Effects of Nitrogen Availability and Cheatgrass Competition on the Establishment of Vavilov Siberian Wheatgrass

Cheatgrass (Bromus tectorum L.) is the most widespread invasive weed in sagebrush ecosystems of North America. Restoration of perennial vegetation is difficult and land managers have often used introduced bunchgrasses to restore degraded sagebrush communities. Our objective was to evaluate the potential of ‘Vavilov’ Siberian wheatgrass (Agropyron fragile [Roth] P. Candargy) to establish on cheatgrass-dominated sites. We examined Vavilov establishment in response to different levels of soil nitrogen availability by adding sucrose to the soil to promote nitrogen (N) immobilization and examined cheatgrass competition by seeding different levels of cheatgrass. We used a blocked split-split plot design with two sucrose levels (0 and 360 g · m⁻²), two levels of Vavilov (0 and 300 seeds · m⁻²), and five levels of cheatgrass (0, 150, 300, 600, and 1 200 seeds · m⁻²). Seeding was conducted in fall 2003 and 2004, and measurements were taken in June 2004, 2005, and 2006. Sucrose addition decreased availability of soil nitrate but not orthophosphate. In the first year after seeding, sucrose reduced cheatgrass density by 35% and decreased both cheatgrass biomass per square meter and seed production per square meter by 67%. These effects were temporary, and by the second year after seeding, there was a sevenfold increase in cheatgrass density. As a result, the effects of sucrose addition were no longer significant. Sucrose affected Vavilov growth, but not density, during the first year after seeding. Vavilov density decreased as cheatgrass seeding density increased. Short-term reductions in N or cheatgrass seed supply did not have long-term effects on cheatgrass and did not increase Vavilov establishment. Longer-term reductions in soil N, higher seeding densities, or more competitive plant materials are necessary to revegetate areas dominated by cheatgrass.     

Native Plant Growth and Seedling Establishment in Soils Influenced by Bromus Tectorum

The invasion of 40 million hectares of the American West by cheatgrass (Bromus tectorum L.) has caused widespread modifications in the vegetation of semi-arid ecosystems and increased the frequency of fires. In addition to well-understood mechanisms by which cheatgrass gains competitive advantage, it has been implicated in reducing arbuscular mycorrhizal fungi (AMF) abundance and taxa diversity. We evaluated this possibility at a high elevation site in a two-pronged approach. To test whether cheatgrass changed native AMF communities in ways that affected subsequent native plant growth, we grew cheatgrass and native plants in native soils and then planted native plants into these soils in a greenhouse experiment. We found that cheatgrass-influenced soils did not inhibit native plant growth or AMF sporulation or colonization. To test whether soils in cheatgrass-dominated areas inhibited establishment and growth of native plants, cheatgrass was removed and six seeding combinations were applied. We found that 14.02 ±  seedlings · m⁻² established and perennial native plant cover increased fourfold over the three years of this study. Glyphosate reduced cheatgrass cover to less than 5% in the year it was applied but did not facilitate native plant establishment or growth compared with no glyphosate. We conclude that cheatgrass influence on the soil community does not appear to contribute to its invasion success in these high elevation soils. It appears that once cheatgrass is controlled on sites with sufficient native plant abundance, there may be few lingering effects to inhibit the natural reestablishment of native plant communities.     

Factors Affecting Bromus Tectorum Seed Bank Carryover in Western Utah

Cheatgrass (Bromus tectorum L.) is a winter annual weed that presents a serious obstacle to rangeland restoration in the Intermountain West. The objective of this study was to evaluate factors regulating the size and persistence of cheatgrass carryover seed banks on semiarid sites in western Utah. We prevented current-year seed production in each of four habitats, then tallied emerging seedlings over the next 44 yr. Twoyr. Two iterations of the study were conducted during consecutive years. One year before initiation of each iteration, we estimated seed rain at each site. Above-average precipitation in 1998–1999 resulted in relatively high seed rain (13942 seeds·m⁻²) for the first iteration, whereas seed rain for the second iteration averaged only 3567·m⁻² because of drought conditions in 1999–2000. Mean total number of seedlings emerging from carryover seeds for the first and second iterations were 1304 and 270 seedlings·m⁻². Seedling emergence from carryover seed was positively correlated with production-year seed rain (R² = 0.69). The fraction of seed rain that carried over tended to be lower when precipitation the year following production favored fall emergence of the transient seed bank. First-year emergence of carryover seeds averaged 96% of total emergence, whereas third-year emergence averaged < 1% and was zero for six of eight cases. Carryover seeds persisted somewhat longer at the xeric black greasewood site than at more upland sites. Our study shows that cheatgrass seeds rarely persist beyond the second carryover year even on semiarid sites. Emergence from the carryover seed bank can be predicted from site attributes and precipitation patterns in previous years.     

Fall-Grazing and Grazing-Exclusion Effects on Cheatgrass (Bromus tectorum) Seed Bank Assays in Nevada,United States

Cheatgrass (Bromus tectorum L.) seedlings suffer mortality if they do not occupy safe sites that provide establishment requirements. Previous research demonstrated that fall cattle grazing has strong potential for reducing invasive annual grass species dominance in winter-dominated precipitation areas of the Intermountain West. Fall cattle grazing reduces the volume of safe sites through the removal of standing dead biomass in the fall and early winter, when cheatgrass can actively germinate. This study continued an assessment of cheatgrass seed bank characteristics under fall-grazing and grazing exclusion treatments initiated by a previous study. A seed bank assay was organized into a randomized complete block, repeated measure design to assess cheatgrass seed bank characteristics from 2014 to 2017 in central Nevada. Across years, fall-grazed areas had about half the assayed seed bank levels (3 432 ± 2 513 seeds ∙ m⁻²) of ungrazed areas (7 187 ± 1 569), (P <0.0001). There was also a difference among years with 2015 producing higher assayed numbers in both grazing treatments. Combined plotted data from this and the previous study indicated that after several years of fall-grazing treatments, removal of fall cattle grazing for only 1 yr can result in significant increases in cheatgrass seed bank size. Conversely, reapplication of fall cattle grazing can quickly decrease cheatgrass seed bank potential.     

Reproductive Effort and Seed Establishment in Grazed Tussock Grass Populations of Patagonia

The importance of sexual reproduction in tussock grasses that regenerate through vegetative growth is unclear. Festuca gracillima Hook. f. was studied as a model because it is a perennial tussock-forming grass that produces abundant seed but rarely regenerates through seedlings. The Study area was the Magellanic Steppe, Patagonia, Argentina (182 mm rainfall), managed with sheep-grazing regimes of 0.65 (high), 0.21 (low), and 0 (exclosure) ewe equivalents · ha^?1 · yr^?1. Tussock size and spikelet production of 358 individuals were recorded over 5 yr. Yearly models of reproductive effort in relation to plant size were tested using a maximum likelihood procedure. Seed was collected and soil cores were tested for germination and viability. Survival and growth of cohorts of seedlings sown in nylon bags were recorded. Eighteen experimental plots were cleared, and seed establishment under protected and grazed conditions was registered. Reproductive effort varied with years and plant size, with a mean of 2.41%. Florets were produced at mean density of 544 ± 217 · m^?2. Predispersal losses reduced viable seed production to 187 ± 48 seeds · m^?2. Seed weighed 2–2.5 mg, with 65–95% germination. Postdispersal losses reduced the seed bank in spring to 33 ± 1.3 seeds · m^?2. Seedling survival curves were negatively exponential, with 95% mortality in the first year. Up to 5% of resources were used for sexual reproduction in favorable years and a recruitment of 1–3 new seedlings · m^?2 · yr^?1 was expected. These new plants were not observed in undisturbed plots, but established naturally in cleared plots and reached a density of 1 plant · m^?2 after 10 yr, together with 44 plants · m^?2 of other species. Competition might block the final establishment in these grasslands. Grazing does not appear to interfere in any stage of seed reproduction. Seed production may not maintain population numbers but could enhance genetic variation in these clonal plant populations and enable dispersal and recolonization of disturbed areas.     

Defoliation Timing Effects on Spotted Knapweed Seed Production and Viability

Spotted knapweed (Centaurea stoebe L.), a perennial invasive forb that reproduces largely by seed, often forms new flowers after prescribed sheep grazing or mowing is applied during the bolting or flowering stage. It is unknown if these new flowers produce viable seeds by the end of the growing season. The purpose of this 2-yr study was to determine the appropriate timing (or timings) or combination (or combinations) of timings of defoliation on spotted knapweed to reduce its viable seed production. Spotted knapweed plants on foothill rangeland in west-central Montana were hand-clipped at seven different timings and frequencies of defoliation: June (bolting stage); July (late-bud–early flowering stage); August (full-flowering stage); June + July; June + August; July + August; or June + July + August. Unclipped plants were controls. Plants clipped in the bolting stage were defoliated at 35–40% relative utilization. Plants clipped at all other timings had 100% of their buds and flowers removed, plus 3 cm of each bud or flower stem. Plant response was evaluated from mid-August through September, whenever the seed heads of each treatment’s plants reached maturity but while their seed-head bracts remained tightly closed. Clipping at any timing or combination of timings reduced the number of buds and flower heads per plant (P < 0.01), number of seeds per plant (P < 0.01), percentage of viability of seeds (P < 0.01), and number of viable seeds per plant (P < 0.01) compared with no clipping. Clipping during the bolting stage reduced the number of viable seeds by nearly 90% compared with no clipping. Clipping during the late-bud–early-flower or full-flower stage reduced the number of viable seeds by nearly 100% compared with no clipping. Spotted knapweed defoliation via prescribed sheep grazing or mowing in summer should suppress viable seed production of spotted knapweed.     

Can Imazapic Increase Native Species Abundance in Cheatgrass (Bromus tectorum) Invaded Native Plant Communities?

Native plant communities invaded by cheatgrass (Bromus tectorum L.) are at risk of unnatural high intensity fires and conversion to cheatgrass monocultures. Management strategies that reduce cheatgrass abundance may potentially allow native species to expand and minimize further cheatgrass invasion. We tested whether the selective herbicide imazapic is effective in reducing cheatgrass and “releasing” native species in a semiarid grassland and shrub steppe in north-central Oregon. The experiment consisted of a completely randomized design with two treatments (sprayed with 70 g ai · ha^?1 of imazapic and unsprayed) and three replicates of each treatment applied to either 2.5 or 4 ha plots. We repeated this experiment in three different sites dominated by the following native species: 1) bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Löve ssp. spicata) and needle and thread (Hesperostipa comata [Trin. & Rupr.] Barkworth), 2) needle and thread and Sandberg bluegrass (Poa secunda J. Presl), and 3) big sagebrush (Artemisia tridentata Nutt.). Nested frequency of all plant species in 1-m² quadrats was collected for 1  yr pretreatment and 4  yr posttreatment. In all three sites, cheatgrass frequencies were significantly lower in sprayed plots than unsprayed plots for 3–4  yr posttreatment (P < 0.1). Other annual plant species were also impacted by imazapic, but the effects were highly variable by species and site. Only two native perennial species, hoary tansyaster (Machaeranthera canescens [Pursh] Gray) and big sagebrush, increased in sprayed plots, and increases occurred only at two sites. These results suggest that a short-term reduction in cheatgrass alone is not an effective strategy for increasing the abundance of most native perennial plant species.     

Suppression of Cheatgrass by Perennial Bunchgrasses

Long-term control of the invasive annual grass cheatgrass is predicated on its biological suppression. Perennial grasses vary in their suppressive ability. We compared the ability of a non-native grass (“Hycrest” crested wheatgrass) and two native grasses (Snake River wheatgrass and bluebunch wheatgrass) to suppress cheatgrass. In a greenhouse in separate tubs, 5 replicates of each perennial grass were established for 96 d, on which two seeds of cheatgrass, 15 cm apart, were then sown in a semicircular pattern at distances of 10 cm, 30 cm, and 80 cm from the established perennial bunchgrasses. Water was not limiting. After 60 d growth, cheatgrass plants were harvested, dried, weight recorded, and tissue C and N quantified. Soil N availability was quantified at each location where cheatgrass was sown, both before sowing and after harvest. Relative to cheatgrass grown at 80 cm, all perennial grasses significantly reduced aboveground biomass at 30 cm (68% average reduction) and at 10 cm (98% average reduction). Sown at 10 cm from established perennial grasses, cheatgrass aboveground biomass was inversely related with perennial grass root mass per unit volume of soil. All cheatgrass sown at 10 cm from “Hycrest” crested wheatgrass died within 38 d. Before sowing of cheatgrass, soil 10 cm from established perennial grasses had significantly less mineral N than soil taken at 30 cm and 80 cm. Relative to cheatgrass tissue N for plants grown at 80 cm, cheatgrass nearest to the established perennial grasses contained significantly less tissue N. All perennial grasses inhibited the NO₂⁻ to NO₃⁻ nitrification step; for “Hycrest” crested wheatgrass, soil taken at 10 cm from the plant had a molar proportion of NO₂⁻ in the NO₂⁻ + NO₃⁻ pool of > 90%. In summary, a combination of reduced nitrogen availability, occupation of soil space by perennial roots, and attenuation of the nitrogen cycle all contributed to suppression of cheatgrass.     

Biomass and Defoliation Tolerance of 12 Populations of Pseudoroegneria spicata at Two Densities

Pseudoroegneria spicata(Pursh.) A. Löve is an important native grass of the rangelands of the Intermountain West, USA and is widely used in this region for restoration applications. High grazing preference, together with high grazing sensitivity, has reduced the abundance of this species, indicating the need for the development of grazing-tolerant plant materials. While a genotype may be defoliation tolerant at low density, e.g., in an experimental setting, an effective grazing-tolerant genotype must also display this trait at higher densities resembling those found in natural and restoration settings. We compared 12 restoration plant materials for response to spring-defoliation at high (25 plants · m^?2) and low (8 plants · m^?2) plant densities in a field experiment. Two consecutive years of spring-defoliation reduced shoot biomass 19% compared to the nonspring-defoliated control, and this reduction was similar for the two densities examined. Two populations, P-3 and Acc:238, were able to compensate for shoot biomass after 2 yr of spring-defoliation, while the remaining 10 populations undercompensated, as is commonly reported for cool-season grasses in arid and semiarid regions. While the association between control and spring-defoliated shoot biomass was marginally positive (R² = 0.26; P < 0.10), we found a stronger negative association (R² = 0.36; P < 0.05) between spring-defoliation tolerance and control shoot-biomass production. This suggests a possible trade-off between growth and defoliation tolerance (calculated as percentage of control biomass) among populations. Of the four commercially available plant materials in our study, the more recent prevariety germplasm, P-7, exhibited higher control shoot biomass and higher spring-defoliation tolerance than the older cultivars, Whitmar and Goldar. Anatone germplasm was intermediate but not statistically different from these other plant materials for these two traits.     

Medusahead Dispersal and Establishment in Sagebrush Steppe Plant Communities

Medusahead (Taeniatherum caput-medusae [L.] Nevski) is an invasive annual grass that reduces biodiversity and production of rangelands. To prevent medusahead invasion land managers need to know more about its invasion process. Specifically, they must know about 1) the timing and spatial extent of medusahead seed dispersal and 2) the establishment rates and interactions with plant communities being invaded. The timing and distance medusahead seeds dispersed from invasion fronts were measured using seed traps along 23 35-m transects. Medusahead establishment was evaluated by introducing medusahead at 1, 10, 100, 1 000, and 10 000 seeds · m⁻² at 12 sites. Most medusahead seeds dispersed less than 0.5 m from the invasion front (P < 0.01) and none were captured beyond 2 m. Medusahead seeds dispersed from the parent plants from early July to the end of October. More seeds were trapped in August than in the other months (P < 0.01). Medusahead establishment increased with higher seed introduction rates (P < 0.01). Medusahead density was negatively correlated to tall tussock perennial grass density and positively correlated to annual grass density of the preexisting plant communities (P = 0.02). Medusahead cover was also negatively correlated with tall tussock perennial grass density (P = 0.03). The results suggest that containment barriers around medusahead infestations would only have to be a few meters wide to be effective. This study also suggests that promoting or maintaining tall tussock perennial grass in areas at risk of invasion can reduce the establishment success of medusahead. Tall tussock perennial grass and annual grass density, in combination with soil data, may be useful in predicting susceptibility to medusahead invasion.     

Cheatgrass Invasion in Salt Desert Shrublands: Benefits of Postfire Reclamation

In 1998, fires burned more than 11 330 ha of rangeland on Dugway Proving Ground in Utah's west desert. Postfire revegetation was implemented in 2 affected salt desert shrub communities (greasewood; Sarcobatus vermiculatus Hook. and black sagebrush/shadscale; Artemisia nova A. Nels; Atriplex confertifolia Torr. & Frem.) to deter cheatgrass (Bromus tectorum L.) encroachment. We monitored cheatgrass densities for 3 years after the fire in burned drill seeded, burned not-seeded, and unburned plots to assess the rate of invasion and determine the impact on cheatgrass of drill seeding perennial species. Cheatgrass invaded quickly in both shrub sites following the fires. In the greasewood site, drill seeded species germinated but did not establish. This was likely due to a combination of soil salinity and extremely dry weather conditions during the second year of the study. Drill seeded species in the black sagebrush site germinated and established well, resulting in the establishment of 16.5 perennial grasses · m^-2 and 1 356 shrubs · ha^-1. Cheatgrass densities were consistently lower in drill seeded versus not-seeded plots, although these were not always statistically different when Bonferroni comparisons were considered. The initial decrease in cheatgrass densities in drill seeded plots may have resulted from soil disturbance coupled with extremely low precipitation rather than competitive effects. Nevertheless, as seeded species mature and increase their competitive ability, we predict long-term suppression of cheatgrass in the absence of further disturbance.     

Fire Effects on the Cheatgrass Seed Bank Pathogen Pyrenophora semeniperda

The generalist fungal pathogen Pyrenophora semeniperda occurs primarily in cheatgrass (Bromus tectorum) seed banks, where it causes high mortality. We investigated the relationship between this pathogen and its cheatgrass host in the context of fire, asking whether burning would facilitate host escape from the pathogen or increase host vulnerability. We used a series of laboratory and field experiments to address the ability of host seeds and pathogen life stages to survive fire. First, we determined the thermal death point (TDP₅₀; temperature causing 50% mortality) of seeds and pathogen propagules at two time intervals using a muffle furnace. We then measured peak fire temperatures in prescribed burns at sites in Utah and Washington and quantified seed and fungal propagule survival using pre- and postburn seed bank sampling and inoculum bioassays. Finally, we investigated the survival of both seeds and pathogen after wildfires. We found that radiant heat generated by both prescribed and wild cheatgrass monoculture fires was generally not sufficient to kill either host seeds or pathogen propagules; most mortality was apparently due to direct consumption by flames. The 5-min mean TDP₅₀ was 164°C for pathogen propagules and 148°C for host seeds, indicating that the pathogen is more likely to survive fire than the seeds. Peak fire temperature at the surface in the prescribed burns averaged 130°C. Fire directly consumed 85–98% of the viable seed bank, but prescribed burns and wildfires generally did not lead to dramatic reductions in pathogen inoculum loads. We conclude that the net effect of fire on this pathosystem is not large. Rapid postburn recovery of both host and associated pathogen populations is the predicted outcome. Postfire management of residual cheatgrass seed banks should be facilitated by the persistent presence of this seed bank pathogen.     

Comparison of Seed Bank Estimation Techniques Using Six Weed Species in Two Soil Types

Evaluation of the viable seeds in a soil, otherwise known as the seed pool or seed bank, is a crucial component of many weed dynamic and plant ecology studies. Seed bank estimation is used to predict the possibility of future weed infestations in rangelands as well as the nascent native plant diversity within them. However, there is no standardized method of reporting seed bank evaluation techniques, limiting the ability to compare across studies. After sowing known quantities of cheatgrass, Bromus tectorum (L.); brome fescue, Vulpia bromoides (L., S.F. Gray); pigweed, Amaranthus retroflexus (L.); kochia, Kochia scoparia (L. Schrad.); lambsquarters, Chenopodium album (L.); and field pepperweed, Lepidium campestre (L. R. Br.) into sterile soil, we compared two different watering regimes in two soil types to Petri plate germination of these seeds. Seed bank estimations from the emergence method were lower compared to estimations from the Petri plate germination. Top-and-bottom watering increased absolute abundance, and the rank order of abundance among species changed with watering method. Emergence levels were the same between the two soil types. The higher water availability of the top-and-bottom watering method resulted in greater seedling emergence (26.3% ± 10% SD vs. 9.1% ± 7.5% SD). Lower emergence compared to germination (62.3% ± 24.4%) may indicate that emergence is an important postgermination barrier to seedling establishment. While emergence techniques may not accurately portray the volume of seeds in the soil, they may more accurately predict which plants can become established in field conditions. Our different species abundances between watering methods show that multiple emergence methods may need to be employed to forecast a range of future rangeland conditions from the soil seed bank.     

Growth of Chickasaw Plum in Oklahoma

Management of rangelands for wildlife and livestock entails understanding growth of clonal shrubs such as Chickasaw plum (Prunus angustifolia Marsh.). We studied growth of this species in one county in north-central (Payne) and two counties in northwestern Oklahoma (Ellis, Harper) during 2006 and 2007. We estimated age of stems and roots by growth rings and area of stands with the use of a handheld GPS unit. Based on zero-intercept regression models, stands grew at similar rates (overlapping 95% confidence intervals [CIs]) among counties with a pooled estimate of 31.0 m² · yr⁻¹ (95% CI = 26.5–35.6 m² · yr⁻¹; n = 95). This rate showed considerable variability within and among study sites (r = 0.52). Stem diameter increased (zero-intercept models) more rapidly in north-central Oklahoma (5.27 mm · yr⁻¹; 95% CI = 5.01–5.53 mm · yr⁻¹; r = 0.90; n = 53) than in northwestern Oklahoma (3.68 mm · yr⁻¹; 95% CI = 3.55–3.81 mm · yr⁻¹; r = 0.91; n = 102); data were pooled because of similar rates in Ellis and Harper counties. Stem height was a power function of stem age (y = 0.97x^0.28; r = 0.56), indicating rate of growth in height (m · yr⁻¹) declined with age according to dy/dx = 0.27x^−0.72. Knowledge of the area expansion rate of Chickasaw plum clones aids in management planning to increase or decrease canopy coverage by this shrub.     

Hydrologic Impacts of Mechanical Seeding Treatments on Sagebrush Rangelands

In and around the Great Basin, United States, restoration of shrub steppe vegetation is needed where rangelands are transitioning to annual grasslands. Mechanical seedbed preparation can aid native species recovery by reducing annual grass competition. This study was designed to investigate the nature and persistence of hydrologic and erosion impacts caused by different mechanical rangeland seeding treatments and to identify interactions between such impacts and related soil and vegetation properties. A cheatgrass (Bromus tectorum L.)–dominated site was burned and seeded with native grasses and shrubs in the fall of the year. An Amazon-drill and a disk-chain seeder were used to provide varying levels of surface soil disturbance. An undisturbed broadcast seeding was used as a control. Simulated rainfall was applied to 6 large (32.5-m²) plots per treatment over 3 growing seasons at a rate of 63.5 mm · h^-1. Rainfall was applied for 60 minutes under dry antecedent moisture conditions and for 30 minutes, 24 hours later under wet antecedent moisture conditions. The disk-chain created the largest reduction in infiltration and increase in sediment yield, which lasted for 3 growing seasons posttreatment. The Amazon-drill had a lesser impact, which was insignificant after the second growing season posttreatment. Surface soil properties showed little correlation with treatment-induced hydrologic and erosion impacts. Hydrologic recovery was strongly correlated with litter dynamics. The seeding treatments were unsuccessful at establishing seeded plant species, and the site once again became dominated by cheatgrass. A continuous upward trend in biomass production and surface litter cover was observed for all treatments between the beginning and end of the study because of cheatgrass invasion. Although the initial goal of using mechanical seeding treatments to enhance recovery of native grass species failed, cheatgrass production provided sufficient biomass to rapidly replenish surface litter cover necessary for rapid hydrologic stability of the site.     

Rough Soil Surface Lessens Annual Grass Invasion in Disturbed Rangeland

Effective manipulations to prevent the spread of invasive species are needed. Downy brome (Bromus tectorum L.) is an annual invader that often expands after disturbances, compromising restoration of big sagebrush (Artemisia tridentata Nutt.) communities in western North America. This study examined the effects of two manipulations that may slow seed dispersal: soil microtopography (roughened with 50-cm relief or flat) and woody debris (0.024 m³·m^? 2 or none) on restoration of four disturbed mountain big sagebrush (A. tridentata Nutt. ssp. vaseyana) sites in Colorado. Treatments were crossed with seeding in a fully factorial experiment (n = 3). Microtopography and woody debris treatments were also crossed in a seed dispersal experiment using fluorescently marked downy brome seeds. In the restoration study, downy brome invaded two sites, one pervasively and one patchily. Seeding limited downy brome cover at both of these sites and also increased perennial grass and forb cover while limiting shrub cover. At the pervasively invaded site, the rough surface reduced unseeded plot downy brome cover from 13% to 3% by 5 yr post treatment. Woody debris increased shrub and perennial grass cover but had little effect on downy brome. In the seed dispersal experiment, the rough surface reduced downy brome mean dispersal distance twofold to threefold and 95% quantile distance threefold to sixfold. Woody debris slightly reduced downy brome dispersal only within rough surface plots. A rough surface may aid restoration by trapping downy brome seeds near the parent plant, limiting their spatial distribution, increasing intraspecific competition, and reducing propagule pressure. Designing landscapes to slow seed dispersal may help control invasives and promote establishment of seeded species.     

Fire,Defoliation, and Competing Species Alter Aristida purpurea Biomass,Tiller, and Axillary Bud Production

Aristida purpurea (purple threeawn) is a competitive native perennial grass with monoculturistic tendencies and poor palatability. We examined effects of fire, defoliation, and interspecific/intraspecific planting for 1) threeawn responses in the presence of threeawn, Bouteloua gracilis, or Pascopyrum smithii, and 2) B. gracilis and P. smithii response with threeawn. Biomass, aboveground production, tillers, and axillary buds were analyzed following two fire and four clipping treatments applied to three species–pair combinations in a completely randomized factorial design with nine replications. Fire killed 36% of threeawn. Fire reduced surviving threeawn biomass 61% and reduced production 27%. Threeawn production was greatest when neither plant was clipped and least when competing species were moderately clipped, or when both plants were severely clipped. Tiller counts of burned threeawn were similar among clipping treatments, and less than non-clipped or moderately clipped plants not burned. Fire decreased threeawn axillary buds on average by 25%. Moderately clipped plants had greater production than those from other clipping treatments across species. Average threeawn percentage of pot biomass was greater with B. gracilis (46 ± 3% SE) than P. smithii (38 ± 3% SE). Fire reduced threeawn from 60 ± 3% to 23 ± 3% of pot biomass, indicating good potential for rapid reductions in threeawn dominance and restoration of plant diversity with fire.     

A Basis for Relative Growth Rate Differences Between Native and Invasive Forb Seedlings

The ability of invasive plants to achieve higher relative growth rates (RGR) than their native counterparts has been widely documented. However, the mechanisms allowing invasives to achieve higher RGR are poorly understood. The objective of this study was to determine the basis for RGR differences between native and invasive forbs that have widely invaded nutrient-poor soils of the Intermountain West. Six native and 6 invasive forbs were seeded in pots in a greenhouse, and 4 harvests were conducted over a 2-month period. These 4 harvests were used to calculate RGR and the components of RGR, net assimilation rate (rate of dry matter production per unit leaf area), leaf area ratio (LAR, leaf area per unit total plant mass), leaf mass ratio (the proportion of biomass allocated to leaves), and specific leaf area (SLA, leaf area per unit leaf biomass). Mean RGR of the 12 study species ranged between 0.04 and 0.15 g · g⁻¹ · d⁻¹ but was significantly higher for invasive forbs compared to native forbs (P = 0.036). The higher RGR achieved by invasive forbs was due mainly to a greater SLA and LAR. This indicates that invasive forbs achieved higher RGR than natives primarily by creating more leaf area per unit leaf mass, not by allocating more biomass to leaf tissue or by having a higher net rate of dry matter production. A high degree of variation in RGR, SLA, and LAR was observed in native forbs, suggesting that the ability to design weed-resistant plant communities may be improved by managing for specific functional traits as opposed to functional groups.     

Leafy Spurge Suppression by Flea Beetles in The Little Missouri Drainage Basin,USA

The Ecological Area-wide Management Leafy Spurge, or TEAM Leafy Spurge, began collecting and redistributing flea beetles (Aphthona spp.) to research/demonstration sites and landowners throughout the Little Missouri River drainage basin to control leafy spurge in 1998. A study to evaluate the change over time of leafy spurge (Euphorbia esula L.) phytosociological characteristics following release of flea beetles was initiated in 2002 on leafy spurge–infested pasture and rangeland in the Little Missouri River drainage of Montana, North Dakota, South Dakota, and Wyoming. A total of 292 flea beetle release sites were analyzed in June and July 2002 and 2003 for leafy spurge stem density, foliar cover, flea beetle density, and vegetation composition. Leafy spurge stem density suppression was evident at 91% of the study sites. On two-thirds of the study sites stem density was reduced from greater than 100 stems·m⁻¹ to less than 25 stems·m⁻¹. Leafy spurge foliar cover was less than 5% on approximately two-thirds of the flea beetle release sites and less than 25% on over 90% of the release sites. Area of observed leafy spurge suppression ranged from 0 m² to 30000 m². Approximately 40% of the release sites had leafy spurge suppression ranging from 1000 m² to 5000 m², and 14% of the release sites had greater than 10000 m² of leafy spurge control. Plant community composition following leafy spurge suppression was characteristic of native plant communities that had not been burned or grazed. Flea beetles effectively reduced leafy spurge stem density and cover in 4–5 yr across a variety of locations and corresponding environmental conditions, both within the Little Missouri River drainage and in selected nearby locations.