Soil Resources Influence Vegetation and Response to Fire and Fire-Surrogate Treatments in Sagebrush-Steppe Ecosystems

Current paradigm suggests that spatial and temporal competition for resources limit an exotic invader, cheatgrass (Bromus tectorum L.), which once established, alters fire regimes and can result in annual grass dominance in sagebrush steppe. Prescribed fire and fire surrogate treatments (mowing, tebuthiuron, and imazapic) are used to reduce woody fuels and increase resistance to exotic annuals, but may alter resource availability and inadvertently favor invasive species. We used four study sites within the Sagebrush Steppe Treatment Evaluation Project (SageSTEP) to evaluate 1) how vegetation and soil resources were affected by treatment, and 2) how soil resources influenced native herbaceous perennial and exotic annual grass cover before and following treatment. Treatments increased resin exchangeable NH₄⁺, NO₃^-, H₂PO₄^-, and K⁺, with the largest increases caused by prescribed fire and prolonged by application of imazapic. Burning with imazapic application also increased the number of wet growing degree days. Tebuthiuron and imazapic reduced exotic annual grass cover, but imazapic also reduced herbaceous perennial cover when used with prescribed fire. Native perennial herbaceous species cover was higher where mean annual precipitation and soil water resources were relatively high. Exotic annual grass cover was higher where resin exchangeable H₂PO₄^- was high and gaps between perennial plants were large. Prescribed fire, mowing, and tebuthiuron were successful at increasing perennial herbaceous cover, but the results were often ephemeral and inconsistent among sites. Locations with sandy soil, low mean annual precipitation, or low soil water holding capacity were more likely to experience increased exotic annual grass cover after treatment, and treatments that result in slow release of resources are needed on these sites. This is one of few studies that correlate abiotic variables to native and exotic species cover across a broad geographic setting, and that demonstrates how soil resources potentially influence the outcome of management treatments.     

Resilience and Resistance of Sagebrush Ecosystems: Implications for State and Transition Models and Management Treatments

In sagebrush ecosystems invasion of annual exotics and expansion of piñon (Pinus monophylla Torr. and Frem.) and juniper (Juniperus occidentalis Hook., J. osteosperma [Torr.] Little) are altering fire regimes and resulting in large-scale ecosystem transformations. Management treatments aim to increase resilience to disturbance and enhance resistance to invasive species by reducing woody fuels and increasing native perennial herbaceous species. We used Sagebrush Steppe Treatment Evaluation Project data to test predictions on effects of fire vs. mechanical treatments on resilience and resistance for three site types exhibiting cheatgrass (Bromus tectorum L.) invasion and/or piñon and juniper expansion: 1) warm and dry Wyoming big sagebrush (WY shrub); 2) warm and moist Wyoming big sagebrush (WY PJ); and 3) cool and moist mountain big sagebrush (Mtn PJ). Warm and dry (mesic/aridic) WY shrub sites had lower resilience to fire (less shrub recruitment and native perennial herbaceous response) than cooler and moister (frigid/xeric) WY PJ and Mtn PJ sites. Warm (mesic) WY Shrub and WY PJ sites had lower resistance to annual exotics than cool (frigid to cool frigid) Mtn PJ sites. In WY shrub, fire and sagebrush mowing had similar effects on shrub cover and, thus, on perennial native herbaceous and exotic cover. In WY PJ and Mtn PJ, effects were greater for fire than cut-and-leave treatments and with high tree cover in general because most woody vegetation was removed increasing resources for other functional groups. In WY shrub, about 20% pretreatment perennial native herb cover was necessary to prevent increases in exotics after treatment. Cooler and moister WY PJ and especially Mtn PJ were more resistant to annual exotics, but perennial native herb cover was still required for site recovery. We use our results to develop state and transition models that illustrate how resilience and resistance influence vegetation dynamics and management options.     

Vegetation Response to Mowing Dense Mountain Big Sagebrush Stands

A decrease in fire frequency and past grazing practices has led to dense mountain big sagebrush (Artemisia tridentata Nutt. subsp. vaseyana [Rydb.] Beetle) stands with reduced herbaceous understories. To reverse this trend, sagebrush-reducing treatments often are applied with the goal of increasing herbaceous vegetation. Mechanical mowing is a sagebrush-reducing treatment that commonly is applied; however, information detailing vegetation responses to mowing treatments generally are lacking. Specifically, information is needed to determine whether projected increases in perennial grasses and forbs are realized and how exotic annual grasses respond to mowing treatments. To answer these questions, we evaluated vegetation responses to mowing treatments in mountain big sagebrush plant communities at eight sites. Mowing was implemented in the fall of 2007 and vegetation characteristics were measured for 3 yr post-treatment. In the first growing season post-treatment, there were few vegetation differences between the mowed treatment and untreated control (P > 0.05), other than sagebrush cover being reduced from 28% to 3% with mowing (P < 0.001). By the second growing season post-treatment, perennial grass, annual forb, and total herbaceous vegetation were generally greater in the mowed than control treatment (P < 0.05). Total herbaceous vegetation production was increased 1.7-fold and 1.5-fold with mowing in the second and third growing seasons, respectively (P < 0.001). However, not all plant functional groups increased with mowing. Perennial forbs and exotic annual grasses did not respond to the mowing treatment (P > 0.05). These results suggest that the abundance of sagebrush might not be the factor limiting some herbaceous plant functional groups, or they respond slowly to sagebrush-removing disturbances. However, this study suggests that mowing can be used to increase herbaceous vegetation and decrease sagebrush in some mountain big sagebrush plant communities without promoting exotic annual grass invasion.     

Plant Community and Target Species Affect Responses to Restoration Strategies

Increases in Kentucky bluegrass and smooth bromegrass on northern Great Plains rangelands have the potential to negatively impact ecosystem function, lower plant diversity, and alter seasonal forage distribution, but control strategies are lacking in the region. A project was initiated on a heavily invaded 16-ha grassland that had not been grazed or hayed for at least 20 yr. Five restoration treatments and a control were initiated in 2003 on communities dominated by 1) smooth bromegrass (Bromus inermis Leyss.), 2) Kentucky bluegrass (Poa pratensis L.), 3) warm-season native grasses, 4) a mix of introduced species, and 5) smooth bromegrass and Kentucky bluegrass. Restoration treatments were 1) late-April burn, 2) late-April burn followed by imazapic at 511.62 mL ai · ha^?1, 3) imazapic only at the same rate, 4) mowing, 5) mowing followed by litter removal, and 6) control. We found that treatment responses were affected by target species, community category, and year. Generally, burning followed by the herbicide imazapic reduced Kentucky bluegrass in the species composition, but smooth brome was reduced by mowing followed by raking. Burning followed by imazapic reduced live grass biomass in all community categories except the native the year following treatment, but by the third year of the study live grass biomass was maintained across all treatments. In the third year of the study, responses of Kentucky bluegrass, other invasive species, and native grasses to restoration treatment differed depending on community. The use of burning plus imazapic was promising for control of Kentucky bluegrass but its use by producers may be limited by yield reductions in early years. Our data suggest management strategies should vary depending on whether the goal is to reduce one or several invaders, specific invader identity, and community type in which the invader is growing. We also found that the most effective strategy was an adaptive management approach, one where treatments are chosen in response to changes in community composition and depending on resource conditions.     

Mowing Wyoming Big Sagebrush Communities With Degraded Herbaceous Understories: Has a Threshold Been Crossed?

Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis &lsqb;Beetle & A. Young] S.L. Welsh) plant communities with degraded native herbaceous understories occupy vast expanses of the western United States. Restoring the native herbaceous understory in these communities is needed to provide higher-quality wildlife habitat, decrease the risk of exotic plant invasion, and increase forage for livestock. Though mowing is commonly applied in sagebrush communities with the objective of increasing native herbaceous vegetation, vegetation response to this treatment in degraded Wyoming big sagebrush communities is largely unknown. We compared mowed and untreated control plots in five Wyoming big sagebrush plant communities with degraded herbaceous understories in eastern Oregon for 3 yr posttreatment. Native perennial herbaceous vegetation did not respond to mowing, but exotic annuals increased with mowing. Density of cheatgrass (Bromus tectorum L.), a problematic exotic annual grass, was 3.3-fold greater in the mowed than untreated control treatment in the third year posttreatment. Annual forb cover, largely consisting of exotic species, was 1.8-fold greater in the mowed treatment compared to the untreated control in the third year posttreatment. Large perennial grass cover was not influenced by mowing and remained below 2%. Mowing does not appear to promote native herbaceous vegetation in degraded Wyoming big sagebrush plant communities and may facilitate the conversion of shrublands to exotic annual grasslands. The results of this study suggest that mowing, as a stand-alone treatment, does not restore the herbaceous understory in degraded Wyoming big sagebrush plant communities. We recommend that mowing not be applied in Wyoming big sagebrush plant communities with degraded understories without additional treatments to limit exotic annuals and promote perennial herbaceous vegetation.     

A Synopsis of Short-Term Response to Alternative Restoration Treatments in Sagebrush-Steppe: The SageSTEP Project

The Sagebrush Steppe Treatment Evaluation Project (SageSTEP) is an integrated long-term study that evaluates ecological effects of alternative treatments designed to reduce woody fuels and to stimulate the herbaceous understory of sagebrush steppe communities of the Intermountain West. This synopsis summarizes results through 3 yr posttreatment. Woody vegetation reduction by prescribed fire, mechanical treatments, or herbicides initiated a cascade of effects, beginning with increased availability of nitrogen and soil water, followed by increased growth of herbaceous vegetation. Response of butterflies and magnitudes of runoff and erosion closely followed herbaceous vegetation recovery. Effects on shrubs, biological soil crust, tree cover, surface woody fuel loads, and sagebrush-obligate bird communities will take longer to be fully expressed. In the short term, cool wet sites were more resilient than warm dry sites, and resistance was mostly dependent on pretreatment herbaceous cover. At least 10 yr of posttreatment time will likely be necessary to determine outcomes for most sites. Mechanical treatments did not serve as surrogates for prescribed fire in how each influenced the fuel bed, the soil, erosion, and sage-obligate bird communities. Woody vegetation reduction by any means resulted in increased availability of soil water, higher herbaceous cover, and greater butterfly numbers. We identified several trade-offs (desirable outcomes for some variables, undesirable for others), involving most components of the study system. Trade-offs are inevitable when managing complex natural systems, and they underline the importance of asking questions about the whole system when developing management objectives. Substantial spatial and temporal heterogeneity in sagebrush steppe ecosystems emphasizes the point that there will rarely be a “recipe” for choosing management actions on any specific area. Use of a consistent evaluation process linked to monitoring may be the best chance managers have for arresting woodland expansion and cheatgrass invasion that may accelerate in a future warming climate.     

Integrated Grazing and Prescribed Fire Restoration Strategies in a Mesquite Savanna: I. Vegetation Responses

This study evaluated the efficacy of prescribed fire applied within landscape-scale rotational grazing treatments to reduce mesquite (Prosopis glandulosa Torr.) encroachment and restore herbaceous productivity and cover. One-herd, multiple-paddock rotational grazing was used to accumulate herbaceous fine fuel for fires via prefire deferment and to provide periodic postfire deferment for grass recovery. Treatments were an unburned continuous-grazed control, a four-paddock-1 herd system with fire (4:1F), and an eight-paddock-1 herd system with fire (8:1F), with two replicates per treatment (1 294–2 130 ha per replicate). The management plan was to burn 25% of each system (one paddock in the 4:1F; two paddocks in the 8:1F treatments) and defer grazing during all or portions of the 9 mo (May to January) prior to burning. Deferral was “internalized” by grazing on the remaining 75% of each treatment without reducing stocking rate determined for the entire system. Mesquite cover increased on clay-loam soils from 22% to 40% in unburned paddocks over 7 yr (1995–2001). This increase, coupled with extended drought, reduced fine fuel amounts for fire and limited the number and intensity of fires that were applied. It was possible to burn one paddock in the 8:1F treatment (12.5% of total area), but not in the 4:1F treatment (25% of total area) during drought. Fires reduced mesquite and cactus (Opuntia spp.) cover by 25–79% and 24–56%, respectively, but cover of these species increased to prefire levels within 6 yr. All fires reduced (P ≤ 0.05) total herbaceous biomass for 1 yr postfire. The 8:1F treatment increased (P ≤ 0.05) grass biomass on loamy-bottom soils and reduced (P ≤ 0.05) bare ground on clay-loam and loamy-bottom soils in unburned paddocks compared to the unburned continuously grazed control. The 8:1F treatment, through internalized grazing deferment, facilitated the application of fire to reduce woody cover during extended drought without degrading the herbaceous understory.     

Herbicide-Assisted Restoration of Great Basin Sagebrush Steppe Infested With Medusahead and Downy Brome

Downy brome or cheatgrass (Bromus tectorum) and medusahead (Taeniatherum caput-medusae) are the most problematic invasive annual grasses in rangelands of the western United States, including sagebrush communities that provide habitat to sage grouse. Rehabilitation of infested sites requires effective weed control strategies combined with seeding of native plants or desirable competitive species. In this study, we evaluated the effect of three fall-applied pre-emergence herbicides (imazapic, rimsulfuron, and chlorsulfuron + sulfometuron), and one spring-applied postemergence herbicide (glyphosate) on the control of downy brome and medusahead and the response of seeded perennial species and resident vegetation in two sagebrush communities in northeastern California. All pre-emergence treatments gave > 93% control of both invasive species at both sites in the first year. Glyphosate was less consistent, giving > 94% control at one site and only 61% control of both species at the other site. Imazapic was the only herbicide to maintain good control (78–88%) of both species 2 yr after treatment. No herbicide caused detectible long-term damage to either perennial grasses or annual forbs, and imazapic treatment resulted in an increase in resident native forb cover 3 yr after treatment. Broadcast seeding with or without soil incorporation did not result in successful establishment of perennial species, probably due to below-average precipitation in the year of seeding. These results indicate that several chemical options can give short-term control of downy brome and medusahead. Over the course of the study, imazapic provided the best management of both invasive annual grasses while increasing native forb cover.     

Effects of Fire Exclusion on Previously Fire-Managed Semiarid Savanna Ecosystem

Long-term (> 100 yr) fire exclusion is associated with numerous ecological consequences in grasslands and savannas, including transitions into shrub- or tree-dominated systems. Several studies have reported differences in woody vegetation after multiple fires among burned and unburned rangelands, but none have reported the impacts of fire exclusion after a period of fire management. We evaluated effects of fire exclusion on herbaceous and woody canopy cover and herbaceous biomass in semiarid savanna of southwest Texas in pastures with known burn histories. Pastures were burned in summer and winter in 1994, 2000, and 2006, followed by 11 yr of fire exclusion. Between 2006 and 2017, woody subcanopy increased (5–21%) in all treatments while overstory canopy remained unchanged. Herbaceous cover decreased (5–18%) in all treatments but remained higher in burned treatments. From 2006 to 2017, herbaceous biomass declined in all treatments by > 650 kg·ha⁻¹ and was not statistically different among treatments. These trends support other research demonstrating the importance of historical mean fire return interval in maintaining grasslands and savannas.     

Evaluating Mechanical Treatments and Seeding of a Wyoming Big Sagebrush Community 10 Yr Post Treatment

Increased cover of perennial grasses and forbs would increase the wildlife and forage value of many Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis Beetle & Young) communities, as well as increase their resistance to weeds. We compared six mechanical treatments in conjunction with seeding a Wyoming big sagebrush community in northern Utah over a 10-yr period. The treatments included disk plow followed by land imprinter, one-way Ely chain, one- and two-way pipe harrow, all applied in fall, and meadow aerator applied in fall and spring. A mixture of native and introduced grasses and forbs was broadcast seeded at 18.3 kg PLS ha^? 1 after the disk and before the imprinter and all other treatments. The experiment was installed in three randomized blocks, and density and cover data were collected before treatment in 2001 and 1, 2, 5, and 10 yr after treatment. All treatments initially reduced sagebrush and residual herbaceous cover and increased seeded species cover compared with the untreated control. By 10 yr after treatment, sagebrush cover was 24.5% ± 0.35% on the control, 1.6% ± 0.28% on the disk imprinter treatment, and 11.7% ± 0.79% on all other treatments. At that time, seeded grass cover was 16.5% ± 1.22% on the disk imprinter treatment and an average of 2% ± 0.1% on all other mechanical treatments. Sagebrush seedlings were recruited in all of the mechanical treatments, but least in the disk imprinter treatment. After 10 yr, the untreated control was dominated by decadent sagebrush and rabbitbrush, the disk imprinter treatment was dominated by seeded perennial grasses, and the other mechanical treatments shared dominance of sagebrush and native perennial grasses. Mechanical treatments changed the composition of this community while retaining sagebrush, but greatest understory increases were associated with greatest control of sagebrush and establishment of seeded species by disk imprinting.     

Longer-Term Evaluation of Sagebrush Restoration After Juniper Control and Herbaceous Vegetation Trade-offs

Degradation of shrublands around the world from altered fire regimes, overutilization, and anthropogenic disturbance has resulted in a widespread need for shrub restoration. In western North America, reestablishment of mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana [Rydb.] Beetle) is needed to restore ecosystem services and function. Western juniper (Juniperus occidentalis ssp. occidentalis Hook) encroachment is a serious threat to mountain big sagebrush communities in the northern Great Basin and Columbia Plateau. Juniper trees can be controlled with fire; however, sagebrush recovery may be slow, especially if encroachment largely eliminated sagebrush before juniper control. Short-term studies have suggested that seeding mountain big sagebrush after juniper control may accelerate sagebrush recovery. Longer-term information is lacking on how sagebrush recovery progresses and if there are trade-offs with herbaceous vegetation. We compared seeding and not seeding mountain big sagebrush after juniper control (partial cutting followed with burning) in fully developed juniper woodlands (i.e., sagebrush had been largely excluded) at five sites, 7 and 8 yr after seeding. Sagebrush cover averaged ~ 30% in sagebrush seeded plots compared with ~ 1% in unseeded plots 8 yr after seeding, thus suggesting that sagebrush recovery may be slow without seeding after juniper control. Total herbaceous vegetation, perennial grass, and annual forb cover was less where sagebrush was seeded. Thus, there is a trade-off with herbaceous vegetation with seeding sagebrush. Our results suggest that seeding sagebrush after juniper control can accelerate the recovery of sagebrush habitat characteristics, which is important for sagebrush-associated wildlife. We suggest land manager and restoration practitioners consider seeding sagebrush and possibly other shrubs after controlling encroaching trees where residual shrubs are lacking after control.     

Understory Cover Responses to Piñon–Juniper Treatments Across Tree Dominance Gradients in the Great Basin

Piñon (Pinus spp.) and juniper (Juniperus spp.) trees are reduced to restore native vegetation and avoid severe fires where they have expanded into sagebrush (Artemisia tridentata Nutt.) communities. However, what phase of tree infilling should treatments target to retain desirable understory cover and avoid weed dominance? Prescribed fire and tree felling were applied to 8–20-ha treatment plots at 11 sites across the Great Basin with a tree-shredding treatment also applied to four Utah sites. Treatments were applied across a tree infilling gradient as quantified by a covariate tree dominance index (TDI = tree cover/&lsqb;tree + shrub + tall perennial grass cover]). Mixed model analysis of covariance indicated that treatment × covariate interactions were significant (P &spilt; 0.05) for most vegetation functional groups 3 yr after treatment. Shrub cover was most reduced with fire at any TDI or by mechanical treatment after infilling resulted in over 50% shrub cover loss (TDI &spigt; 0.4). Fire increased cheatgrass (Bromus tectorum L.) cover by an average of 4.2% for all values of TDI. Cutting or shredding trees generally produced similar responses and increased total perennial herbaceous and cheatgrass cover by an average of 10.2% and 3.8%, at TDIs ≥ 0.35 and ≥ 0.45. Cheatgrass cover estimated across the region was &spilt; 6% after treatment, but two warmer sites had high cheatgrass cover before (19.2% and 27.2%) and after tree reduction (26.6% and 50.4%). Fuel control treatments are viable management options for increasing understory cover across a range of sites and tree cover gradients, but should be accompanied by revegetation on warmer sites with depleted understories where cheatgrass is highly adapted. Shrub and perennial herbaceous cover can be maintained by mechanically treating at lower TDI. Perennial herbaceous cover is key for avoiding biotic and abiotic thresholds in this system through resisting weed dominance and erosion.     

Models for Predicting Fuel Consumption in Sagebrush-Dominated Ecosystems

Fuel consumption predictions are necessary to accurately estimate or model fire effects, including pollutant emissions during wildland fires. Fuel and environmental measurements on a series of operational prescribed fires were used to develop empirical models for predicting fuel consumption in big sagebrush (Artemisia tridentata Nutt.) ecosystems. Models are proposed for predicting fuel consumption during prescribed fires in the fall and the spring. Total prefire fuel loading ranged from 5.3–23.6 Mg · ha^?1; between 32% and 92% of the total loading was composed of live and dead big sagebrush. Fuel consumption ranged from 0.8–22.3 Mg · ha^?1, which equates to 11–99% of prefire loading (mean = 59%). Model predictors include prefire shrub loading, proportion of area burned, and season of burn for shrub fuels (R² = 0.91). Models for predicting proportion of area burned for spring and fall fires were also developed (R² = 0.64 and 0.77 for spring and fall fire models, respectively). Proportion of area burned, an indicator of the patchiness of the fire, was best predicted from the coverage of the herbaceous vegetation layer, wind speed, and slope; for spring fires, day-of-burn 10-h woody fuel moisture content was also an important predictor variable. Models predicted independent shrub consumption measurements within 8.1% (fall) and 12.6% (spring) for sagebrush fires.     

Response of Planted Sagebrush Seedlings to Cattle Grazing Applied to Decrease Fire Probability

Restoration of non-sprouting shrubs after wildfire is increasingly becoming a management priority. In the western U.S., Wyoming big sagebrush (Artemisia tridentata Nutt. ssp. wyomingensis Beetle & Young) restoration is a high priority, but sagebrush establishment from seed is sporadic. In contrast, planting seedlings often successfully restores sagebrush, but is expensive and time consuming. After planting, hence, there is a need to protect the investment from disturbances such as fire that will erase gains in sagebrush recovery. Grazing is likely the only tool that can be applied feasibly across the landscape to decrease wildfire probability, but there are concerns that grazing and associated activities (e.g. trampling) may negatively impact sagebrush seedlings. We investigated effects of grazing by cattle, applied as a fine fuel management strategy, on planted sagebrush seedlings at five blocks for five years. Grazing substantial reduced exotic annual grasses, large perennial bunchgrasses, and total herbaceous cover, thus achieving fuel management goals. Sagebrush cover and reproductive efforts were almost 2-fold greater in grazed compared to non-grazed areas in the final year of the study. This suggests that grazing favored sagebrush, a generally unpalatable shrub, recovery, likely by reducing competition from highly palatable herbaceous vegetation. Density of sagebrush, however, was similar between grazed and non-grazed areas. This research demonstrates that grazing can be strategically applied to reduce the probability of wildfire in areas with planted sagebrush seedlings; thereby, protecting the investment in sagebrush recovery. With more refinement, it also appears that grazing can be utilized to accelerate the recovery of sagebrush and potentially other woody vegetation habitat by modifying the competitive relationship between herbaceous and woody vegetation.     

Ventenata and Other Coexisting Exotic Annual Grass Control and Plant Community Response to Increasing Imazapic Application Rates

Ventenata (Ventenata dubia [Leers] Coss.) is an exotic annual grass that can invade intermountain rangeland plant communities, where it can form monotypic stands, degrade wildlife habitat, and reduce livestock forage. There is limited information on ventenata control in rangelands as it has only recently been identified as a substantial problem. Imazapic is a pre-emergent herbicide commonly used to control other exotic annual grasses and, therefore, is likely to control ventenata in rangelands. We evaluated five application rates of imazapic (0 − 175 g ae ∙ ha^− 1) on ventenata and other exotic annual grass control and plant community response at two rangeland sites in 2 yr (2014 and 2015). Imazapic reduced exotic annual grass (largely ventenata) cover and density, with greater control with increasing imazapic rates. Exotic annual grass density at the highest levels of control (82%−94%) was 184 − 299 plants ∙ m^− 2 the first yr after imazapic application. Exotic annual grasses fully recovered in the second or third yr after imazapic application. Bare ground generally increased with imazapic application. However, density of perennial vegetation (grasses and forbs) did not vary among treatments. Perennial vegetation cover generally did not increase with imazapic control of ventenata and other exotic annual grasses. Imazapic can control ventenata; however, even at the highest rates, control was not enough to shift the dominance from exotic annual species to perennial species. Integrating other treatments with imazapic application may be a strategy to improve ventenata control and increase perennial vegetation and will require further investigation. The difficulty and likely expense of achieving substantial and lasting control of ventenata suggest, similar to other exotic annual grasses, that preventing ventenata invasion and dominance should be a high management priority.     

Effects of Long-Term Livestock Grazing on Fuel Characteristics in Rangelands: An Example From the Sagebrush Steppe

Livestock grazing potentially has substantial influence on fuel characteristics in rangelands around the globe. However, information quantifying the impacts of grazing on rangeland fuel characteristics is limited, and the effects of grazing on fuels are important because fuel characteristics are one of the primary factors determining risk, severity, continuity, and size of wildfires. We investigated the effects of long-term (70+ yr) livestock grazing exclusion (nongrazed) and moderate levels of livestock grazing (grazed) on fuel accumulations, continuity, gaps, and heights in shrub-grassland rangelands. Livestock used the grazed treatment through 2008 and sampling occurred in mid- to late summer in 2009. Nongrazed rangelands had over twofold more herbaceous standing crop than grazed rangelands (P < 0.01). Fuel accumulations on perennial bunchgrasses were approximately threefold greater in nongrazed than grazed treatments. Continuity of fuels in nongrazed compared to grazed treatments was also greater (P < 0.05). The heights of perennial grass current year’s and previous years’ growth were 1.3-fold and 2.2-fold taller in nongrazed compared to grazed treatments (P < 0.01). The results of this study suggest that moderate livestock grazing decreases the risk of wildfires in sagebrush steppe plant communities and potentially other semi-arid and arid rangelands. These results also suggest wildfires in moderately grazed sagebrush rangelands have decreased severity, continuity, and size of the burn compared to long-term nongrazed sagebrush rangelands. Because of the impacts fuels have on fire characteristics, moderate levels of grazing probably increase the efficiency of fire suppression activities. Because of the large difference between fuel characteristics in grazed and nongrazed sagebrush rangelands, we suggest that additional management impacts on fuels and subsequently fires need to be investigated in nonforested rangelands to protect native plant communities and prioritize management needs.     

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.     

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.     

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.