus sinensis), an Asian species under consideration for biomass production in the Midwest, has escaped ornamental plantings in the United States to form naturalized populations. Evidence suggests that U.S. populations are able to tolerate relatively shady conditions, but it is unclear whether U.S. populations have greater shade tolerance than the relatively shade-intolerant populations within the species' native range in Asia. Increased shade tolerance could result in a broader range of invaded light environments within the introduced range of M. sinensis. However, results from our common garden experiment do not support the hypothesis of increased shade tolerance in introduced U.S. populations compared with seedlings from native Asian populations. Our results do demonstrate that for both U.S. and Japanese populations under low light conditions, M. sinensis seeds germinate and seedlings gain mass and leaf area; therefore, land managers should carefully monitor or eradicate M. sinensis within these habitats.
We investigated nonindustrial private forest (NIPF) owners' invasive plant risk perceptions and mitigation practices using statistical analysis of mail survey data and qualitative analysis of interview data collected in Oregon's ponderosa pine zone. We found that 52% of the survey sample was aware of invasive plant species considered problematic by local natural resource professionals; 70% was concerned about these species; and 46% had treated invasive plants on their parcels. Owners' perceptions of invasive plant risks fell along a spectrum ranging from a lack of awareness or concern, to the view that invasive plant infestations have discrete causes and controllable consequences, to the perception that incursions by invasive plants have diffuse causes and uncontrollable effects. Being aware or concerned about invasive plant species were predictors (p ≤ 0.001) of whether owners treat their parcels to control invasive plants. Holding wildlife habitat and/or biodiversity as an important forest management goal was also a predictor (p ≤ 0.08) of whether owners treated their parcels to control invasive plants. Some owners were sensitive to the risks of invasive plant infestations from nearby properties, and a surprisingly high percentage of respondents had cooperated with others in forest management activities previously. Our findings suggest three approaches to increasing the frequency of invasive plant mitigation by NIPF owners that hold promise: (1) raising awareness and concern about invasive plants and their impacts on forest management goals that owners care about, such as wildlife habitat and/or biodiversity; (2) providing assistance to help owners mitigate invasive plants they feel unable to control; and (3) engaging owners in coordinated efforts across ownership boundaries to address invasive plant risks.
Nomenclature: Ponderosa pine, Pinus ponderosa Dougl. ex C. Lawson.
Management Implications: Understanding the invasive plant risk perceptions and mitigation practices of nonindustrial private forest (NIPF) owners in the U.S. is important because they control a substantial portion of the forestland that invasive plants can affect, and their lands are generally located in areas where transmission via waterways, roads, game, and livestock is common. Our study found that NIPF owners in Oregon's ponderosa pine zone have the potential to help mitigate invasive plants: 52% of survey respondents were aware of invasive plants on their parcels, 70% were concerned about them, and 46% had worked to control them. Awareness and concern explained the likelihood of mitigation, as did holding wildlife habitat and/or biodiversity as an important forest management goal.
Using education and assistance programs to increase awareness, concern, and capacity among NIPF owners may be a worthwhile strategy for increasing the frequency of mitigation efforts. Understanding owners' priority management goals and how invasive species affect them, and targeting such programs at owners whose values are at risk, such as those with wildlife habitat and/or biodiversity goals, may be especially effective. Owners whose primary residence is located on their forest parcels are more likely to treat invasive plants on those parcels. Thus, outreach to nonresidents could be an opportunity for increasing invasive plant mitigation activities. Research to better understand why owners who are aware of and concerned about invasive plants do not attempt to control them may reveal the unique constraints faced by owners having different management goals, which is important for policy development. Finally, engaging owners in cross-boundary, landscape-scale efforts at invasive plant control is important and may be well received.
Control options for the nonnative common mullein are of increasing interest to land managers in the west. Common mullein is a prolific seed producer, with a single plant able to produce well over 100,000 seeds. We found that mechanical control of common mullein before mature seed capsules developed along the raceme significantly reduced viable seed production. Seeds from immature capsules had very low viability (early reproductive stage = 0.08%, 95% CI = 0.06%, 0.67%; mid reproductive stage = 1.52%, 95% CI = 0.49%, 3.11%). This information allows managers to time their management efforts so that they can reduce the amount of plant material that must be disposed of in order to control the spread of common mullein seeds.
Nomenclature: Common mullein, Verbascum thapsus L.
Management Implications: Common mullein is an increasing priority for control in natural areas of the western United States. This biennial species produces a large number of seeds that can lay dormant in the seed bank for at least 100 years. Managers detach reproductive flowering stalks in an effort to limit seed production but then must remove the reproductive portion from the field or risk adding seeds to the seed bank. At the request of land managers, the relationship between plant phenology and seed viability was studied to facilitate timing of the removal of reproductive stalks to minimize additions of common mullein seeds to the seed bank.
This study provides specific information about the timing of removal of the reproductive inflorescences of common mullein to best limit seed set. If management efforts are carried out before seedpods turn brown on the stalk, seeds will not develop and the risk of adding additional seed to the seed bank is diminished. However, if seedpods have begun to mature, land managers should take precautionary measures by removal of stalks from the field, followed by proper disposal.