Annual Grass Cover Research Articles

Oil and gas development influences potential for dust emission from the Upper Colorado River Basin, USA

AbstractWind erosion and dust emission from drylands have large consequences for ecosystem function and human health. Wind erosion is naturally reduced by soil crusting and sheltering by non‐erodible roughness elements such as plants. Land uses that reduce surface roughness and disturb the soil surface can dramatically increase dust emission. Extraction of oil and gas is a common and growing land use in the western United States (US) that removes vegetation and other roughness elements for construction of well pads and unpaved access roads, resulting in thousands of small (1–4 ha), discrete patches of unprotected soil. Here, we use a satellite albedo‐based model to assess the effect of oil/gas activity on surface roughness in the Uinta‐Piceance Basin, an area of the Upper Colorado River Basin (UCRB) with dense oil and natural gas development and modelled how the change in surface roughness could impact aeolian sediment flux and dust emission. We also investigated how regional drought influences the response of surface roughness to well pads and access roads. Oil/gas activity reduced surface roughness and increased modelled aeolian sediment flux at the landscape scale across much of the study region, resulting in a modest increase of 10 139 kg of dust per year, which is small relative to dust loads from a single regional dust event observed in the region, but downwind impact could be significant. The magnitude of surface roughness reductions by oil/gas activity was generally consistent among land cover types. However, in parts of the basin that had high cover of annual forbs and grasses, oil/gas activity was associated with larger surface roughness and smaller potential dust emission. Drought decreased surface roughness across disturbed and undisturbed sites, but there was no interactive effect of oil/gas activity and drought on surface roughness. These results suggest that oil/gas activity may increase sediment fluxes and likely contributes to dust emission from landscapes in the UCRB. Understanding how drought and land use change contribute to dust emissions will benefit mitigation of undesirable impacts of wind erosion and dust transport.

Cross-scale analysis reveals interacting predictors of annual and perennial cover in Northern Great Basin rangelands.

Exotic annual grass invasion is a widespread threat to the integrity of sagebrush ecosystems in Western North America. Although many predictors of annual grass prevalence and native perennial vegetation have been identified, there remains substantial uncertainty about how regional-scale and local-scale predictors interact to determine vegetation heterogeneity, and how associations between vegetation and cattle grazing vary with environmental context. Here, we conducted a regionally extensive, one-season field survey across burned and unburned, grazed, public lands in Oregon and Idaho, with plots stratified by aspect and distance to water within pastures to capture variation in environmental context and grazing intensity. We analyzed regional-scale and local-scale patterns of annual grass, perennial grass, and shrub cover, and examined to what extent plot-level variation was contingent on pasture-level predictions of site favorability. Annual grasses were widespread at burned and unburned sites alike, contrary to assumptions of annual grasses depending on fire, and more common at lower elevations and higher temperatures regionally, as well as on warmer slopes locally. Pasture-level grazing pressure interacted with temperature such that annual grass cover was associated positively with grazing pressure at higher temperatures but associated negatively with grazing pressure at lower temperatures. This suggests that pasture-level temperature and grazing relationships with annual grass abundance are complex and context dependent, although the causality of this relationship deserves further examination. At the plot-level within pastures, annual grass cover did not vary with grazing metrics, but perennial cover did; perennial grasses, for example, had lower cover closer to water sources, but higher cover at higher dung counts within a pasture, suggesting contrasting interpretations of these two grazing proxies. Importantly for predictions of ecosystem response to temperature change, we found that pasture-level and plot-level favorability interacted: perennial grasses had a higher plot-level cover on cooler slopes, and this difference across topography was starkest in pastures that were less favorable for perennial grasses regionally. Understanding the mechanisms behind cross-scale interactions and contingent responses of vegetation to grazing in these increasingly invaded ecosystems will be critical to land management in a changing world.

Satellite‐derived prefire vegetation predicts variation in field‐based invasive annual grass cover after fire

AbstractAimsInvasion by annual grasses (IAGs) and concomitant increases in wildfire are impacting many drylands globally, and an understanding of factors that contribute to or detract from community resistance to IAGs is needed to inform postfire restoration interventions. Prefire vegetation condition is often unknown in rangelands but it likely affects variation in postfire invasion resistance across large burned scars. Whether satellite‐derived products like the Rangeland Analysis Platform (RAP) can fulfill prefire information needs and be used to parametrize models of fire recovery to inform postfire management of IAGs is a key question.MethodsWe used random forests to ask how IAG abundances in 669 field plots measured in the 2‐3 years following megafires in sagebrush steppe rangelands of western USA responded to RAP estimates of annual:perennial prefire vegetation cover, the effects of elevation, heat load, postfire treatments, soil moisture–temperature regimes, and land‐agency ratings of ecosystem resistance to invasion and resilience to disturbance.ResultsPostfire IAG cover measured in the field was % and RAP‐estimated prefire annual herbaceous cover was %. The random forest model had an R2 of 0.36 and a root‐mean‐squared error (RMSE) of 4.41. Elevation, postfire herbicide treatment, and prefire estimates from RAP for the ratio of annual:perennial and shrub cover were the most important predictors of postfire IAG cover. Threshold‐like relationships between postfire IAG cover and the predictors indicate that maintaining annual:perennial cover below 0.4 and shrub cover below <10% prior to wildfire would decrease invasion, at low elevations below 1400 m above sea level.ConclusionDespite known differences between RAP and field‐based estimates of vegetation cover, RAP was still a useful predictor of variation in IAG abundances after fire. IAG management is oftentimes reactive, but our findings indicate impactful roles for more inclusively addressing the exotic annual community, and focusing on prefire maintenance of annual:perennial herbaceous and shrub cover at low elevations.

Managing Medusahead Using Dormant Season Grazing in the Northern Great Basin

The invasive annual grass, medusahead, infests rangelands throughout the West, from the Columbia Plateau to the California Annual Grasslands and the Great Basin. Dominating secondary succession in the sagebrush steppe, medusahead can degrade the habitat of threatened species such as the greater sage-grouse. This research explores the potential of dormant season grazing as an applied management strategy to reduce the negative impacts of medusahead while promoting recovery of perennial vegetation at the landscape scale. In particular, it assessed grazing with four treatments from 2018 to 2020: traditional grazing (May–October), dormant season grazing (October–February), traditional + dormant season grazing (May–February), and no grazing. After 2 yr of grazing treatments, biomass, density, cover, and fuel continuity did not differ between treatments (P > 0.05). However, biomass measurements were significantly different between years, which is likely due to greater than normal precipitation in 2019 and 2020. Between 2018 and 2019, annual grass biomass increased by 81% (666–1 212 kg ha−1) and perennial grass biomass increased by 165% (118–313 kg ha−1). Litter biomass decreased by approximately 15% in every year since 2018 (2 374, 2 012, and 1 678 kg ha−1 in 2018–2020). There were not significant differences in cover or density of annual and perennial grasses between treatments and years. Our results indicate that 2 yr may not be adequate time for dormant season grazing treatments to be effective in reducing the abundance of medusahead and that after 2 yr of treatments, dormant season grazing does not have a detrimental effect on perennial vegetation.

Influence of directional side of sagebrush canopies and interspaces on microhabitats

Shrubs can contribute to spatial heterogeneity in plant communities by creating distinct microsites under their canopies compared to between their canopies (interspaces). This results in distinct microhabitats that differ in understory vegetation characteristics and ground cover. However, microhabitats may also differ under the north and south side of canopies because of differences in shading and other microsite characteristics. We investigated if microhabitats varied among north and south sides of sagebrush canopies, and interspaces in 16 plant communities. Several understory vegetation characteristics and most ground cover variables varied among north sides, south sides, and interspaces. Moss and litter cover were greatest and bare ground was lowest in north sides. Moss and litter cover decreased and bare ground increased from north to south sides and from south sides to interspaces. Exotic annual grass cover and abundance was less in north side microsites compared to south side and interspace microsites, implying that sagebrush creates heterogeneity in resistance to invasion. This may be critical in allowing native herbaceous vegetation to persist under annual grass invasion pressure. Our results provide evidence that sagebrush creates distinct microhabitats. This highlights the pivotal role of shrubs in creating heterogeneity in shrub steppe communities and indicates that preventing the loss of shrubs in these communities should be a management priority. This also suggests that it is critical to restore sagebrush, and potentially other shrubs in similar ecosystems, after they are lost to maintain differences in microhabitats that promote diversity and coexistence.

Landscape and connectivity metrics as a spatial tool to support invasive annual grass management decisions

The spatial patterns and context of invasions are increasingly recognized as important for successful and efficient management actions. Beyond mapping occurrence or percent cover in pixels, spatial summary information that describes the size and arrangement of patches in the context of a larger landscape (e.g., infested regions, connected patch networks) can add a depth of information for managing invasive grasses that threaten native ecosystems. Few invasive annual grass analyses have explored the use of landscape and circuit-based connectivity metrics to characterize and compare spatial patterns of invasion. To assess the transferability and applicability of these landscape ecology analyses, we calculated landscape metrics (4 area-based, 3 configuration) and a connectivity metric (circuit-based centrality), using a weighted-average map of invasive annual grass cover in the Great Basin, USA. We calculated metrics at local and regional scales, allowing invasion statistics to be compared across the landscape and illustrating varying patterns of invasion extent and connectedness. We found the metrics provided additional, complementary information at the sampled local and regional scales beyond abundance measures alone. We also illustrated how key metrics could be used to categorize and map areas needing different management strategies, for example, where strategies could proactively protect uninvaded cores, disconnect fine fuel patches, or contain established invasions. The landscape and connectivity metric approach can be applied across scales to spatially target patches locally, provide broader context within a single region, as well as to compare metrics and spatial variation in patterns among different regions.

Long-Term Trends in Vegetation on Bureau of Land Management Rangelands in the Western United States

The US Bureau of Land Management (BLM) manages nearly 1 million km2 of public lands that support recreation, livestock production, and wildlife habitat. Monitoring the condition of vegetation on these lands is crucial for sound management but has historically been difficult to do at scale. Here we used newly developed remote-sensing tools to conduct an unprecedented assessment of trends in vegetation cover and production for all BLM rangelands from 1991 to 2020. We found widespread increases in cover and production of annual grasses and forbs, declines in herbaceous perennial cover, and expansion of trees. Cover and production of annual plants now exceed that of perennials on > 21 million ha of BLM rangeland, marking a fundamental shift in the ecology of these lands. This trend was most dramatic in the Western Cold Desert of Nevada and parts of surrounding states where aboveground production of annuals has more than tripled. Trends in annuals were negatively correlated with trends in bare ground but not with trends in perennials, suggesting that annuals are filling in bare ground rather than displacing perennials. Tree cover increased in half of ecoregions affecting some 44 million ha and underscoring the threat of woodland expansion for western rangelands. A multiscale variance partitioning analysis found that trends often varied the most at the finest spatial scale. This result reinforces the need to combine plot-level field data with moderate-resolution remote sensing to accurately quantify vegetation changes in heterogeneous rangelands. The long-term changes in vegetation on public rangelands argue for a more hands-on approach to management, emphasizing preventative treatment and restoration to preserve rangeland habitat and functioning. Our work shows the power of new remote-sensing tools for monitoring public rangelands and developing effective strategies for adaptive management and conservation.

Above-Average Rainfall Stimulates Short-Term, Passive Recovery of Ungulate Mediated Soil Scalds in Australian Mesic Savanna

The quality of savanna habitats across the globe has declined due to increased grazing pressure from introduced ungulate species. High grazing pressure can result in a loss of perennial grass cover, which can lead to the formation of “soil scalds” on clay soils. Scalds are maintained by the formation of a soil crust, which impedes water penetration and increases runoff. Scald formation can have significant negative ecological impacts on savanna ecosystems, yet the reversibility of this process remains uncertain. In this study, we examine perennial grass decline in a north Australian savanna, where historic grazing pressure has resulted in formation of soil scalds. We tested the reversibility of scald formation by implementing a soil treatment, which involved ripping the soil to a depth of 5−10 cm and placing erosion matting on the soil surface. In addition, ungulate exclusion fencing was erected to test if the presence of feral ungulates impacted scald recovery. After a year of above-average rainfall, both control and soil treatment plots showed significant recovery in perennial and annual grass cover, as well as a significant decline in bare ground cover. However, the degree of recovery was greatest in soil treatment plots, mainly due to increases in annual grass cover. This short-term study suggests that passive recovery of scalds is possible under favorable conditions and confirms that soil mediation can accelerate revegetation. In contrast, fencing was not found to have a significant effect on vegetation recovery, which is likely due to the low ungulate density present during the study. It is important to note, however, that our results are from a single season and a long-term study is required to confirm ongoing recovery.

Resilience and Resistance Framework Predicts Regional Vegetation Responses to Shrub Reduction Treatments in the Sagebrush Ecosystem

The concepts of ecosystem resilience and resistance (R&R) are being widely used in management planning for the big-sagebrush ecosystem of the western United States. Soil temperature and moisture regimes have been used to map geographic areas that are expected to have high, moderate, or low resilience (recovery of perennial species after disturbance) and resistance (to the spread of invasive annual grasses, most often Bromus tectorum L. or cheatgrass). While the theory underlying this framework is well developed, there have been few empirical tests of the map's utility in predicting on-the-ground management outcomes. We used data from 51 sites in Utah to relate R&R level to changes in plant cover after disturbances, using effect size analyses that standardized the magnitude of postdisturbance change (relative to predisturbance levels) across sites. Sites had been treated mechanically to reduce shrub cover or burned, and most sites were seeded. Perennial grass cover, which included native and non-native species due to the seed mixes used, increased in all R&R classes relative to predisturbance levels, but this increase was greatest at High R&R sites. Perennial forb cover increased only in the High R&R sites and decreased in Low R&R sites. Sagebrush shrub cover decreased across all R&R levels but showed evidence of recovery at High and Moderate R&R sites. These results indicate that outcomes across different disturbance types were generally in line with expectations based on the R&R map. Annual grass cover increased slightly at High and Moderate R&R sites but decreased in the Low R&R sites. These annual grass results illustrate that local site characteristics, and history may be more important factors than broad-scale soil moisture-temperature regime classifications in some cases. Nonetheless, our results overall indicate that the R&R map can be a useful tool for predicting the general outcomes of management actions in the sagebrush ecosystem.

Prefire Vegetation Structure of High Severity Wildfires in Nonherbaceous‐Dominated Rangelands in the Western United States

AbstractHigh severity rangeland wildfires have enormous negative impacts on human safety and infrastructure and may have long‐term consequences on ecosystem structure, function, and services, across the vast rangelands in the western United States. We explored prefire vegetation structure of all high severity wildfires, as well as associated areas with low and moderate burn severity, in nonherbaceous‐dominated rangelands in western United States from 1985 to 2018, based on fractional cover of annual forbs and grasses, perennial forbs and grasses, shrubs and trees from the Rangeland Analysis Platform. These wildfires were grouped into nine clusters using Expectation Maximization algorithms and patterns of prefire vegetation structure and the major vegetation types were assessed. High woody cover is a common characteristic of the prefire vegetation structure. Four of the clusters, which accounted for more than 70% of the areas of high severity wildfires, were characterized by high (∼50%) woody cover in prefire vegetation structure. The areas with high severity burns also had significantly higher woody cover than the areas with low or moderate severity burns in eight of the nine clusters. Some of these ecosystems can have abundant fine fuel, structural connectivity such as fuel laddering, and/or highly flammable materials, which can facilitate the ignition of the woody crowns, leading to high severity wildfires. Management strategies (such as prescribed burning) that can decrease woody cover and reduce standing dead vegetation and fuel laddering could be effective in reducing the risk and extent of high severity wildfires on rangelands in the western United States.

Reducing Exotic Annual Grass Competition did not Improve Shrub Restoration Success During a Drought

Restoration of native shrub species is challenging but direly needed in arid and semiarid rangelands globally as native shrubs provide critical habitat for wildlife and livestock forage. Restoration of antelope bitterbrush (Purshia tridentata Pursh DC), a wildlife-important shrub, is often a priority on western US rangelands. One challenge to bitterbrush restoration is competitive exotic annual grasses. Exotic annual grasses can be successfully controlled with pre-emergent herbicides, but the effects of controlling exotic annual grasses with pre-emergent herbicides on bitterbrush survival and growth are unknown. We evaluated the effects of applying a pre-emergent herbicide, imazapic, to control exotic annual grasses on planted bitterbrush seedlings and existing vegetation for 2 yr post treatment at five sites in southeastern Oregon. Imazapic application reduced exotic annual grass cover and density but did not improve bitterbrush establishment. Exotic annual grass control did lead to an increase in native perennial bunchgrass cover. We suspect the lack of treatment effect was caused by high mortality of bitterbrush seedlings from drought in the first year. By the second year, bitterbrush was largely lost across the study sites with only four individuals surviving. The high bitterbrush seedling mortality observed in this study highlights that multiple barriers to restoration success likely exist in arid and semiarid rangelands. For successful restoration, land managers and restoration practitioners need to have a plan and resources for overcoming multiple barriers, which may require several restoration attempts should initial attempts be unsuccessful.

Invasion of annual grasses following wildfire corresponds to maladaptive habitat selection by a sagebrush ecosystem indicator species

Numerous wildlife species within semi-arid shrubland ecosystems across western North America are experiencing substantial habitat loss and fragmentation. These changes in habitat are often attributed to a diverse suite of factors including prolonged and increasingly severe droughts, conifer expansion, anthropogenic development, domestic and feral livestock grazing, and invasion of exotic annual grasses, which promotes increased wildfire frequency and severity. Greater sage-grouse ( Centrocercus urophasianus ; hereafter, sage-grouse) are considered an indicator of sagebrush ecosystem health and have experienced widespread population decline associated with habitat loss and degradation, as well as changes in predator communities. Our objectives were to model and map sage-grouse habitat selection and survival during the important brood-rearing life stage in relation to landscape-scale environmental predictors. Furthermore, we sought to understand impacts of wildfire and annual grass invasion on brood habitat, as these accelerated disturbance regimes are a primary cause of habitat loss within the Great Basin region of the USA. We used a hierarchical Bayesian modeling framework to estimate resource selection functions and survival for early and late brood-rearing stages of sage-grouse in relation to a broad suite of habitat characteristics evaluated at multiple spatial scales within the Great Basin from 2009 to 2019. Sage-grouse selected for greater perennial grass cover, higher relative elevations, and areas closer to springs and wet meadows during both early and late brood-rearing. Terrain characteristics, including heat load and aspect, were important in survival models, as was variation in shrub height. We also found strong evidence for higher survival for both early and late broods within previously burned areas, but survival within burned areas decreased as annual grass cover (i.e. cheatgrass, Bromus tectorum ) increased. This interaction effect demonstrates how invasion of annual grasses into burned areas, which has become prevalent in Great Basin sagebrush ecosystems, can lead to maladaptive habitat selection by brood-rearing greater sage-grouse. Understanding these complex relationships aids wildlife conservation and habitat management as wildfire and annual grass cycles continue to accelerate across western ecosystems.

Ungulate herbivores as drivers of aspen recruitment and understory composition throughout arid montane landscapes

AbstractHerbivory by wild and domestic ungulates can influence tree recruitment and understory forest communities throughout the world. Herbivore‐driven declines in tree recruitment have been observed for quaking aspen (Populus tremuloides), a foundation species whose health and management is recognized as a critical priority throughout much of its range. Livestock fencing is commonly used to promote aspen regeneration, but its effectiveness is rarely assessed, especially across large spatial scales. Using a livestock‐reduction experiment, we evaluated the effects of ungulate herbivory on aspen in the Great Basin and southern Cascades, an expansive and environmentally heterogeneous region where aspen faces the interacting threats of climate change, conifer encroachment, and herbivory. We found that livestock fencing only reduced the intensity of herbivore browsing on aspen when wild ungulate abundance was low and did not increase stem densities of aspen recruits. Contrary to expectations, wild ungulate abundance was a strong driver of browsing intensity on juvenile aspen within fenced, but not unfenced, aspen stands, and when the abundance of these herbivores was high, browsing intensity in fenced stands exceeded that in unfenced stands. The density of aspen recruits decreased with browsing intensity on juvenile aspen and with the density of both adult aspen and conifers, suggesting that both herbivory and intra‐ and interspecific competition are important drivers of recruitment. Fire history was also an important driver of recruitment, with stands that burned 10–20 years ago having the greatest density of aspen recruits. Finally, in the stand understory, we found that livestock fencing decreased forb cover, increased shrub species richness, and increased the cover of exotic annual grasses, a group dominated by Bromus tectorum. This latter finding suggests that livestock fencing may not be appropriate in areas where controlling the spread of this invader is a priority. In sum, our findings indicate that aspen recruitment is limited by browsing by both wild and domestic ungulates, is mediated by competition with neighboring trees and fire history, and will require management actions beyond livestock fencing, as this approach does not control browsing by wild ungulates.

Using Postfire Spatial Variability to Improve Restoration Success with Seeded Bitterbrush

Seed-based restoration of wildlife-important shrubs following wildfire is a management priority in many ecosystems. However, postfire restoration success is spatiotemporally variable and establishment from seed frequently fails in arid and semiarid rangelands. There may be opportunities to improve restoration success by taking advantage of small-scale spatial variability in environmental characteristics. Woody plants create distinct postfire microsites, which may influence establishment and growth of seeded species, under their canopies (canopies) compared with between their canopies (interspaces). Immediately after fire, former canopies generally have less vegetation and greater soil nutrient concentrations compared with interspaces. Thus, former canopy compared with interspace microsites may be more favorable for establishment and growth of seeded species, but rapid exotic plant invasion of former canopy microsites may hinder success. We evaluated seeding bitterbrush (Purshia tridentata Pursh DC) after wildfire in former western juniper (Juniperus occidentalis ssp. occidentalis Hook) canopy compared with interspace microsites at six locations for 3 yr post seeding. Bitterbrush abundance was 3.6-fold greater in former canopy compared with interspace microsites after 3 yr. Bitterbrush height was 1.5 to 2.5-fold greater in former canopy compared with interspace microsites. The first year after fire, exotic annual grass cover was 15.6-fold greater in interspace compared with canopy microsites. Abundance and cover of other herbaceous vegetation were generally also greater in the interspace. Exotic annual grass and native bunchgrass abundance increased substantially over time in former canopy microsites, suggesting abundant resource availability. Less herbaceous competition and presumably greater resource availability in former canopies probably resulted in greater success of seeded bitterbrush. These results suggest that capitalizing on spatial variability in environments can be used to increase restoration efficiency. After fire in western juniper−encroached rangelands, former juniper canopy microsites are a favorable environment for establishment and growth of seeded bitterbrush and could be targeted for restoration efforts to improve efficiency.

How do accuracy and model agreement vary with versioning, scale, and landscape heterogeneity for satellite-derived vegetation maps in sagebrush steppe?

Maps of the distribution and abundance of dominant plants derived from satellite data are essential for ecological research and management, particularly in the vast semiarid shrub-steppe. Appropriate application of these maps requires an understanding of model accuracy and precision, and how it might vary across space, time, and different vegetation types. For a 113 k Ha burn area, we compared modeled maps of different vegetation cover types created from satellite data to ‘benchmark” models based on intensive field sampling (∼1500–2000 plots resampled annually for 5 years) for three new satellite-derived models: USDA Rangeland Analysis Platform (RAP), the USGS Rangeland Condition Monitoring Assessment and Projection (RCMAP), and USGS fractional estimate of exotic annual grass cover (USGS-fractional-EAG). We assessed out-of-sample point accuracy and asked if and how accuracy changed each year due to vegetation shifts, new images, and model improvements (i.e. model versions). We also assessed how map agreement between satellite-based and field-based models changed with scale of application, topography, and time since fire.Accuracy and map agreement varied considerably among the vegetation types and across time and space (r2 ranging from 0 to 0.53), and some of the variability was predictable. All models tended to over or underestimate cover when field-measured cover was relatively low or high, respectively, i.e. a “false moderating effect”. Accuracy was greater and improved with newer versions of RAP (+0.05 to 0.29 r2) compared to RCMAP and USGS fractional model estimates, and in some cases was greater than field-based models. Variability in map agreement tended to decrease with larger areas sampled (particularly in areas >12 km), and this scale dependency was more evident in RAP and USGS-fractional-EAG models. Creating a “fair” basis for comparison of spatial models of low-statured semiarid vegetation derived from satellite compared to field data is not trivial because scaling the field data to the scale of large satellite pixels (or downscaling satellite-based models to field scale) requires modeling and associated model uncertainty. Accuracy can vary considerably and understanding the variation can help guide application of the models to the appropriate time, place, and variables.

Plant community context controls short‐ versus medium‐term effects of pre‐emergent herbicides on target and non‐target species after fire

AbstractQuestionsSelective herbicide application is a common restoration strategy to control exotic invaders that interfere with native plant recovery after wildfire. Whether spraying with pre‐emergent or bio‐herbicides releases native plants from competition with exotics (“spray‐and‐release” strategy) and makes communities resistant to reinvasion by exotic annual grasses (e.g. cheatgrass, medusahead), without risks to non‐target native plants or secondary invasion, is a major question for land managers of semiarid plant communities.LocationSagebrush steppe of southwest Idaho, USA.MethodsWe applied chemical herbicides (imazapic, rimsulfuron) and weed‐suppressive bacteria (Pseudomonas fluorescens strains MB906 and D7) to three sagebrush steppe communities after fire. We measured plant cover prior to burning and for four years (five for exotic annual grasses) post treatment.ResultsBoth chemical herbicides significantly reduced exotic annual grass cover in all communities in the first post‐spraying year, but rimsulfuron plots were re‐invaded after 1–2 years, while imazapic plots continued to resist reinvasion four and even five years post spraying, well after the herbicide should have degraded. We did not detect any increase in native perennial grass cover with either herbicide, and herbicides had both positive and negative effects on individual bunchgrass basal diameter, depending on species and plant community. Rimsulfuron was more damaging than imazapic to shallow‐rooted perennial bunchgrasses. Moss and lichen cover, key components of soil integrity, increased with chemical herbicide treatments in some communities. Both herbicides increased secondary invaders (exotic forbs or grasses), which varied by plant community and herbicide. Weed‐suppressive bacteria treatments had no significant effects on cover of any functional group.ConclusionsWhile short‐term effects of chemical herbicides that target exotic annual grasses were relatively consistent and predictable, longer‐term effects were specific to the herbicide and plant community. The “spray‐and‐release” strategy may confer resistance to re‐invasion by exotic annual grasses if herbicides prevent re‐invasion for an extended period.

What Is Driving the Proliferation of Exotic Annual Grasses in Sagebrush Communities? Comparing Fire with Off-Season Grazing

Exotic annual grass invasion is a pressing concern in sagebrush rangelands of the western United States. Overgrazing and fire have historically both been implicated in the rise of annual grasses, but experiments that compare the effect of grazing versus fire are lacking, particularly for contemporary grazing practices such as off-season (fall and winter) grazing. We compared 1) burned and ungrazed (burned), 2) off-season, moderately grazed and unburned (grazed), and 3) ungrazed and unburned (control) treatments at five Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis [Beetle & A. Young] S.L. Welsh) sites in southeastern Oregon for half a decade. Fire, but not off-season grazing, substantially increased exotic annual grass cover and abundance. Vegetation cover and density were generally similar between grazed and control areas. In contrast, at the end of the study exotic annual grass cover and density were over fourfold greater in burned areas. Exotic annual grass became the dominant plant group in burned areas, but not in grazed and control areas. Cover and density of annual forbs, predominately non-native species, were generally greater in the burned compared with grazed and control treatments. Fire also decreased soil biological crust cover and sagebrush cover and density compared with grazed and control treatments. This study provides strong evidence that fire is a threat to the sustainability of Wyoming big sagebrush communities at risk of exotic annual grass dominance, but that off-season, moderate grazing poses little risk. However, considering the spatial extent of our study was limited, further evaluations are needed across a larger geographic area. Given that off-season grazing can decrease the probability of fire, off-season grazing may be a valuable tool to reduce the risk of exotic annual grass dominance.

Moderate Grazing During the Off-Season (Fall-Winter) Reduces Exotic Annual Grasses in Sagebrush-Bunchgrass Steppe

Exotic annual grass invasion and dominance of sagebrush-bunchgrass steppe is a concern because it decreases biodiversity and promotes frequent wildfires. Management is needed to reduce exotic annual grasses to prevent sagebrush-bunchgrass communities from transitioning to annual grasslands. Grazing during the off season (fall-winter) has shown promise at reducing exotic annual grasses, but it has not been evaluated in plant communities dominated by sagebrush and native bunchgrasses. We compared moderate grazing during the off season with not grazing in five Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis [Beetle & A. Young] S.L. Welsh)−bunchgrass communities in the northern Great Basin. Treatments were applied annually for 10 yr (2009−2010 through 2018−2019). Plant community characteristics were measured after treatments had been applied from 6 to 10 yr. Off-season grazing reduced exotic annual grass density and cover. After a decade, annual grass cover was twofold greater in ungrazed areas. Sandberg bluegrass (Poa secunda J. Presl) density increased with off-season grazing, but large bunchgrass density was similar between off-season grazed and ungrazed areas. Perennial and annual forb density and cover were similar between off-season grazed and ungrazed treatments. Biological soil crust cover was also similar between off-season grazed and ungrazed areas. The results of this study provide strong evidence that off-season grazing has application for managing exotic annual grasses in sagebrush-bunchgrass steppe. Considering the vast scope of the exotic annual grass problem, properly applied grazing may be the most cost-efficient tool to mediate the impacts of annual grass invasion.

Greater sage-grouse respond positively to intensive post-fire restoration treatments.

Habitat loss is the most prevalent threat to biodiversity in North America. One of the most threatened landscapes in the United States is the sagebrush (Artemisia spp.) ecosystem, much of which has been fragmented or converted to non‐native grasslands via the cheatgrass‐fire cycle. Like many sagebrush obligates, greater sage‐grouse (Centrocercus urophasianus) depend upon sagebrush for food and cover and are affected by changes to this ecosystem. We investigated habitat selection by 28 male greater sage‐grouse during each of 3 years after a 113,000‐ha wildfire in a sagebrush steppe ecosystem in Idaho and Oregon. During the study period, seeding and herbicide treatments were applied for habitat restoration. We evaluated sage‐grouse responses to vegetation and post‐fire restoration treatments. Throughout the 3 years post‐fire, sage‐grouse avoided areas with high exotic annual grass cover but selected strongly for recovering sagebrush and moderately strongly for perennial grasses. By the third year post‐fire, they preferred high‐density sagebrush, especially in winter when sagebrush is the primary component of the sage‐grouse diet. Sage‐grouse preferred forb habitat immediately post‐fire, especially in summer, but this selection preference was less strong in later years. They also selected areas that were intensively treated with herbicide and seeded with sagebrush, grasses, and forbs, although these responses varied with time since treatment. Wildfire can have severe consequences for sagebrush‐obligate species due to loss of large sagebrush plants used for food and for protection from predators and thermal extremes. Our results show that management efforts, including herbicide application and seeding of plants, directed at controlling exotic annual grasses after a wildfire can positively affect habitat selection by sage‐grouse.

At high stocking rates, cattle do not functionally replace wild herbivores in shaping understory community composition.

Over a quarter of the world's land surface is grazed by cattle and other livestock, which are replacing wild herbivores, potentially impairing ecosystem structure, and functions. Previous research suggests that cattle at moderate stocking rates can functionally replace wild herbivores in shaping understory communities. However, it is uncertain whether this is also true under high stocking rates and the effects of wild herbivore on plant communities are moderate, enhanced, or simply additive to the effects of cattle at high stocking rates. To evaluate the influence of cattle stocking rates on the ability of cattle to functionally replace wild herbivores and test for interactive effects between cattle and wild herbivores in shaping understory vegetation, we assessed herbaceous vegetation in a long-term exclosure experiment in a semi-arid savanna in central Kenya that selectively excludes wild mesoherbivores (50-1000 kg) and megaherbivores (elephant and giraffe). We tested the effects of cattle stocking rate (zero/moderate/high) on herbaceous vegetation (diversity, composition, leafiness). We also tested how those effects depend on the presence of wild mesoherbivores and megaherbivores. We found that herbaceous community composition (primary ordination axis) was better explained by the presence/absence of herbivore types than by total herbivory, suggesting that herbivore identity is a more important determinant of community composition than total herbivory at high cattle stocking rates. The combination of wild mesoherbivores and cattle stocked at high rates led to increased bare ground and annual grass cover, reduced perennial grass cover and understory leafiness, and enhanced understory diversity. These shifts were weaker or absent when cattle were stocked at high stocking rates in the absence of wild mesoherbivores. Megaherbivores tempered the effects of cattle stocked at high rates on herbaceous community composition but amplified the effects of high cattle stocking rate on bare ground and understory diversity. Our results show that cattle at high stocking rates do not functionally replace wild herbivores in shaping savanna herbaceous communities contrary to previous findings at moderate stocking rates. In mixed-use rangelands, interactions between cattle stocking rate and wild herbivore presence can lead to non-additive vegetation responses with important implications for both wildlife conservation and livestock production.