Additive effects of aboveground and belowground herbivores on the dominance of spotted knapweed (Centaurea stoebe)

Spotted knapweed (Centaurea stoebe) is found in over 3 million ha of rangeland and forests across North America, and evidence supporting the use of biological control as a regional method to reduce infestations and their associated impacts remains inconclusive. Several species of insects have been reported to reduce plant densities in some areas; however, rigorous studies that test combinations of these species and the influence of resource availability are lacking. We examined the singular and combined effects of herbivory by a root weevil (Cyphocleonus achates) and a flower head weevil (Larinus minutus) on the growth and flower production of C. stoebe. We also manipulated soil resource fertility as an additional factor that could explain the outcomes of contradictory biological control herbivore effects on C. stoebe. In a greenhouse study, herbivory by C. achates decreased flower production for plants across all resource environments. In a caged common garden study, the negative effects of herbivory also did not interact with soil nutrient status. However, the presence of plant competition further decreased knapweed growth, and the negative effects of concurrent herbivory by C. achates and L. minutus on plant biomass and flower production were additive. Derived within the context of variable levels of soil nutrient availability and competing vegetation, these results support the cumulative stress hypothesis and the contention that combined above- and belowground herbivory can reduce spotted knapweed densities and reduce the ecological and economic impacts of this species in rangelands of western North America.     

Testing biological control agent compatibility: Cyphocleonus achates and Larinus minutus on diffuse knapweed

While weed biological control success is typically achieved with one agent, multiple agents are invariably introduced. Biological control agents that share a host–plant may interact either directly or indirectly through changes in host–plant quality. Negative interactions could reduce the impacts of the agents on the density of their host–plant while positive interactions (facilitation) could improve biological control success.In the Okanagan Valley of British Columbia, Canada, initial declines in the invasive rangeland weed, diffuse knapweed (Centaurea diffusa) were attributed to the introduction of the weevil Larinus minutus. A second weevil, Cyphocleonus achates has recently become common on diffuse knapweed. We sought to determine if the recent increase of C. achates could threaten the success of L. minutus. We considered whether L. minutus colonisation or performance remained the same when C. achates was present, and whether the two agents acted independently to reduce plant performance.Neither changes in colonisation rates nor competitive interactions were apparent between C. achates and L. minutus. Both insects reduced plant performance and, for all metrics, the reduction in plant performance by one species was independent of the second. The two agents appear to be compatible and both should contribute to the control of diffuse knapweed. To assess how biological control agents interact requires understanding both their competitive interactions and their joint effects on the shared host.     

How do biological control and hybridization affect enemy escape?

Two mechanisms often linked with plant invasions are escape from enemies and hybridization. Classical biological control aims to reverse enemy escape and impose top-down population control. However, hybridization has the potential to alter interactions with enemies and thus affect biological control. We examined how introductions of biological control agents affect enemy escape by comparing specialist enemy loads between the native and introduced ranges of two noxious weeds (spotted and diffuse knapweed; Centaurea stoebe L. and C. diffusa Lam.) that have been the targets of an extensive biological control program. Hybrids between spotted and diffuse knapweed are often found within diffuse knapweed sites in North America, so we also compared enemy loads on plants that appeared morphologically like diffuse knapweed and hybrids. Finally, we tested the preference for diffuse knapweed, hybrids, and spotted knapweed of one of the agents thought to be instrumental in control of diffuse knapweed (Larinus minutus; Curculionidae). In North America spotted knapweed has largely escaped its root herbivores, while seedhead herbivore loads are comparable in the introduced and native ranges. Diffuse knapweed exhibited seedhead herbivore loads five times higher in the introduced compared to native range. While this pattern of seedhead herbivory is expected with successful biological control, increased loads of specialist insect herbivores in the introduced range have rarely been reported in the literature. This finding may partially explain the better population control of diffuse vs. spotted knapweed. Within North American diffuse knapweed sites, typical diffuse knapweed and hybrid plants carried similar herbivore loads. However, in paired feedings trials, the specialist L. minutus demonstrated a preference for newly created artificial hybrids over North American diffuse knapweed and for European diploid spotted knapweed over North American tetraploid spotted knapweed. Overall though, hybridization does not appear to disrupt biological control in this system.     

Role of plant phenology in mediating interactions between two biological control agents for spotted knapweed

The role of spotted knapweed phenology on the attack rate of two seed-head insects Urophora affinis and Larinus minutus was assessed in a series of field studies at four study sites in south-eastern British Columbia, Canada. Slow or later developing knapweed plants had more seed heads that contained only single or multiple U. affinis whereas early or faster developing plants had more seed heads containing L. minutus alone or in combination with U. affinis. L. minutus did not distinguish between seed heads with or without U. affinis larvae when laying eggs. However, seed heads with multiple U. affinis present, produced fewer L. minutus adults than expected. The probability of single or multiple U. affinis galls being present increased with seed-head diameter but was not affected by seed-head height. Attack by L. minutus increased with seed-head diameters >5 mm and was lower at plant heights above 50 cm. These results demonstrate two mechanisms that enable U. affinis to successfully coexist with L. minutus: differences between the species in their response to the developmental phenology of knapweed heads, and increased survivorship of U. affinis in heads with multiple U. affinis galls through niche interference competition. These mechanisms provide a possible explanation for the persistence of U. affinis populations on spotted knapweed, in spite of high levels of within seed-head mortality that have been observed with increasing L. minutus populations.     

Interactions and effects of multiple biological control insects on diffuse and spotted knapweed in the Front Range of Colorado

Abundances and interactions among biological control insects and their effects on target invasive plants were monitored within the flower heads and roots of diffuse knapweed, Centaurea diffusa, and in spotted knapweed, Centaurea stoebe, along the Colorado Front Range. Flower weevils, (Larinus species) and root-feeders (Cyphocleonus achates and Sphenoptera jugoslavica) were released on knapweed that already supported biological control gall flies (Urophora species). At a single monitoring site, seed production by C. diffusa declined from 4400 seeds m⁻² in 1997 to zero seeds m⁻² on the monitoring sites in 2006, while the flowering stem density of C. diffusa declined from a peak of almost 30 stems m⁻² in 2000 to zero stems m⁻² in 2006. The average abundance of Urophora and Larinus in flower heads fluctuated independently during the 2001–2006 interval, while the relative abundance of C. achates and S. jugoslavica in roots exhibited a weak inverse relationship that appeared driven by climate effects. The relative abundance of insects on a population of C. stoebe was monitored for five years as Larinus species and C. achates became established on spotted knapweed that already supported Urophora species. Spotted knapweed seed production on our monitoring site declined from 4600 seeds m⁻² in 2003 to zero seeds m⁻² in 2006. Unlike C. diffusa, substantial numbers of rosettes of C. stoebe remained present. Larinus consumed almost all Urophora encountered in C. diffusa, and consumed about 40% of the Urophora in co-infested flower heads of C. stoebe (ca. 10–15% of the total Urophora population). No negative correlations between the relative densities of flower head and root-feeding insects were observed. The effects of these insects on target plants have produced results consistent with the ‘cumulative stress hypothesis’ for biological control of Centaurea species.     

Physiological and growth responses of Centaurea maculosa (Asteraceae) to root herbivory under varying levels of interspecific plant competition and soil nitrogen availability

Summary Centaurea maculosa seedlings were grown in pots to study the effects of root herbivory by Agapeta zoegana L. (Lep.: Cochylidae) and Cyphocleonus achates Fahr. (Col.: Curculionidae), grass competition and nitrogen shortage (each present or absent), using a full factorial design. The aims of the study were to analyse the impact of root herbivory on plant growth, resource allocation and physiological processes, and to test if these plant responses to herbivory were influenced by plant competition and nitrogen availability. The two root herbivores differed markedly in their impact on plant growth. While feeding by the moth A. zoegana in the root cortex had no effect on shoot and root mass, feeding by the weevil C. achates in the central vascular tissue greatly reduced shoot mass, but not root mass, leading to a reduced shoot/root ratio. The absence of significant effects of the two herbivores on root biomass, despite considerable consumption, indicates that compensatory root growth occurred. Competition with grass affected plant growth more than herbivory and nutrient status, resulting in reduced shoot and root growth, and number of leaves. Nitrogen shortage did not affect plant growth directly but greatly influenced the compensatory capacity of Centaurea maculosa to root herbivory. Under high nitrogen conditions, shoot biomass of plants infested by the weevil was reduced by 30% compared with uninfested plants. However, under poor nitrogen conditions a 63% reduction was observed compared with corresponding controls. Root herbivory was the most important stress factor affecting plant physiology. Besides a relative increase in biomass allocation to the roots, infested plants also showed a significant increase in nitrogen concentration in the roots and a concomitant reduction in leaf nitrogen concentration, reflecting a redirection of the nitrogen to the stronger sink. The level of fructans was greatly reduced in the roots after herbivore feeding. This is thought to be a consequence of their mobilisation to support compensatory root growth. A preliminary model linking the effects of these root herbivores to the physiological processes of C. maculosa is presented.     

Influence of seed head-attacking biological control agents on spotted knapweed reproductive potential in western Montana over a 30-year period

Five insect biological control agents that attack flower heads of spotted knapweed, Centaurea stoebe L. subsp. micranthos (Gugler) Hayek, became established in western Montana between 1973 and 1992. In a controlled field experiment in 2006, seed-head insects reduced spotted knapweed seed production per seed head by 84.4%. The seed production at two sites in western Montana where these biological control agents were well established was 91.6-93.8% lower in 2004-2005 than 1974-1975, whereas the number of seed heads per square meter was 70.7% lower, and the reproductive potential (seeds/m(2)) was 95.9-99.0% lower. The average seed bank in 2005 at four sites containing robust spotted knapweed populations was 281 seeds/m(2) compared with 19 seeds/m(2) at four sites where knapweed density has declined. Seed bank densities were much higher at sites in central Montana (4,218 seeds/m(2)), where the insects have been established for a shorter period. Urophora affinis Frauenfeld was the most abundant species at eight study sites, infesting 66.7% of the seed heads, followed by a 47.3% infestation by Larinus minutus Gyllenhal and L. obtusus Gyllenhal. From 1974 to 1985, Urophora spp. apparently reduced the number of seeds per seed head by 34.5-46.9%; the addition of Larinus spp. further reduced seed numbers 84.2-90.5% by 2005. Path analysis indicated that both Larinus spp. and U. affinis contributed significantly to reduction of seed production over the 30-yr period. Spotted knapweed density may not decrease significantly until the seed bank falls below a critical threshold.     

Influence of Nutrient Availability on the Interaction Between Spotted Knapweed and Bluebunch Wheatgrass

Centaurea maculosa (Lam.) (spotted knapweed) reduces wildlife and livestock habitat biodiversity and increases erosion. Nutrient availability to plants may be used to accelerate succession away from spotted knapweed. Early‐successional plant communities often have high nutrient availability, whereas late‐successional communities are often found on lower nutrient soils. We hypothesized that removal of nutrients would change the competitive advantage from spotted knapweed to Pseudoroegneria spicatum (bluebunch wheatgrass) (late seral). In two addition series matrices, background densities of Secale cereale (annual rye) and Elymus elimoides (bottlebrush squirreltail) (3,000 seeds/m²) were used to remove nutrients from the soil. In another set of addition series matrices, nitrogen (33 kg/ha) or phosphorus (33 kg/ha) were added to the soil. Nutrient analysis of soil and vegetation indicated that annual rye and bottlebrush squirreltail reduced nutrient availability in soils. In another matrix, neither a background density nor nutrients were added. Data were fit into Watkinson's curvilinear model to determine the competitive relationship between bluebunch wheatgrass and spotted knapweed. This allowed comparison of the equivalence ratios (C) generated from each addition series. The C parameters are the per‐plant equivalent of bluebunch wheatgrass or spotted knapweed and can be interpreted as the ratio of intra‐to‐interspecific competition. The C parameters are also the equivalence ratio of the number of spotted knapweed it takes to have equivalent effect on bluebunch wheatgrass or the number of bluebunch wheatgrass having the equivalent effect on spotted knapweed. Without nutrient manipulation, spotted knapweed was more competitive than bluebunch wheatgrass. The C for bluebunch wheatgrass was 0.17, indicating that 0.17 knapweed plants were competitively equivalent to one wheatgrass. Annual rye changed the competitive balance in favor of bluebunch wheatgrass (C = 9.9). Addition of nitrogen, phosphorus, or the mid‐seral species did not change the competitive relationship between the two species. This preliminary study suggests that succession from spotted knapweed to late‐seral bluebunch wheatgrass community may be accelerated by altering resource availability.     

Herbivory and novel weapons: no evidence for enhanced competitive ability or allelopathy induction of <Emphasis Type="Italic">Centaurea diffusa</Emphasis> by biological controls

Biological control of weeds by arthropod herbivores is thought to work by reducing the competitive ability of the weed relative to the surrounding vegetation. However, the assumption that herbivory reduces plant competitive ability has not been tested in most biological control systems, and counter to expectation, recent research on the impact of biological control agents on invasive Centaurea species suggests that this genus may respond to herbivory by increased competitive ability through enhanced plant re-growth and/or by inducing increased production of phytotoxic allelochemicals. We examined the impact of two biological control agents of the invasive plant diffuse knapweed (C. diffusa) to see if feeding by either of these insects would enhance the plant’s competitive ability or allelochemical output. Sub-lethal herbivory by either of the biological control agents significantly reduced knapweed performance when the plant was grown in competition with either of two native species. Competition with knapweed significantly reduced the performance of both native species (Artemisia frigida and Bouteloua gracilis), and herbivory by one of the biocontrol agents resulted in a small but significant increase in both native species’ performance. Diffuse knapweed’s putative allelochemical 8-hydroxyquinoline was not detected in experimental or field collected soils from knapweed-infested sites. In contrast to other studies on the impacts of biological control on other Centaurea species, these data support the premise that biological control agents may reduce invading plant competitive ability. We find no evidence for diffuse knapweed allelopathy mediated by 8-hydroxyquinoline or enhanced allelopathy in response to herbivory by biological control agents.     

Revised methods for the mass-rearing of the spotted knapweed biological control agent,Cyphocleonus achates (Coleoptera: Curculionidae), in field corrals

Cyphocleonus achates (Fahraeus), a root-feeding weevil introduced from Eurasia, is an effective biological control agent against spotted knapweed, Centaurea stoebe L. ssp. micranthos. Because C. achates is univoltine and does not fly, distribution of the weevil has been slow. To hasten the weevil's distribution, a rearing effort using field corrals was initiated at a facility in Corvallis, Montana. Procedures for mass-rearing the weevil in field corrals are described, with an emphasis on improvements over earlier methods. The described field-corral approach is effective and appropriate for producing C. achates for distribution in the western United States.     

A knapweed biological control perspective

Diffuse and spotted knapweed (Centaurea diffusa Lam. and C. stoebe micranthos (Gugler) Hayek) are Eurasian plants that devastate dry and mesic North American grasslands. They have a mutualistic association with arbuscular mycorrhizal fungal (AMF) phylotypes with hyphal links to nearby plants and a nutrient flux to the strongest sink, usually knapweed. They displace many AMF beneficial to grass and affect knapweed nutrient allocation, biology, knapweed insects and probably root necrosis and emergence of ant buried seed. AMF determined nutrient root or shoot allocation determines nutrient shoot and root allocation and the benefit to root or seed-head insect species and whether C. diffusa is an annual–biannual or a semelparous perennial needing 5 or more years to flower. Both knapweeds do well without its AMF phylotypes without competition in fertile soil. In grass in Eurasia, they have a community of seven seed-head species segregated by head development stage. Prolonged seed dormancy buffered knapweed decline that resulted in release of a surfeit seed-head species. The presence of an eliasome on the seed and vigorous seedling clumps suggests burial by myrmecochorous ants with AMF supplied carbon supporting their growth. The root species community is segregated by habitat, climate, root part, and size. With larval induced compensatory growth and AMF nutrient sharing, the growth of plants with and without a larva was the same. On feeding completion, a nutrient out flux from the attacked plants reduced growth; but without killing. This needs a dual species or a repeated single species attack. Root species packing increases knapweed utilization; but the four approved species are insufficient for maximum utilization. Two additions are suggested. The aim of the paper is to provide enough understanding of the AMF and its plant and insect interactions to facilitate knapweed biological control and avoid past mistakes.     

Growth and development of the knapweed root weevil,Cyphocleonus achates,on a meridic larval diet

Cyphocleonus achates, the knapweed weevil, is an effective biological control agent of the invasive weed, Centaurea maculosa Lam. A meridic diet was developed and tested for the rearing of the larval stage of this insect. Using this diet, C. achates was reared for over three generations, with the adults being offered knapweed plants for feeding and oviposition in greenhouse conditions. Slight or no differences were seen between insects reared on a standard meridic diet formulation and one containing knapweed tissues. The following life history parameters were monitored over the three generations: percent egg hatch (ranging from 42.9 to 59.1%), time to egg hatch (20.0–23.2 days), time to adult emergence (52.0–54.1 days), adult weights 3 days post-eclosion (101.9–117.0 mg), percent adult emergence (48.3–58.6%), and percent mortality/deformity in the different stages (with mortality occurring primarily in the early larval stages). Additionally, a study involving low temperature and short day conditions suggested that C. achates could be maintained for longer periods of time in larval diet cells when placed in growth-retarding conditions, although percent adult emergence was lower. External morphology was also studied in order to distinguish between the sexes to ensure that each adult cage had a similar ratio of females to males. Abdominal features were found to be the most dependable characteristics for use when determining the sex of adult C. achates.     

Plant community changes after the reduction of an invasive rangeland weed, diffuse knapweed, Centaurea diffusa

The expected outcome of weed control in natural systems is that the decline of a dominant weed will result in an increase in diversity of the plant community but this has seldom been tested. Here we evaluate the response of the plant community following the decline of diffuse knapweed (Centaurea diffusa) in six different pastures at White Lake, BC, Canada over five years. This period followed the establishment, spread and high levels of attack by the introduced European weevil, Larinus minutus, as part of a biological control program. Knapweed declined immediately before and during the study period, but, contrary to expectations, the species richness and diversity of the rangeland plant community did not increase. The absolute cover of native and introduced forbs and grasses increased following knapweed decline, but only the introduced grasses showed a consistent increase in cover relative to the other life-forms. However, unlike in other studies, the native plants dominated the study site. We conclude that the changes in plant communities following successful biological control are variable among programs and that the impact of replacement species must be evaluated in assessing the success of ecological restoration programs that use biological control to manage an undesirable weed.     

Phenology and dispersal of Larinus minutus Gyllenhal and Larinus obtusus Gyllenhal (Coleoptera: Curculionidae), two biological control agents of Centaurea stoebe ssp. micranthos (spotted knapweed) in Michigan

Centaurea stoebe L. ssp. micranthos (Gugler) (spotted knapweed) is an invasive plant that has been the target of classical biological control in North America for more than four decades. The seedhead-feeding weevils Larinus minutus Gyllenhal and Larinus obtusus Gyllenhal (Coleoptera: Curculionidae) are two of the most-widely released C. stoebe control agents, and have more recently been introduced into the eastern US. While there have been many studies focusing on their ability to impact C. stoebe in the western US and Canada, there have been few studies from eastern North America, and basic knowledge of important aspects of their biology is lacking. Here we report on the phenology and dispersal of L. minutus and L. obtusus in Michigan. We regularly sampled two established Larinus spp. populations in southern Michigan in 2012 and 2013, and found that while adult abundance fluctuates during the growing season, they remained at easily detectable levels from mid-June through the end of August. We also used previously established populations of L. minutus and L. obtusus released in 1996 (n = 1), 2007 (n = 2), and 2010 (n = 5) to determine how dispersal of Larinus spp. into the surrounding landscape changes with time since release. Populations of Larinus spp. weevils showed little dispersal for 2 years post-release. However, after initial establishment dispersal rates increased rapidly, resulting in average dispersal rates that increased exponentially with time since release. These findings can inform future biological control release and sampling programs for Larinus spp. in eastern North America.     

Increased seed consumption by biological control weevil tempers positive CO2 effect on invasive plant (Centaurea diffusa) fitness

Predicted increases in atmospheric CO₂ and temperature may benefit some invasive plants, increasing the need for effective invasive plant management. Biological control can be an effective means of managing invasive plants, but the anticipated range in responses of plant–insect interactions to climate change make it difficult to predict how effective biological control will be in the future. Field experiments that manipulate climate within biological control systems could improve predictive power, but are challenging to implement and therefore rare to date. Here, we show that free air CO₂ enrichment in the field increased the fitness of Centaurea diffusa Lam., a problematic invader in much of the western United States. However, CO₂ enrichment also increased the impact of the biological control agent Larinus minutus (Coleoptera: Curculionidae) on C. diffusa fitness. C. diffusa plants flowered earlier and seed heads developed faster with both elevated CO₂ and increased temperature. Natural dispersal of L. minutus into the experimental plots provided a unique opportunity to examine weevil preference for and effects on C. diffusa grown under the different climate change treatments. Elevated CO₂ increased both the proportion of seed heads infested by L. minutus and, correspondingly, the amount of seed removed by weevils. Warming had no detectable effect on weevil utilization of plants. Higher weevil densities on elevated CO₂ plants reduced, but did not eliminate, the positive effects of CO₂ on C. diffusa fitness. Correlations between plant development time and weevil infestation suggest that climate change increased weevil infestation by hastening plant phenology. Phenological mismatches are anticipated with climate change in many plant–insect systems, but in the case of L. minutus and C. diffusa in mixed-grass prairie, a better phenological match may make the biological control agent more effective as CO₂ levels rise.     

Additive effects of genotype,nutrient availability and type of tissue damage on the compensatory response of Salix planifolia ssp. planifolia to simulated herbivory

Plant responses to herbivory include tolerance (i.e. compensatory growth) and defense. Several factors influence the tolerance of a plant following herbivory, including plant genetic identity, site nutrient availability, and previous and/or concurrent herbivory. We studied the effects of these factors on the compensatory response of Salix planifolia ssp. planifolia, a shrub species common in the boreal and subarctic regions of North America. We cloned several genets of S. planifolia and submitted them to simulated root and/or leaf herbivory while varying the nutrient availability. Simulated leaf herbivory was more detrimental to the plant than simulated root herbivory, reducing both above- and below-ground tissue production. Leaf demography was unaffected by either simulated herbivory treatment. There was some compensatory growth following simulated leaf and root herbivory, but only the root compartment responded to increased nutrient availability. Simulated leaf herbivory increased leaf transpiration and reduced stomatal resistance, suggesting increased carbon fixation. The unexpected finding of the experiment was the absence of interactions among factors (genotype, nutrient availability and type of tissue damage) on the compensatory response of S. planifolia. These factors thus have additive effects on the species' compensatory ability.     

History of exposure to herbivores increases the compensatory ability of an invasive plant

Release from natural enemies is frequently cited as an important factor contributing to plant invasions. But such effects are likely to be temporary—native herbivores can form new plant-herbivore associations and co-evolved insects might reach the new range. While the potential effects of the initial enemy release have been well studied, the consequences of any resumption of herbivory are poorly understood. Alternanthera philoxeroides is one of the most widespread invasive plants in China and is attacked both by a specialist herbivore introduced from the native range, Agasicles hygrophila, and a native beetle Cassida piperata Hope which has formed a new association. However, these insects are not found throughout the invaded range. To test the effect of the history of population exposure to herbivory on compensatory ability, plants were cultured from 14 populations around China that differed in whether A. hygrophila or C. piperata were present. Treatment plants were exposed to herbivory by A. hygrophila for a week until 50% of the leaf area was defoliated, then grown for 80 days. Plants from populations with prior exposure to herbivory (of any kind) accumulated more root mass than populations without prior exposure, indicating that prior exposure to insects can stimulate plant compensation to herbivory. We would recommend that potential changes in plant tolerance in response to prior exposure to herbivory are considered in invasive plant management plans that employ bio-control agents.     

Is spotted knapweed (<Emphasis Type="Italic">Centaurea stoebe</Emphasis> L.) patch size related to the effect on soil and vegetation properties?

Spotted knapweed (Centaurea stoebe L. subsp. Micranthos (Gugler) Hayek) was first introduced in the 1890s from Europe into western North America, where it now occupies over three million hectares of rangeland and pasture in 14 states and two Canadian provinces, reducing forage production and causing economic damage. Despite many reported effects spotted knapweed can have on soils and native vegetation, it is not known whether patch size is correlated with these ecosystem-level effects. The objective of our study was to determine whether the effects of spotted knapweed on plant composition and soil properties was related to spotted knapweed patch size. We asked the following questions: (1) Are there differences in plant species richness and diversity between small and large knapweed patches? and (2) Do soil water and soil mineral nutrient properties change depending on knapweed patch size? Twenty-four knapweed patches, and paired natural grassland plots, were randomly selected within Lac du Bois Provincial Park, British Columbia, Canada. Knapweed patch size ranged from 6 to 366 m². Sampling and analysis revealed a significant effect of knapweed patch size on soil and vegetation properties. Soil P, soil temperature, and total dry plant biomass (g/0.25 m²) increased, while soil N, soil C, and soil moisture decreased with patch size. Since our results show that spotted knapweed patch size is related to degree of soil alteration, it is important to consider size of patch when modeling the impact of spotted knapweed in North America. Since large patches of spotted knapweed seem to have a proportionately greater effect on soil chemistry properties, large patches may move the system further away from a point where it is possible to restore the site to pre-invasion conditions.     

Do invasive plants structure microbial communities to accelerate decomposition in intermountain grasslands?

Invasive plants are often associated with greater productivity and soil nutrient availabilities, but whether invasive plants with dissimilar traits change decomposer communities and decomposition rates in consistent ways is little known. We compared decomposition rates and the fungal and bacterial communities associated with the litter of three problematic invaders in intermountain grasslands; cheatgrass (Bromus tectorum), spotted knapweed (Centaurea stoebe) and leafy spurge (Euphorbia esula), as well as the native bluebunch wheatgrass (Pseudoroegneria spicata). Shoot and root litter from each plant was placed in cheatgrass, spotted knapweed, and leafy spurge invasions as well as remnant native communities in a fully reciprocal design for 6 months to see whether decomposer communities were species‐specific, and whether litter decomposed fastest when placed in a community composed of its own species (referred to hereafter as home‐field advantage–HFA). Overall, litter from the two invasive forbs, spotted knapweed and leafy spurge, decomposed faster than the native and invasive grasses, regardless of the plant community of incubation. Thus, we found no evidence of HFA. T‐RFLP profiles indicated that both fungal and bacterial communities differed between roots and shoots and among plant species, and that fungal communities also differed among plant community types. Synthesis. These results show that litter from three common invaders to intermountain grasslands decomposes at different rates and cultures microbial communities that are species‐specific, widespread, and persistent through the dramatic shifts in plant communities associated with invasions.