Roger L. Sheley

Nitrogen Enhances the Competitive Ability of Cheatgrass (Bromus tectorum) Relative to Native Grasses

Abstract Invasion by cheatgrass and the associated high fire frequency can displace native plant communities from a perennial to an annual grass driven system. Our overall objective of this study was to determine the potential to favor desired native perennial bunchgrasses over annual grasses by altering plant available mineral nitrogen (N). In the first study, we grew cheatgrass and three native bunch grasses (native grasses were combined in equal proportions) in an addition series experimental design and applied one of three N treatments (0, 137, and 280 mg N/kg soil). Regression models were used to derive the effects of intra- and interspecific competition on individual plant yield of cheatgrass and the native bunch grasses (combined). In our second study, we compared the absolute growth rate of the four plant species grown in isolation in a randomized complete block design for 109 days under the same soil N treatments as the competition study. Predicted mean average weight of isolated individuals increased with increasing soil N concentrations for both cheatgrass and the three native perennials (P < 0.05). Biomass of cheatgrass and its competitive ability increased with increasing soil N concentrations (P < 0.0001) compared to the combined native bunchgrasses. However, the greatest resource partitioning occurred at the 137 mg N/kg soil N treatment compared to the 0 (control) and 280 mg N/kg soil treatments, suggesting there may be a level of N that minimizes competition. In the second study, the absolute growth of cheatgrass grown in isolation also increased with increasing N levels (P  =  0.0297). Results and ecological implications of this study suggest that increasing soil N leads to greater competitive ability of cheatgrass, and that it may be possible to favor desired plant communities by modifying soil nutrient levels. Nomenclature: Bluebunch wheatgrass, Pseudoroegneria spicata (Pursh) A. Love PSSP6; Idaho fescue, Festuca idahoensis Elmer FEID; needle and thread, Hesperostipa comata (Trin. and Rupr.) Barkworth HECO26; cheatgrass, Bromus tectorum L BRTE.

Creating Invasion Resistant Soils via Nitrogen Management

Abstract Invasion by annual grasses, such as cheatgrass, into the western U.S. sagebrush-steppe is a major concern of ecologists and resource managers. Maintaining or improving ecosystem health depends on our ability to protect or re-establish functioning, desired plant communities. In frequently disturbed ecosystems, nutrient status and the relative ability of species to acquire nutrients are important drivers of invasion, retrogression, and succession. Thus, these processes can potentially be modified to direct plant community dynamics toward a desired plant community. The overall objective of this review paper is to provide the ecological background of invasion by exotic plants and propose a concept to facilitate the use of soil nitrogen (N) management to achieve desired plant communities that resist invasion. Based on the literature, we propose a model that predicts the outcome of community dynamics based on N availability. The model predicts that at low N levels, native mid- and late-seral species are able to successfully out-compete early-seral and invasive annual species up to some optimal level. However, at some increased level of N, early-seral species and invasive annual grasses are able to grow and reproduce more successfully than native mid- and late-seral species. At the high end of N availability to plants, the community is most susceptible to invasion and ultimately, increased fire frequency. Soil N level can be managed by altering microbial communities, grazing, mowing, and using cover crops and bridge species during restoration. In these cases, management may be more sustainable since the underlying cause of invasion and succession is modified in the management process. Nomenclature: Cheatgrass, Bromus tectorum L. BROTE.

Herbicide Effects on Density and Biomass of Russian Knapweed (Acroptilon repens) and Associated Plant Species1

Sustainable invasive weed management must address treatment effects on desired vegetation. Our objective was to determine the influence of clopyralid plus 2,4-D, glyphosate, and fosamine, at various application rates and timing, on the density and biomass of Russian knapweed and desired plant groups growing in association with this invasive weed. In a randomized complete block design with four replications, three herbicides by three herbicide rates by three herbicide application timings and a nontreated control were factorially applied to two sites located along the Missouri River riparian corridor in Montana. Clopyralid plus 2,4-D, glyphosate, and fosamine were applied during the spring rosette stage of Russian knapweed (June), the bud to bloom stage of Russian knapweed (July), or the flowering stage of Russian knapweed (August). Herbicide rates were considered low, medium, and high based on label rates of clopyralid plus 2,4-D, glyphosate, or fosamine. Density and biomass of all species were sampled 3 yr after treatment. Russian knapweed biomass decreased from 125 to about 25 g/m2 using clopyralid plus 2,4-D, irrespective of rate or timing of application. Russian knapweed density was reduced by about half by this mixture of herbicides. Nonnative grass density and biomass were maintained, whereas native grasses increased using clopyralid plus 2,4-D at medium or high rates. Neither glyphosate nor fosamine provided substantial Russian knapweed control or increases in grasses. Too few forbs were present to analyze their response to the treatments. We believe that herbicides must be combined with revegetation in areas lacking a diverse mixture of desired species capable of capturing resources made available by controlling Russian knapweed. Nomenclature: Clopyralid; 2,4-D; fosamine; glyphosate; Russian knapweed, Acroptilon repens (L) DC. #3 CENRE. Additional index words: Native plants, restoration. Abbreviation: SE, standard error.

CONTROLLING PERFORMANCE OF BLUEBUNCH WHEATGRASS AND SPOTTED KNAPWEED USING NITROGEN AND SUCROSE AMENDMENTS

Abstract Range- and wildlands are being invaded by nonindigenous plants, resulting in an unprecedented, rapid change in plant community composition across the United States. Successional management predicts that species performance may be modified by resource availability. The objective of this study was to determine whether species performance could be altered by modifying soil nitrate (NO3–) and ammonium (NH4+) concentrations within an Idaho fescue (Festuca idahoensis)/bluebunch wheatgrass (Pseudoroegneria spicata) plant association. We planted bluebunch wheat-grass and spotted knapweed (Centaurea maculosa) in an addition series at 2 sites in southwestern Montana. Each plot in the addition series matrix was divided into thirds, and we applied nitrogen (N) to a subplot and sucrose to a 2nd subplot. The remaining subplot was not amended and considered a control. Nitrogen amendment tended to enhance the performance of spotted knapweed, while sucrose amendment had no effect. Bluebunch wheatgrass performance was not affected by either amendment. Sucrose treatments only decreased soil NO3– at the more productive site. Regression models for predicting bluebunch wheatgrass and spotted knapweed biomass accounted for only about 30% of the variation, suggesting other processes in addition to interference were responsible for explaining relative plant performance. We recommend that land managers prevent activities that increase soil N concentration while the effectiveness of carbon amendments as a means to decrease soil N concentrations and shift interference relationships is further investigated.

Integrating 2,4-D and sheep grazing to rehabilitate spotted knapweed infestations

Abstract Effective integrated weed management requires understanding the impacts of management strategies applied alone and in combination. The objectives of this study were to determine the effects of 2,4-D and repeated sheep (Ovis aries) grazing applied alone and in combination on spotted knapweed (Centaurea maculosa Lam.) and perennial grasses. We hypothesized that integrating a single spring 2,4-D application would remove the adult plants, repeated sheep grazing would control rosettes, and spotted knapweed density, cover and biomass would decrease, allowing residual grasses to reoccupy the sites. A single spring 2,4-D application at 2.1 kg a.i. ha−1, repeated annual sheep grazing of 95% spotted knapweed or 60% grass utilization, and repeated sheep grazing and 2,4-D combined were applied to small pastures at 2 sites in western Montana beginning in 1997 and continued through 2001. Spotted knapweed rosette and flowering plant density, and spotted knapweed and perennial grass cover and biomass were sampled from 1998 through 2001. Spotted knapweed rosette density was 61.7, 34.3, 44.3, and 0.3 m−2 in the control, sheep grazing, 2,4-D, and combined sheep grazing and 2,4-D treatments, respectively, at 1 site in 2001. Spotted knapweed flowering plant density increased from 3.7 in 1998 to 10.7 m−2 in 2002 in the 2,4-D treatment whereas there was no increase in the 2,4-D combined with sheep grazing treatment from 1998 to 2002. Perennial grass biomass was 6.9, 8.4, 25.7, and 19.7 in the control, sheep grazing, 2,4-D, and combined sheep grazing and 2,4-D treatments, respectively, averaged for both sites and 4 years of sampling. Herbicides released perennial grasses from weed competition and changed the weed population from mature, less palatable plants to juvenile plants that were preferred by sheep.

Integrated Management of Leafy Spurge-Infested Rangeland

Abstract Leafy spurge is an invasive Eurasian weed on pastures and rangeland in North America where it reduces grass forage production. Our objective was to determine the effects of multispecies grazing combined with Aphthona flea beetles on leafy spurge-infested rangeland. On two western North Dakota sites divided into four 25- to 79-ha pastures, two grazing duration treatments were applied: season-long with 7 to 10 cow calf pairs and 20 to 25 sheep from late May through mid-September, and rotation with 18 to 21 cow calf pairs and 45 to 50 sheep for 3 weeks twice per year. Grazing treatments started in 1998 and continued through 2002. Aphthona spp. were released beginning in 1991 and were widespread in both pastures by 1998. Four grazing exclosures were randomly located in each pasture in the spring of 1998. Cover of leafy spurge, grass, and forbs, as well as density of vegetative and flowering leafy spurge stems were measured in July 1998 through 2002. Aphthona densities were counted July 1999 through 2002. Grazing initially increased leafy spurge vegetative stem density, but grazing decreased vegetative stem density from 104 in 1999 to 20 stems · m−2 in 2002. Season-long grazing reduced vegetative stem density by over 30 stems · m−2 compared to rotation grazing. Leafy spurge flowering stems decreased from 80 stems · m−2 in 1998 to 4 stems · m−2 in 2002 in all treatments. The decrease was more rapid when grazing was combined with Aphthona. Initially, Aphthona densities were greater in the grazed areas than the exclosures, but by 2002 more Aphthona were found in the exclosures than the grazed areas. Grazing reduced grass cover and increased forb cover. Results suggest combining multispecies grazing and Aphthona when restoring spurge-infested grasslands produces a synergistic effect.

Using ecological theory to guide the implementation of augmentative restoration

Abstract Successful control of invasive plants can have unexpected effects on native plants and wildland systems. Therefore, it is important for managers of invasive species to be concerned with ecological mechanisms and processes like invasion resistance, environmental heterogeneity, and succession that direct plant community dynamics. Augmentative restoration is a management approach for restoring desired species on wildlands dominated by invasive plants, where functioning ecological processes are maintained by selectively augmenting only those processes that are not operating sufficiently. The study was conducted within the Mission Valley, Montana, in an area where meadow vole disturbance provided site availability for colonization. In a split-plot design with four replications, eight factorial treatment combinations from three factors (shallow tilling, watering, and seeding) were applied to whole plots, and 2,4-D was applied to subplots. Cover and density of seeded species, spotted knapweed, and sulfur cinquefoil were sampled in July 2002 and 2003 to produce pretreatment and posttreatment data. Analysis of covariance was used to analyze cover and density data using pretreatment data as a baseline covariate. Data indicated that in areas with adequate site availability due to meadow vole disturbance, seeding and watering without tilling were required to increase seeded species. Spotted knapweed and sulfur cinquefoil decreased in response to 2,4-D. These data provided evidence that augmentative restoration may improve our ability to establish desired species on invasive plant–dominated wildlands. Nomenclature: 2,4-D; spotted knapweed, Centaurea maculosa Lam.; sulfur cinquefoil, Potentilla recta L. Additional index words: Environmental heterogeneity, invasive weeds, native plant establishment, successional management. Abbreviations: ANCOVA, analysis of covariance.

Restoring species richness and diversity in a russian knapweed (Acroptilon repens)-infested riparian plant community using herbicides

Abstract Species richness and diversity are important indicators of ecosystem function and may be related to plant community resistance to invasion by nonindigenous species. Our specific objective was to determine the influence of clopyralid plus 2,4-D, glyphosate, and fosamine, at different application rates and timings, on richness and diversity of total species, total native species, and total nonnative species within a Russian knapweed–infested plant community. Twenty-eight treatments (3 herbicides by 3 rates by 3 application timings, and an untreated control) were applied to two sites located along the Missouri River riparian corridor in Montana. Clopyralid plus 2,4-D, glyphosate, and fosamine were applied in June (spring rosette stage of Russian knapweed), July (bud to bloom stage of Russian knapweed), and August (flowering stage of Russian knapweed). Herbicide rates were clopyralid plus 2,4-D at 0.08 (clopyralid) + 0.42 (2,4-D), 0.13 + 0.67, and 0.18 + 0.92 kg ai ha−1; glyphosate at 0.6, 1.2, and 1.8 kg ai ha−1; fosamine at 3.6, 7.2, and 10.8 kg ai ha−1. Density of each species was recorded during June and August of 2001 and 2002. Species richness and Simpsons Reciprocal Index (1/D) were calculated. By August 2002, only the glyphosate treatment (4.6 species m−2) yielded greater total richness over that of the control (3.5 species m−2). At that time, diversity after applying clopyralid plus 2,4-D remained similar to that of the control (1.4), but glyphosate (2.3) and fosamine (2.0) increased total species diversity. Nonnative grasses and forbs accounted for the increases in richness and diversity. Glyphosate may be appropriate for enhancing ecosystem function and possibly niche occupation to preempt reinvasion by Russian knapweed, but restoring native species seems unlikely using any of these herbicides alone. Nomenclature: Clopyralid, fosamine, glyphosate, 2,4-D, Russian knapweed, Acroptilon repens (L.) DC. CENRE

Establishment of Native and Invasive Plants along a Rangeland Riparian Gradient

Environmental resource gradients affect plant establishment during invasive weed management and ecological restoration. We investigated how hydrology and nitrogen (N) availability influence establishment of native and invasive species along a dry-wet gradient. We hypothesized that 1) emergence and growth would be higher in subirrigated environments than in saturated or dry environments; 2) annuals and invasive exotics would respond more to N availability than late-successional perennials; and 3) N effects would be greatest in subirrigated environments. In the greenhouse, seedling emergence was uniform across dry to subirrigated pots for upland species and across subirrigated to saturated, but not flooded, pots for riparian species. Seedling biomass was greatest in subirrigated environments. Seedling emergence in the field was at or near zero for most species in subirrigated sites. In the greenhouse, biomass increased with N availability, and responses to N were greatest in subirrigated pots. In both experiments, the upland invasive, spotted knapweed (Centaurea maculosa), responded more to N availability than bluebunch wheatgrass (Agropyron spicatum). However, such differences were not found for the riparian species, possibly due to low emergence. Possible explanations for low emergence in subirrigated field sites include predation by small rodents, which were more active in the transition than upland or wetland areas. Establishment of spotted knapweed and Canada thistle (Cirsium arvense) in riparian sites was limited by poor germination and seedling survival. Once established, growth was good in subirrigated environments, indicating that weeds may invade riparian sites in spite of poor initial establishment, which suggests caution when contemplating ecological restoration practices that increase N availability.

Using Light Attenuation to Estimate Leafy Spurge Impacts on Forage Production

Abstract Rangeland managers often must decide whether to suppress dicotyledonous weed populations with expensive and time-consuming management strategies. Often, the underlying goal of weed suppression efforts is to increase production of native forage plants. Many managers suppress weeds only when they feel the unwanted plants are substantially impacting their forage base. Currently, intuition and guesswork are used to determine whether weed impacts are severe enough to warrant action. We believe scientific impact assessments could be more effective than these casual approaches to decision making. Scientific approaches will necessitate data on weed abundances because the severity of a weeds impact is highly correlated with its abundance. The need for weed abundance data poses major obstacles because gathering these data with readily available techniques is time consuming. Most managers cannot or will not spend a lot of time gathering vegetation data. In this paper, we explore a rapidly measured index (<2 minutes per sample location) that is highly correlated with weed (i.e., leafy spurge Euphorbia esula L.) abundance per unit area. This index is based on the light attenuation leafy spurge causes. After measuring light attenuation in plots planted to leafy spurge and grasses, we developed a probabilistic model that predicts leafy spurge impacts on forage production. Data from experiments where herbicides suppressed leafy spurge provided an opportunity to evaluate prediction accuracy of the model. In each case herbicide experiment data fell within the range of values (i.e., credibility intervals) the model predicted, even though the model development experiments were separated from the herbicide experiments by several hundred kilometers in space and 4 years in time. Therefore, we conclude that the model successfully accounts for spatial and temporal variation. We believe light attenuation could help natural resource managers quickly quantify some kinds of weed impacts.