Restoration of Sagebrush in Crested Wheatgrass Communities: Longer-Term Evaluation in Northern Great Basin

Crested wheatgrass (Agropyron cristatum [L] Gaertm. and Agropyron desertorum [Fisch.] Schult.), an introduced bunchgrass, has been seeded on millions of hectares of sagebrush steppe. It can establish near-monocultures; therefore, reestablishing native vegetation in these communities is often a restoration goal. Efforts to restore native vegetation assemblages by controlling crested wheatgrass and seeding diverse species mixes have largely failed. Restoring sagebrush, largely through planting seedlings, has shown promise in short-term studies but has not been evaluated over longer timeframes. We investigated the reestablishment of Wyoming big sagebrush (Artemisia tridentata spp. wyomingensis [Beetle & A. Young] S.L. Welsh) in crested wheatgrass communities, where it had been broadcast seeded (seeded) or planted as seedlings (planted) across varying levels of crested wheatgrass control with a herbicide (glyphosate) for up to 9 yr post seeding/planting. Planting sagebrush seedlings in crested wheatgrass stands resulted in full recovery of sagebrush density and increasing sagebrush cover over time. Broadcast seeding failed to establish any sagebrush, except at the highest levels of crested wheatgrass control. Reducing crested wheatgrass did not influence density, cover, or size of sagebrush in the planted treatment, and therefore, crested wheatgrass control is probably unnecessary when using sagebrush seedlings. Herbaceous cover and density were generally less in the planted treatment, probably as a result of increased competition from sagebrush. This trade-off between sagebrush and herbaceous vegetation should be considered when developing plans for restoring sagebrush steppe. Our results suggest that planting sagebrush seedlings can increase the compositional and structural diversity in near-monocultures of crested wheatgrass and thereby improve habitat for sagebrush-associated wildlife. Planting native shrub seedlings may be a method to increase diversity in other monotypic stands of introduced grasses.     

Restoring the Sagebrush Component in Crested Wheatgrass–Dominated Communities

Monotypic stands of crested wheatgrass (Agropyron cristatum [L] Gaertm. and Agropyron desertorum [Fisch.] Schult.), an introduced grass, occupy vast expanses of the sagebrush steppe. Efforts to improve habitat for sagebrush-associated wildlife by establishing a diverse community of native vegetation in crested wheatgrass stands have largely failed. Instead of concentrating on a diversity of species, we evaluated the potential to restore the foundation species, Wyoming big sagebrush (Artemisia tridentata spp. wyomingensis [Beetle & A. Young] S. L. Welsh), to these communities. We investigated the establishment of Wyoming big sagebrush into six crested wheatgrass stands (sites) by broadcast seeding and planting seedling sagebrush across varying levels of crested wheatgrass control with glyphosate. Planted sagebrush seedlings survived at high rates (～ 70% planted sagebrush survival 3 yr postplanting), even without crested wheatgrass control. However, most attempts to establish sagebrush by broadcast seeding failed. Only at high levels of crested wheatgrass control did a few sagebrush plants establish from broadcasted seed. Sagebrush density and cover were greater with planting seedlings than broadcast seeding. Sagebrush cover, height, and canopy area were greater at higher levels of crested wheatgrass control. High levels of crested wheatgrass control also created an opportunity for exotic annuals to increase. Crested wheatgrass rapidly recovered after glyphosate control treatments, which suggests multiple treatments may be needed to effectively control crested wheatgrass. Our results suggest that planting sagebrush seedlings can structurally diversify monotypic crested wheatgrass stands to provide habitat for sagebrush-associated wildlife. Though this is not the full diversity of native functional groups representative of the sagebrush steppe, it is a substantial improvement over other efforts that have largely failed to alter these plant communities. We also hypothesize that planting sagebrush seedlings in patches or strips may provide a relatively inexpensive method to facilitate sagebrush recovery across vast landscapes where sagebrush has been lost.     

Longer-Term Evaluation of Revegetation of Medusahead-Invaded Sagebrush Steppe

Medusahead (Taeniatherum caput-medusae [L.] Nevski) and other exotic annual grasses have invaded millions of hectares of sagebrush (Artemisia L.) steppe. Revegetation of medusahead-invaded sagebrush steppe with perennial vegetation is critically needed to restore productivity and decrease the risk of frequent wildfires. However, it is unclear if revegetation efforts provide long-term benefits (fewer exotic annuals and more perennials). The limited literature available on the topic questions whether revegetation efforts reduce medusahead abundance beyond 2 or 3 yr. We evaluated revegetation of medusahead-invaded rangelands for 5 yr after seeding introduced perennial bunchgrasses at five locations. We compared areas that were fall-prescribed burned immediately followed by an imazapic herbicide treatment and then seeded with bunchgrasses 1 yr later (imazapic-seed) with untreated controls (control). The imazapic-seed treatment decreased exotic annual grass cover and density. At the end of the study, exotic annual grass cover and density were 2-fold greater in the control compared with the imazapic-seed treatment. The imazapic-seed treatment had greater large perennial bunchgrass cover and density and less annual forb (predominately exotic annuals) cover and density than the untreated control for the duration of the study. At the end of the study, large perennial bunchgrass density average 10 plant ? m^? 2 in the imazapic-seed treatment, which is comparable with intact sagebrush steppe communities. Plant available soil nitrogen was also greater in the imazapic-seed treatment compared with the untreated control for the duration of the study. The results of this study suggest that revegetation of medusahead-invaded sagebrush steppe can provide lasting benefits, including limiting exotic annual grasses.     

Eighty Years of Grazing by Cattle Modifies Sagebrush and Bunchgrass Structure

Grazing by cattle is ubiquitous across the sagebrush steppe; however, little is known about its effects on sagebrush and native bunchgrass structure. Understanding the effects of long-term grazing on sagebrush and bunchgrass structure is important because sagebrush is a keystone species and bunchgrasses are the dominant herbaceous functional group in these communities. To investigate the effects of long-term grazing on sagebrush and bunchgrass structure, we compared nine grazing exclosures with nine adjacent rangelands that were grazed by cattle in southeast Oregon. Grazing was moderate utilization (30 ? 45%) with altering season of use and infrequent rest. Long-term grazing by cattle altered some structural aspects of bunchgrasses and sagebrush. Ungrazed bunchgrasses had larger dead centers in their crowns, as well as greater dead fuel depths below and above the crown level compared with grazed bunchgrasses. This accumulation of dry fuel near the meristematic tissue may increase the probability of fire-induced mortality during a wildfire. Bunchgrasses in the ungrazed treatment had more reproductive stems than those in the long-term grazed treatment. This suggests that seed production of bunchgrasses may be greater in ungrazed areas. Sagebrush height and longest canopy diameter were 15% and 20% greater in the ungrazed compared with the grazed treatment, respectively. However, the bottom of the sagebrush canopy was closer to the ground in the grazed compared with the ungrazed treatment, which may provide better hiding cover for ground-nesting avian species. Sagebrush basal stem diameter, number of stems, amount of dead material in the canopy, canopy gap size, and number of canopy gaps did not differ between ungrazed and grazed treatments. Moderate grazing does not appear to alter the competitive relationship between a generally unpalatable shrub and palatable bunchgrasses. Long-term, moderate grazing appears to have minimal effects to the structure of bunchgrasses and sagebrush, other than reducing the risk of bunchgrass mortality during a fire event.     

Herbicide Protection Pods (HPPs) Facilitate Sagebrush and Bunchgrass Establishment under Imazapic Control of Exotic Annual Grasses

Revegetation of exotic annual grass−invaded rangelands is a primary objective of land managers following wildfires. Controlling invasive annual grasses is essential to increasing revegetation success; however, preemergent herbicides used to control annual grasses prohibit immediate seeding due to nontarget herbicide damage. Thus, seeding is often delayed 1 yr following herbicide application. This delay frequently allows for reinvasion of annual grasses, decreasing the success of revegetation efforts. Incorporating seeds into herbicide protection pods (HPPs) containing activated carbon (AC) permits concurrent high preemergent herbicide application and seeding because AC adsorbs and renders herbicides inactive. While HPPs have, largely in greenhouse studies, facilitated perennial bunchgrass emergence and early growth, their effectiveness in improving establishment of multiple species and functional groups in the field has not been assessed. Five bunchgrass species and two shrub species were seeded at two field sites with high imazapic application rates as bare seed and seed incorporated into HPPs. HPPs significantly improved establishment of sagebrush (Artemesia tridentata Nutt. Spp. wyomingensis Beetle & Young) and crested wheatgrass (Agropyron cristatum [L.] Gaertn.) over the 2-yr study. Three native perennial grass species were protected from herbicide damage by HPPs but had low establishment in both treatments. The two remaining shrub and grass species did not establish sufficiently to determine treatment effects. While establishment of native perennial bunchgrasses was low, this study demonstrates that HPPs can be used to protect seeded bunchgrasses and sagebrush from imazapic, prolonging establishment time in the absence of competition with annual grasses.     

Revegetation of Medusahead-Invaded Sagebrush Steppe

Medusahead (Taeniatherum caput-medusae [L.] Nevski) is an exotic annual grass invading western rangelands. Invasion by medusahead is problematic because it decreases livestock forage production, degrades wildlife habitat, reduces biodiversity, and increases fire frequency. Revegetation of medusahead-invaded sagebrush steppe is needed to increase ecosystem and economic productivity. Most efforts to revegetate medusahead-infested plant communities are unsuccessful because perennial bunchgrasses rarely establish after medusahead control. The effects of prescribed burning (spring or fall), fall imazapic application, and their combinations were evaluated for medusahead control and the establishment of seeded large perennial bunchgrasses. One growing season after treatments were applied, desert wheatgrass (Agropyron desertorum [Fisch. ex Link] Schult.) and squirreltail (Elymus elymoides [Raf.] Swezey) were drill seeded into treatment plots, except for the control treatment. Vegetation characteristics were measured for 2 yr postseeding (second and third year post-treatment). Medusahead was best controlled when prescribed burned and then treated with imazapic (P < 0.05). These treatments also had greater large perennial bunchgrass cover and density compared to other treatments (P < 0.05). The prescribed burned followed by imazapic application had greater than 10- and 8-fold more perennial bunchgrass cover and density than the control treatment, respectively. Prescribed burning, regardless of season, was not effective at controlling medusahead or promoting establishment of perennial bunchgrasses. The results of this study question the long-term effectiveness of using imazapic in revegetation efforts of medusahead-infested sagebrush steppe without first prescribed burning the infestation. Effective control of medusahead appears to be needed for establishment of seeded perennial bunchgrasses. The results of this study demonstrate that seeding desert wheatgrass and squirreltail can successfully revegetate rangeland infested with medusahead when medusahead has been controlled with prescribed fire followed by fall application of imazapic.     

Site,Competition, and Plant Stock Influence Transplant Success of Wyoming Big Sagebrush

Within the sagebrush steppe ecosystem, sagebrush plants influence a number of ecosystem properties, including nutrient distribution, plant species diversity, soil moisture, and temperature, and provide habitat for a wide variety of wildlife species. Recent increases in frequency and size of wildfires and associated annual grass expansion within the Wyoming big sagebrush alliance have increased the need for effective sagebrush restoration tools and protocols. Our objectives were to quay the success of Wyoming big sagebrush transplants relative to transplant stock (nursery seedlings vs. wildlings) across different ecological sites and vegetation types and to test the hypothesis that reduction of herbaceous vegetation would increase survival of transplanted sagebrush. We used a randomized block (reps = 5) design at each of three sites—1) cheatgrass dominated, 2) native plant dominated, and 3) crested wheatgrass dominated—near Elko, Nevada. Treatments included plant stock (nursery stock or locally harvested wildlings) and herbicide (glyphosate) to reduce competition from herbaceous vegetation. Transplants were planted in the spring of 2009 and 2010 and monitored for survival. Data were analyzed for site and treatment effects using mixed-model ANOVA. Surviving plant density at and 2 yr postplanting was generally highest (up to 3-fold) on the native site (P < 0.05). Density of surviving transplants was almost 3-fold higher for nursery stock on most sites for the 2009 planting, but differences in survival by planting stock were minimal for the 2010 planting. Glyphosate application increased surviving plant density up to 300% (depending on site) for both years of planting. High labor and plant material investments (relative to traditional drilling or broadcasting) may limit the size of projects for which sagebrush transplants are practical, but these costs may be partially offset by high success relative to traditional methods. Our data indicate that sagebrush transplants can be effective for establishing sagebrush on depleted sites.     

Native Vegetation Composition in Crested Wheatgrass in Northwestern Great Basin

Crested wheatgrass, an introduced perennial bunchgrass, has been seeded extensively on the rangelands of western North America. There is a perception that this species is very competitive and that it forms monoculture or low diversity stands where successfully seeded. However, there is limited information on species composition in sites previously seeded to crested wheatgrass. We measured native vegetation and environmental characteristics in areas seeded with crested wheatgrass across the northwestern Great Basin. Plant community composition within these crested wheatgrass stands was variable, from seedings that were near monocultures of crested wheatgrass to those that contained more diverse assemblages of native vegetation, especially shrubs. Environmental factors explained a range of functional group variability from 0% of annual grass density to 56% of large native bunchgrass density. Soil texture appeared to be an important environmental characteristic in explaining vegetation cover and density. Native vegetation was, for all functional groups, positively correlated with soils lower in sand content. Our results suggest environmental differences explain some of the variability of native vegetation in crested wheatgrass stands, and this information will be useful in assessing the potential for native vegetation to co-occupy sites seeded with crested wheatgrass. This research also suggests that crested wheatgrass seedings do not always remain in near monoculture vegetation states as seedings substantially varied in native vegetation composition and abundance with some seeded areas having a more diverse assemblage of native vegetation. In half the sites, there were five or more perennial herbaceous species and 63% of sites contained Wyoming big sagebrush. Although not exclusively true, species most commonly encountered in crested wheatgrass seedings are those that are able to minimize competition with crested wheatgrass via temporal (i.e., Sandberg bluegrass, annual forbs, annual grasses) or spatial (i.e., shrubs) differentiation in resource use.     

Crested Wheatgrass Control and Native Plant Establishment in Utah

Effective control methods need to be developed to reduce crested wheatgrass (Agropyron cristatum [L.] Gaertner) monocultures and promote the establishment of native species. This research was designed to determine effective ways to reduce crested wheatgrass and establish native species while minimizing weed invasion. We mechanically (single- or double-pass disking) and chemically (1.1 L · ha^?1 or 3.2 L · ha^?1 glyphosate–Roundup Original Max) treated two crested wheatgrass sites in northern Utah followed by seeding native species in 2005 and 2006. The study was conducted at each site as a randomized block split plot design with five blocks. Following wheatgrass-reduction treatments, plots were divided into 0.2-ha subplots that were either unseeded or seeded with native plant species using a Truax Rough Rider rangeland drill. Double-pass disking in 2005 best initially controlled wheatgrass and decreased cover from 14% to 6% at Lookout Pass and from 14% to 4% at Skull Valley in 2006. However, crested wheatgrass recovered to similar cover percentages as untreated plots 2–3 yr after wheatgrass-reduction treatments. At the Skull Valley site, cheatgrass cover decreased by 14% on herbicide-treated plots compared to an increase of 33% on mechanical-treated plots. Cheatgrass cover was also similar on undisturbed and treated plots 2 yr and 3 yr after wheatgrass-reduction treatments, indicating that wheatgrass recovery minimized any increases in weed dominance as a result of disturbance. Native grasses had high emergence after seeding, but lack of survival was associated with short periods of soil moisture availability in spring 2007. Effective wheatgrass control may require secondary treatments to reduce the seed bank and open stands to dominance by seeded native species. Manipulation of crested wheatgrass stands to restore native species carries the risk of weed invasion if secondary treatments effectively control the wheatgrass and native species have limited survival due to drought.     

Evaluating the Effectiveness of Low Soil-Disturbance Treatments for Improving Native Plant Establishment in Stable Crested Wheatgrass Stands

Past seedings of crested wheatgrass (Agropyron cristatum [L.] Gaertn. and A. desertorum [Fisch. ex Link] Schult.) have the potential to persist as stable, near-monospecific stands, thereby necessitating active intervention to initiate greater species diversity and structural complexity of vegetation. However, the success of suppression treatments and native species seedings is limited by rapid recovery of crested wheatgrass and the influx of exotic annual weeds associated with herbicidal control and mechanical soil disturbances. We designed a long-term study to evaluate the efficacy of low-disturbance herbicide and seed-reduction treatments applied together or alone and either once or twice before seeding native species. Consecutive herbicide applications reduced crested wheatgrass density for up to 6 ? 7 yr depending on study site, but seed removal did not reduce crested wheatgrass abundance; however, in some cases combining herbicide application with seed removal significantly increased densities of seeded species relative to herbicide alone, especially for the site with a more northern aspect. Although our low-disturbance treatments avoided the pitfalls of secondary exotic weed influx, we conclude that crested wheatgrass suppression must reduce established density to values much lower than 4 ? 7 plants/m², a range that has not been obtained by ours or any previous study, in order to diminish its competitive influence on seed native species. In addition, our results indicated that site differences in environmental stress and land-use legacies exacerbate the well-recognized limitations of native species establishment and persistence in the Great Basin region.     

Establishment and Trends in Persistence of Selected Perennial Cool-Season Grasses in Western United States

Restoring western US rangelands from a site dominated by invasive annuals, such as cheatgrass and medusahead, to a diverse, healthy, perennial plant ? dominated ecosystem can be difficult with native grasses. This study describes the establishment and trends in persistence (plant/m²) of native grass cultivars and germplasm compared with typically used crested and Siberian wheatgrasses at four locations in Idaho (one), Wyoming (one), and Utah (two) that range in mean average annual precipitation (MAP) from 290 to 415 mm. Sites were cultivated and fallowed 1 yr before planting using two glyphosate applications to control weeds. We monitored seedling establishment of 10 perennial cool-season grass species and plant persistence over 5 yr. Precipitation during the seeding year varied with the Utah sites locations reviving below MAP (4% and 14%), while the Wyoming and Idaho sites received above MAP at 8% and 26%, respectively. Across these four sites, native grass seedling establishment of bottlebrush squirreltail (29 ± 0.08 [standard error] seedling/m²), bluebunch (28 ± 0.05), slender (30 ± 0.05), and Snake River wheatgrasses (28 ± 0.08) was similar to “Vavilov II” Siberian wheatgrass (36 ± 3.20). By yr 5, western, Snake River, and thickspike wheatgrasses were the only native grasses to have plant densities similar to Vavilov II (37 ± 0.29) Siberian and “Hycrest II” (36 ± 0.29) crested wheatgrasses. On sites receiving between 290 and 415 mm MAP, our data suggest that native grasses are able to establish but in general lack the ability to persist except for western, Snake River, and thickspike wheatgrasses, which had plant densities similar to crested and Siberian wheatgrasses after 5 yr.     

Bunchgrass Root Abundances and Their Relationship to Resistance and Resilience of Burned Shrub-Steppe Landscape

Invasion of exotic annual grasses (EAG) and increased wildfire have led to an emphasis on managing rangeland plant communities for resistance to invasion and resilience to disturbances. In sagebrush steppe and similar rangelands, perennial bunchgrasses and particularly their roots are hypothesized to be primary contributors to resistance and resilience. We asked how bunchgrass root abundance relates to annual grass invasion and aboveground indicators of bunchgrass vigor that are more readily measured, such as plant height. We used a standardized US Department of Agriculture protocol for root measurement in 445 excavations made in 2016 ? 2018 across a topographically and ecologically varied region of sagebrush steppe burned in the 2015 Soda fire in the Northern Great Basin, United States. Nearly all (99%) bunchgrasses, including seedlings, had deeper roots than the surrounding annual grasses (mean depth of annuals = 6.8 ± 3.3 cm), and 88% of seedlings remained rooted in response to the “tug test” (uprooting resistance to ~ 1 kg of upward pull on shoot), with smaller plants (mean height and basal diameters < 20 cm and < 2 cm, respectively) more likely to fail the test regardless of their root abundance. Lateral roots of bunchgrasses were scarcer in larger basal gaps (interspace between perennials) but were surprisingly not directly related to cover of surrounding EAG. However, EAG cover increased with the size of basal gaps and decreased with greater basal diameter of bunchgrass (in addition to prefire EAG abundance), albeit with a low r². These results provide some support for 1) the importance of basal gaps and bunchgrass diameters as indicators of both vulnerability to annual grass invasion and bunchgrass root abundance and 2) the need for more detailed methods for root measurement than used here in order to substantiate their usefulness in understanding rangeland resistance and resilience.     

Herbaceous Biomass Response to Prescribed Fire in Juniper-Encroached Sagebrush Steppe

Western juniper (Juniperus occidentalis Hook.) has expanded into sagebrush steppe plant communities the past 130 ? 150 yr in the northern Great Basin. The increase in juniper reduces herbage and browse for livestock and big game. Information on herbaceous yield response to juniper control with fire is limited. We measured herbaceous standing crop and yield by life form in two mountain big sagebrush communities (MTN1, MTN2) and a Wyoming/basin big sagebrush (WYOBAS) community for 6 yrs following prescribed fire treatments to control western juniper. MTN1 and WYOBAS communities were early-successional (phase 1) and MTN2 communities were midsuccessional (phase 2) woodlands before treatment. Prescribed fires killed all juniper and sagebrush in the burn units. Total herbaceous and perennial bunchgrass yields increased 2 to 2.5-fold in burn treatments compared with unburned controls. Total perennial forb yield did not differ between burns and controls in all three plant communities. However, tall perennial forb yield was 1.6- and 2.5-fold greater in the WYOBAS and MTN2 burned sites than controls. Mat-forming perennial forb yields declined by 80 ? 90% after burning compared with controls. Cheatgrass yield increased in burned WYOBAS and MTN2 communities and at the end of the study represented 10% and 22% of total yield, respectively. Annual forbs increased with burning and were mainly composed of native species in MTN1 and MTN2 communities and non-natives in WYOBAS communities. Forage availability for livestock and wild ungulates more than doubled after burning. The additional forage provided on burned areas affords managers greater flexibility to rest and treat additional sagebrush steppe where juniper is expanding, as well as rest or defer critical seasonal habitat for wildlife.     

Seedling Interference and Niche Differentiation Between Crested Wheatgrass and Contrasting Native Great Basin Species

Interference from crested wheatgrass (Agropyron cristatum [L.] Gaertn.) seedlings is considered a major obstacle to native species establishment in rangeland ecosystems; however, estimates of interference at variable seedling densities have not been defined fully. We conducted greenhouse experiments using an addition-series design to characterize interference between crested wheatgrass and four key native species. Crested wheatgrass strongly interfered with the aboveground growth of Wyoming big sagebrush (Artemisia tridentata Nutt. subsp. wyomingensis Beetle & Young), rubber rabbitbrush (Ericameria nauseosa [Pall. ex Pursh] G. L. Nesom & Baird subsp. consimilis [Greene] G. L. Nesom & Baird), and to a lesser extent with bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Löve). Alternatively, bottlebrush squirreltail (Elymus elymoides [Raf.] Swezey subsp. californicus [J. G. Sm.] Barkworth) and crested wheatgrass had similar effects on each other’s growth, and interference ratios were near 1.0. Results indicate that the native grasses more readily establish in synchrony with crested wheatgrass than these native shrubs, but that once established, the native shrubs are more likely to coexist and persist with crested wheatgrass because of high niche differentiation (e.g., not limited by the same resource). Results also suggest that developing strategies to minimize interference from crested wheatgrass seedlings emerging from seed banks will enhance the establishment of native species seeded into crested wheatgrass–dominated communities.     

Freezing Stress Influences Emergence of Germinated Perennial Grass Seeds

In sagebrush rangelands perennial bunchgrasses are typically seeded in fall and a high proportion of planted seeds germinate prior to winter onset but fail to emerge in spring. Our objectives were to evaluate freezing tolerance of germinated but nonemergent bluebunch wheatgrass seeds under laboratory conditions. We used data from a 2-yr pilot study to determine overwinter freezing temperature and duration for soils in southeastern Oregon. We then conducted two experiments to assess freezing tolerance. In experiment 1, bluebunch wheatgrass seeds were planted in control pots and compared to seeds planted at early, mid, or late postgermination stages. Pots from each treatment were placed in a grow room maintained at 12 h 40 min light/11 h 20 min dark photoperiod, with a constant temperature of 22°C for 30 d either immediately or following a 30-d freeze. In experiment 2, germinated bluebunch wheatgrass seeds were planted in pots that were left nonfrozen or were frozen for a specified duration prior to a 30-d period in the grow room. Emergence density and tillers · seedling^?1 were quantified for both experiments. The number of days per year for freezing soil conditions in the pilot study ranged yearly from 25 to 51; maximum duration of continuous freezing was 16.5 and 11.2 d. Freezing reduced or eliminated seedling emergence at all postgermination stages (P < 0.001) and tiller density was reduced by at least 50% (P < 0.001). Maximum reduction in seedling density (P < 0.001) was realized within 4 d of initiation of freezing and tillers · seedling^?1 were reduced 30–70% with > 6 d of freezing (P = 0.001). Our data indicate that freezing-associated mortality of germinated but nonemergent bluebunch wheatgrass seedlings can be extremely high and suggest that management practices to reduce prewinter germination of seeds could improve subsequent emergence and seeding success.     

Long-Term Persistence of Cool-Season Grasses Planted to Suppress Broom Snakeweed,Downy Brome,and Weedy Forbs

Invasive plants are spreading throughout arid and semiarid rangelands of western North America. Long-lived perennial plants that can persist under harsh environmental conditions are needed to compete with invasive species. The objective of this study was to conduct a long-term evaluation of native and introduced grass species planted to suppress and prevent reinvasion of downy brome (Bromus tectorum L.), snakeweed (Gutierrezia sarothrae [Pursh] Britt. & Rusby), and annual forbs. Seeding treatments comprised three introduced grasses: crested wheatgrass (Agropyron cristatum [L.] Gaertner × A. desertorum [Fisch. Ex Link] Schultes), pubescent wheatgrass (Elytrigia intermedia spp. trichophorum [Host] Beauv.), and Russian wildrye (Psathyrostachys junceus [Fisch.] Nevski); a mix of these introduced grass species, three native grasses: bluebunch wheatgrass (Pseudoroegneria spicata [Pursh]), western wheatgrass (Pascopyrum smithii [Rybd.] A. Löve), and squirreltail (Elymus multisetus [J.G. Sm.] Jones); and a mix of these native grass species, or forage kochia (Bassia prostrata [L.] A.J. Scott). The treatments were seeded in October 2003. Frequency and biomass were measured in 2015 and 2017 in Howell, Utah and in 2015 and 2016 in Nephi, Utah. Crested wheatgrass persisted at both locations (> 62% frequency) along with the rhizomatous grass species, pubescent (> 65%) and western wheatgrasses (> 72%). Russian wildrye was still present at Howell (30%) with little remaining at Nephi (7%). Squirreltail frequency was 13% at Howell and 12% at Nephi. Bluebunch wheatgrass was no longer present at either location (< 1%). Forage kochia remained at Nephi (36%) with little remaining at Howell (4%). Downy brome was present at both locations and was suppressed relative to control plots, at Nephi, by crested wheatgrass and the introduced grass mix (< 9%). Downy brome was > 93% in all plots, at Howell, in 2017. In summary, crested, pubescent, and western wheatgrasses were able to persist over 12 yr at both locations.     

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.     

Grass Yields Under Clipping and Watering Explain Varied Efficacy of Management Intensive Grazing on Rangelands

Management intensive grazing (MIG) may not maximize plant productivity on rangelands because of morphophysiological traits of grassland vegetation. We examined defoliation and moisture effects on the biomass yield of rhizomatous and caespitose grass pairs that were either phylogenetically similar or of similar agroclimatic adaptation, including two agronomic grasses. From relatively low to high moisture regime adaptation, species pairs included western wheatgrass (Pascopyrum smithii [Rydb.] A. Love) and needle-and-thread (Hesperostipa comata [Trin. & Rupr.] Barkw.), northern wheatgrass (Elymus lanceolatus [Scribn. & J.G. Sm.]) and western porcupine grass (H. curtiseta [Hitchc.] Barkw.), plains and foothills rough fescue (Festuca hallii [Vasey] and F. campestris Rydb.), and smooth and meadow brome (Bromus inermis Leyss. and B. riparius Rehm). Response variables were shoot yield, root-shoot ratio, and water-use efficiency. We hypothesized that caespitose grasses, regardless of their origin or adaptation to agroclimate regime, would respond more determinately in biomass accumulation. Defoliation effects on shoot biomass were more pronounced under high moisture. Low intensity ? high frequency defoliation yielded similarly to deferred controls in all grasses, and the same was true for high-intensity ? low-frequency (HILF) defoliation in 1 rhizomatous grass. Three of the 4 rhizomatous grasses and 1 caespitose grass yielded greater under HILF defoliation compared with high-intensity ? high-frequency defoliation. Caespitose grasses allocated more biomass to roots under low moisture conditions. Water-use efficiency decreased under high moisture conditions and more intense and/or frequent defoliation and peaked in agronomic grasses. Overall, our results suggested that growth patterns corresponded more with phylogenetic similarity as opposed to growth form. A conceptual model from these results showed that across all species, only the introduced bromes generated greater biomass under HILF defoliation, and this may explain why past research consistently concludes that MIG does not enhance plant productivity on rangelands.     

Prescribed Summer Fire and Seeding Applied to Restore Juniper-Encroached and Exotic Annual Grass-Invaded Sagebrush Steppe

Western juniper (Juniperus occidentalis Hook.) encroachment and exotic annual grass (medusahead [Taeniatherum caput-medusae L. Nevski] and cheatgrass [Bromus tectorum L.]) invasion of sagebrush (Artemisia L.) communities decrease ecosystem services and degrade ecosystem function. Traditionally, these compositional changes were largely confined to separate areas, but more sagebrush communities are now simultaneously being altered by juniper and exotic annual grasses. Few efforts have evaluated attempts to restore these sagebrush communities. The Crooked River National Grassland initiated a project to restore juniper-encroached and annual grass-invaded sagebrush steppe using summer (mid-July) applied prescribed fires and postfire seeding. Treatments were unburned, burned, burned and seeded with a native seed mix, and burned and seeded with an introduced seed mix. Prescribed burning removed all juniper and initially reduced medusahead cover but did not influence cheatgrass cover. Neither the native nor introduced seed mix were successful at increasing large bunchgrass cover, and 6 yr post fire, medusahead cover was greater in burned treatments compared with the unburned treatment. Large bunchgrass cover and biological soil crusts were less in treatments that included burning. Exotic forbs and bulbous bluegrass (Poa bulbosa L.), an exotic grass, were greater in burned treatments compared with the unburned treatment. Sagebrush communities that are both juniper encroached and exotic annual grass invaded will need specific management of both juniper and annual grasses. We suggest that additional treatments, such as pre-emergent herbicide control of annuals and possibly multiple seeding events, are necessary to restore these communities. We recommend an adaptive management approach in which additional treatments are applied on the basis of monitoring data.