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.     

Simulating Current Successional Trajectories in Sagebrush Ecosystems With Multiple Disturbances Using a State-and-Transition Modeling Framework

Disturbances and their interactions play major roles in sagebrush (Artemisia spp. L.) community dynamics. Although impacts of some disturbances, most notably fire, have been quantified at the landscape level, some have been ignored and rarely are interactions between disturbances evaluated. We developed conceptual state-and-transition models for each of two broad sagebrush groups—a warm-dry group characterized by Wyoming big sagebrush (Artemisia tridentata Nutt. subsp. wyomingensis Beetle & Young) communities and a cool-moist group characterized by mountain big sagebrush (Artemisia tridentata Nutt. subsp. vaseyana [Rydb.] Beetle) communities. We used the Vegetation Dynamics Development Tool to explore how the abundance of community phases and states in each conceptual model might be affected by fire, insect outbreak, drought, snow mold, voles, sudden drops in winter temperatures (freeze-kill), livestock grazing, juniper (Juniperus occidentalis var. occidentalis Hook.) expansion, nonnative annual grasses such as cheatgrass (Bromus tectorum L.), and vegetation treatments. Changes in fuel continuity and loading resulted in average fire rotations of 12 yr in the warm-dry sagebrush group and 81 yr in the cool-moist sagebrush group. Model results in the warm-dry sagebrush group indicated postfire seeding success alone was not sufficient to limit the area of cheatgrass domination. The frequency of episodes of very high utilization by domestic livestock during severe drought was a key influence on community phase abundance in our models. In the cool-moist sagebrush group, model results indicated at least 10% of the juniper expansion area should be treated annually to keep juniper in check. Regardless, juniper seedlings and saplings would remain abundant.     

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.     

Postfire Succession in Big Sagebrush Steppe With Livestock Grazing

Prescribed fire in rangeland ecosystems is applied for a variety of management objectives, including enhancing productivity of forage species for domestic livestock. In the big sagebrush (Artemisia tridentata Nutt.) steppe of the western United States, fire has been a natural and prescribed disturbance, temporarily shifting vegetation from shrub–grass codominance to grass dominance. There is limited information on the impacts of grazing to community dynamics following fire in big sagebrush steppe. This study evaluated cattle grazing impacts over four growing seasons after prescribed fire on Wyoming big sagebrush (Artemisia tridentata subsp. Wyomingensis [Beetle & Young] Welsh) steppe in eastern Oregon. Treatments included no grazing on burned and unburned sagebrush steppe, two summer-grazing applications after fire, and two spring-grazing applications after fire. Treatment plots were burned in fall 2002. Grazing trials were applied from 2003 to 2005. Vegetation dynamics in the treatments were evaluated by quantifying herbaceous canopy cover, density, annual yield, and perennial grass seed yield. Seed production was greater in the ungrazed burn treatments than in all burn–grazed treatments; however, these differences did not affect community recovery after fire. Other herbaceous response variables (cover, density, composition, and annual yield), bare ground, and soil surface litter did not differ among grazed and ungrazed burn treatments. All burn treatments (grazed and ungrazed) had greater herbaceous cover, herbaceous standing crop, herbaceous annual yield, and grass seed production than the unburned treatment by the second or third year after fire. The results demonstrated that properly applied livestock grazing after low-severity, prescribed fire will not hinder the recovery of herbaceous plant communities in Wyoming big sagebrush steppe.     

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.     

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.     

Cattle Grazing and Vegetation Succession on Burned Sagebrush Steppe

There is limited information about the effects of cattle grazing to longer-term plant community composition and herbage production following fire in sagebrush steppe. This study evaluated vegetation response to cattle grazing over 7 yr (2007–2013) on burned Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis [Beetle & Young] Welsh) steppe in eastern Oregon. Treatments, replicated four times and applied in a randomized complete block design, included no grazing on burned (nonuse) and unburned (control) sagebrush steppe; and cattle grazing at low (low), moderate (moderate), and high (high) stocking on burned sagebrush steppe. Vegetation dynamics were evaluated by quantifying herbaceous (canopy and basal cover, density, production, reproductive shoot weight) and shrub (canopy cover, density) response variables. Aside from basal cover, herbaceous canopy cover, production, and reproduction were not different among low, moderate, and nonuse treatments. Perennial bunchgrass basal cover was about 25% lower in the low and moderate treatments than the nonuse. Production, reproductive stem weight, and perennial grass basal cover were greater in the low, moderate, and nonuse treatments than the control. The high treatment had lower perennial bunchgrass cover (canopy and basal) and production than other grazed and nonuse treatments. Bunchgrass density remained unchanged in the high treatment, not differing from other treatments, and reproductive effort was comparable to the other treatments, indicating these areas are potentially recoverable by reducing stocking. Cover and production of Bromus tectorum L. (cheatgrass) did not differ among the grazed and nonuse treatments, though all were greater than the control. Cover and density of A.t. spp. wyomingensis did not differ among the burned grazed and nonuse treatments and were less than the control. We concluded that light to moderate stocking rates are compatible to sustainable grazing of burned sagebrush steppe rangelands.     

Disturbance Type and Sagebrush Community Type Affect Plant Community Structure After Shrub Reduction

Treatments to reduce shrub cover are commonly implemented with the objective of shifting community structure away from shrub dominance and toward shrub and perennial grass codominance. In sagebrush (Artemisia L.) ecosystems, shrub reduction treatments have had variable effects on target shrubs, herbaceous perennials, and non-native annual plants. The factors mediating this variability are not well understood. We used long-term data from Utah’s Watershed Restoration Initiative project to assess short-term (1 − 4 yr post-treatment) and long-term (5 − 12 yr post-treatment) responses of sagebrush plant communities to five shrub reduction treatments at 94 sites that span a range of abiotic conditions and sagebrush community types. Treatments were pipe harrow with one or two passes, aerator, and fire with and without postfire seeding. We analyzed effect sizes (log of response ratio) to assess responses of sagebrush, perennial and annual grasses and forbs, and ground cover to treatments. Most treatments successfully reduced sagebrush cover over the short and long term. All treatments increased long-term perennial grass cover in Wyoming big sagebrush (A. tridentata Nutt. ssp. wyomingensis Beetle & Young) communities, but in mountain big sagebrush (ssp. vaseyana [Rydb.] Beetle) communities, perennial grasses increased only when seeded after fire. In both sagebrush communities, treatments generally resulted in short-term, but not long-term, increases in perennial forb cover. Annual grasses (largely invasive cheatgrass, Bromus tectorum L.) increased in all treatments on sites dominated by mountain big sagebrush but stayed constant or decreased on sites dominated by Wyoming big sagebrush. This result was unexpected because sites dominated by Wyoming big sagebrush are typically thought to be less resilient to disturbance and less resistant to invasion than sites dominated by mountain big sagebrush. Together, these results indicate some of the benefits, risks, and contingent outcomes of sagebrush reduction treatments that should be considered carefully in any future decisions about applying such treatments.     

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.     

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.     

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.     

Ecological Scale of Bird Community Response to Piñon-Juniper Removal

Piñon (Pinus spp.) and juniper (Juniperus spp.) removal is a common management approach to restore sagebrush (Artemisia spp.) vegetation in areas experiencing woodland expansion. Because many management treatments are conducted to benefit sagebrush-obligate birds, we surveyed bird communities to assess treatment effectiveness in establishing sagebrush bird communities at study sites in Utah, Nevada, Idaho, and Oregon. Our analyses included data from 1 or 2 yr prior to prescribed fire or mechanical treatment and 3 to 5 yr posttreatment. We used detrended correspondence analysis to 1) identify primary patterns of bird communities surveyed from 2006 to 2011 at point transects, 2) estimate ecological scale of change needed to achieve treatment objectives from the relative dissimilarity of survey points to the ordination region delineating sagebrush bird communities, and 3) measure changes in pre- and posttreatment bird communities. Birds associated with sagebrush, woodlands, and ecotones were detected on our surveys; increased dissimilarity of survey points to the sagebrush bird community was characterized by a gradient of increased juniper and decreased sagebrush cover. Prescribed fires burned between 30% and 97% of our bird survey points. However, from 6% to 24% cover of piñon-juniper still remained posttreatment on the four treatment plots. We measured only slight changes in bird communities, which responded primarily to current vegetation rather than relative amount of change from pretreatment vegetation structure. Bird communities at survey points located at greater ecological scales from the sagebrush bird community changed least and will require more significant impact to achieve changes. Sagebrush bird communities were established at only two survey points, which were adjacent to a larger sagebrush landscape and following almost complete juniper removal by mechanical treatment. Our results indicate that management treatments that leave residual woodland cover and are not adjacent to extensive sagebrush stands are unlikely to establish sagebrush birds.     

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.     

Sage Grouse Groceries: Forb Response to Piñon-Juniper Treatments

Juniper and piñon coniferous woodlands have increased 2- to 10-fold in nine ecoregions spanning the Intermountain Region of the western United States. Control of piñon-juniper woodlands by mechanical treatments and prescribed fire are commonly applied to recover sagebrush steppe rangelands. Recently, the Sage Grouse Initiative has made conifer removal a major part of its program to reestablish sagebrush habitat for sage grouse (Centrocercus urophasianus) and other species. We analyzed data sets from previous and ongoing studies across the Great Basin characterizing cover response of perennial and annual forbs that are consumed by sage grouse to mechanical, prescribed fire, and low-disturbance fuel reduction treatments. There were 11 sites in western juniper (Juniperus occidentalis Hook.) woodlands, 3 sites in singleleaf piñon (Pinus monophylla Torr. & Frém.) and Utah juniper (Juniperus osteosperma [Torr.] Little), 2 sites in Utah juniper, and 2 sites in Utah juniper and Colorado piñon (Pinus edulis Engelm). Western juniper sites were located in mountain big sagebrush (A. tridentata ssp. vaseyana) steppe associations, and the other woodlands were located in Wyoming big sagebrush (A. tridentata ssp. wyomingensis) associations. Site potential appears to be a major determinant for increasing perennial forbs consumed by sage grouse following conifer control. The cover response of perennial forbs, whether increasing (1.5- to 6-fold) or exhibiting no change, was similar regardless of conifer treatment. Annual forbs favored by sage grouse benefitted most from prescribed fire treatments with smaller increases following mechanical and fuel reduction treatments. Though forb abundance may not consistently be enhanced, mechanical and fuel reduction conifer treatments remain good preventative measures, especially in phase 1 and 2 woodlands, which, at minimum, maintain forbs on the landscape. In addition, these two conifer control measures, in the short term, are superior to prescribed fire for maintaining the essential habitat characteristics of sagebrush steppe for sage grouse.     

Absence of a Grass/Fire Cycle in a Semiarid Grassland: Response to Prescribed Fire and Grazing

Many nonnative invasive grasses alter fire regimes to their own benefit and the detriment of native organisms. In southern Arizona the nonnative Lehmann lovegrass (Eragrostis lehmanniana Nees) dominates many semiarid grasslands where native grasses were abundant. Managers are wary of using prescribed fire in this fire-prone community partly due to the perceived effects of a grass/fire cycle. However, examples of the grass/fire cycle originate in ecosystems where native plants are less fire-tolerant than grasses and the invasive plant does not mimic the physiognomy of the native community. We investigate the effects of prescribed fire and livestock grazing on a semiarid grassland community dominated by a nonnative invasive grass. Lehmann lovegrass does not appear to alter the fire regime of semiarid grasslands to the detriment of native plants. Prescribed fire reduced the abundance of Lehmann lovegrass for 1 to 2 yr while increasing abundance of native grasses, herbaceous dicotyledons and fall richness, and diversity. Effects of livestock grazing were less transformative than the effects of fire in this long-grazed area, but grazing negatively affected native plants as did the combination of prescribed fire and livestock grazing. Although Lehmann lovegrass produces more fuel than native plants, fire frequency in semiarid grasslands appears to be limited by the paucity of above-average precipitation, which constrains high fuel loads. In addition, many native grasses tolerate high temperatures produced by Lehmann lovegrass fires. Consistent with previous research, fire does not promote the spread of Lehmann lovegrass, and more importantly human alteration of the fire regime is greater than the nominal effects of Lehmann lovegrass introduction on the fire regime.     

放牧对天山北坡中段草原群落结构和功能群生产力的影响

通过对天山北坡中段典型羊茅草原的动态监测,分析了放牧对草原群落结构和功能群生产力的影响。结果表明：放牧不能改变草原植物生活型功能群的组成结构,且多年生禾草占据主导地位,放牧与不放牧条件下所占比重依次为30．74％～44．999,5、30．819／6～35．71％;中度放牧能够增加草原植物种数,但不能改变羊茅、针茅、草原苔草的优势地位,其优势度分别为11．64％～19．40％,11．05％～19．70％和11．65％～23．79％;植物生活型功能群组成中,半灌木、灌木与多年生禾草、杂类草,多年生豆科草与杂类草间植物量显著正相关,生态互补效应显著。     

Runoff and Erosion After Cutting Western Juniper

Western juniper (Juniperus occidentalis spp. occidentalis Hook.) has encroached on and now dominates millions of acres of sagebrush/bunchgrass rangeland in the Great Basin and interior Pacific Northwest. On many sites western juniper has significantly increased exposure of the soil surface by reducing density of understory species and surface litter. We used rainfall and rill simulation techniques to evaluate infiltration, runoff, and erosion on cut and uncut field treatments 10 years after juniper removal. Juniper-dominated hillslopes had significantly lower surface soil cover of herbaceous plants and litter and produced rapid runoff from low-intensity rainfall events of the type that would be expected to occur every 2 years. Direct exposure of the soil to rainfall impacts resulted in high levels of sheet erosion (295 kg · ha^-1) in juniper-dominated plots. Large interconnected patches of bare ground concentrated runoff into rills with much higher flow velocity and erosive force resulting in rill erosion rates that were over 15 times higher on juniper-dominated plots. Cutting juniper stimulated herbaceous plant recovery, improved infiltration capacity, and protected the soil surface from even large thunderstorms. Juniper-free plots could only be induced to produce runoff from high-intensity events that would be expected to occur once every 50 years. Runoff events from these higher-intensity simulations produced negligible levels of both sheet and rill erosion. While specific inferences drawn from the current study are limited to juniper-affected sites in the Intermountain sagebrush steppe, the scope of ecosystem impacts are consistent with woody-plant invasion in other ecosystems around the world.     

Short-Term Butterfly Response to Sagebrush Steppe Restoration Treatments

As part of the Sagebrush Steppe Treatment Evaluation Project (SageSTEP), butterflies were surveyed pretreatment and up to 4 yr posttreatment at 16 widely distributed sagebrush steppe sites in the interior West. Butterfly populations and communities were analyzed in response to treatments (prescribed fire, mechanical, herbicide) designed to restore sagebrush steppe lands encroached by piñon-juniper woodlands (Pinus, Juniperus spp.) and invaded by cheatgrass (Bromus tectorum). Butterflies exhibited distinct regional patterns of species composition, with communities showing marked variability among sites. Some variation was explained by the plant community, with Mantel's test indicating that ordinations of butterfly and plant communities were closely similar for both woodland sites and lower-elevation treeless (sage-cheat) sites. At woodland sites, responses to stand replacement prescribed fire, clear-cutting, and tree mastication treatments applied to 10–20-ha plots were subtle: 1) no changes were observed in community structure; 2) Melissa blues (Plebejus melissa) and sulfurs (Colias spp.) increased in abundance after either burning or mechanical treatments, possibly due to increase in larval and nectar food resource, respectively; and 3) the juniper hairstreak (Callophrys gryneus) declined at sites at which it was initially present, probably due to removal of its larval food source. At sage-cheat sites, after prescribed fire was applied to 25–75-ha plots, we observed 1) an increase in species richness and abundance at most sites, possibly due to increased nectar resources for adults, and 2) an increase in the abundance of skippers (Hesperiidae) and small white butterflies. Linkages between woody species removal, the release of herbaceous vegetation, and butterfly response to treatments demonstrate the importance of monitoring an array of ecosystem components in order to document the extent to which management practices cause unintended consequences.     

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.     

Vegetation,Hydrologic, and Erosion Responses of Sagebrush Steppe 9 Yr Following Mechanical Tree Removal

Land managers across the western United States are faced with selecting and applying tree-removal treatments on pinyon (Pinus spp.) and juniper (Juniperus spp.) woodland-encroached sagebrush (Artemisia spp.) rangelands, but current understanding of long-term vegetation and hydrological responses of sagebrush sites to tree removal is inadequate for guiding management. This study applied a suite of vegetation and soil measures (0.5 ? 990 m²), small-plot rainfall simulations (0.5 m²), and overland flow experiments (9 m²) to quantify the effects of mechanical tree removal (tree cutting and mastication) on vegetation, runoff, and erosion at two mid- to late-succession woodland-encroached sagebrush sites in the Great Basin, United States, 9 yr after treatment. Low amounts of hillslope-scale shrub (3 ? 15%) and grass (7 ? 12%) canopy cover and extensive intercanopy (area between tree canopies) bare ground (69 ? 88% bare, 75% of area) in untreated areas at both sites facilitated high levels of runoff and sediment from high-intensity (102 mm ? h^? 1, 45 min) rainfall simulations in interspaces (~ 45 mm runoff, 59 ? 381 g ? m^? 2 sediment) between trees and shrubs and from concentrated overland flow experiments (15, 30, and 45 L ? min^? 1, 8 min each) in the intercanopy (371 ? 501 L runoff, 2 342 ? 3 015 g sediment). Tree cutting increased hillslope-scale density of sagebrush by 5% and perennial grass cover by twofold at one site while tree cutting and mastication increased hillslope-scale sagebrush density by 36% and 16%, respectively, and perennial grass cover by threefold at a second more-degraded (initially more sparsely vegetated) site over nine growing seasons. Cover of cheatgrass (Bromus tectorum L.) was < 1% at the sites pretreatment and 1 ? 7% 9 yr after treatment. Bare ground remained high across both sites 9 yr after tree removal and was reduced by treatments solely at the more degraded site. Increases in hillslope-scale vegetation following tree removal had limited impact on runoff and erosion for rainfall simulations and concentrated flow experiments at both sites due to persistent high bare ground. The one exception was reduced runoff and erosion within the cut treatments for intercanopy plots with cut-downed-trees. The cut-downed-trees provided ample litter cover and tree debris at the ground surface to reduce the amount and erosive energy of concentrated overland flow. Trends in hillslope-scale vegetation responses to tree removal in this study demonstrate the effectiveness of mechanical treatments to reestablish sagebrush steppe vegetation without increasing cheatgrass for mid- to late-succession woodland-encroached sites along the warm-dry to cool-moist soil temperature ? moisture threshold in the Great Basin. Our results indicate improved hydrologic function through sagebrush steppe vegetation recruitment after mechanical tree removal on mid- to late-succession woodlands can require more than 9 yr. We anticipate intercanopy runoff and erosion rates will decrease over time at both sites as shrub and grass cover continue to increase, but follow-up tree removal will be needed to prevent pinyon and juniper recolonization. The low intercanopy runoff and erosion measured underneath isolated cut-downed-trees in this study clearly demonstrate that tree debris following mechanical treatments can effectively limit microsite-scale runoff and erosion over time where tree debris settles in good contact with the soil surface.