Search

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. [ABSTRACT FROM AUTHOR]

There are concerns about the negative consequences of non‐native livestock grazing of sagebrush communities, especially since these communities are experiencing unpreceded threats from invasive annual grasses, altered fire regimes, and climate change. The narrative around grazing often focuses on the effects of heavy, repeated growing season use that were common historically but now are rare or localized (e.g., near water sources). At the same time, the potential for ecological benefits of strategically applied grazing is often overlooked, limiting management options that may promote desired outcomes. To improve management in the face of unprecedented threats, we synthesized the literature to investigate and identify potential ecological benefits of strategically applied livestock grazing in sagebrush communities. We found that grazing can be used to modify fine fuel characteristics in ways that decrease fire probability and severity in sagebrush communities. Pre‐fire moderate grazing may be especially important because it decreases fire severity and, thereby, promotes biodiversity and reduces postfire annual grass invasion, fire‐induced mortality of native bunchgrasses, and fire damage to soil biocrusts. Grazing can create and maintain fine fuel breaks to improve firefighter safety and fire suppression efficiency. Strategic grazing can also be used to promote desirable plant community composition. Grazing can be a valuable tool, that is currently underutilized, for achieving desired management outcomes in the sagebrush and likely other ecosystems. Improper grazing can generate severe negative consequences; therefore, successful application of grazing to achieve desired outcomes will require careful attention to plant community response and balancing management objectives with community constraints. [ABSTRACT FROM AUTHOR]

Aim: In the western United States, sagebrush (Artemisia spp.) and salt desert shrublands are rapidly transitioning to communities dominated by exotic annual grasses, a novel and self‐reinforcing state that threatens the economic sustainability and conservation value of rangelands. Climate change is predicted to favour annual grasses, potentially pushing transitions to annual grass dominance into higher elevations and north‐facing aspects. We sought to quantify expansion of annual grass‐dominated vegetation communities along topographic gradients over the past several decades. Location: Our analysis focused on rangelands among three ecoregions in the Great Basin of the western United States, where several species of exotic annual grasses are widespread among shrub and perennial grass‐dominated vegetation communities. Methods: We used recently developed remote sensing‐based rangeland vegetation data to produce yearly maps of annual grass‐dominated vegetation communities spanning 1990–2020. With these maps, we quantified the rate of spread and characterized changes in the topographic distribution (i.e. elevation and aspect) of areas transitioning to annual grass dominance. Results: We documented more than an eightfold increase in annual grass‐dominated area since 1990, occurring at an average rate of >2,300 km2 year−1 (0.6% of the area of Great Basin rangelands). In 2020, annual grasses dominated approximately one‐fifth (>77,000 km2) of Great Basin rangelands. This rapid expansion was associated with a broadening topographic niche, with widespread movement into higher elevations and north‐facing aspects consistent with predicted effects of a warming climate. Main conclusions: More than a century after first appearing in the region, exotic annual grasses continue to proliferate and establish dominance in new environments across the Great Basin. Accelerated, strategic intervention is critically needed to conserve vulnerable sagebrush and salt desert shrub communities not yet heavily invaded. In this era of warming, future climate provides important context for selecting from among alternative management actions and judging long‐term prospects of success. [ABSTRACT FROM AUTHOR]

Understanding the effects of contemporary cattle grazing on herbaceous perennial communities in big sagebrush steppe is important for managing for wildlife habitat, plant diversity, and productivity, yet potentially complicated by legacy impacts of historic, often higher intensity, livestock grazing. Here, we evaluate whether recovery of herbaceous communities in eastern Oregon, USA, after the cessation of intense spring sheep grazing (1935) was affected by moderate cattle grazing in paired plots with or without grazing over the past 75 yr (1936–2011). We tested for the effects of cattle grazing on herbaceous community recovery, as indicated by changes over time in plant density, and composition, as measured by Bray–Curtis dissimilarity. We also included current and prior to sampling year precipitation anomalies, to account for the weather effects, and a random term for pasture location of plot pairs to include potential subtle differences in abiotic environment and grazing management. We further tested whether time since cessation of intense sheep grazing and moderate cattle grazing were associated with convergence or divergence in community composition indicated by changes in evenness, richness, species relative abundance (rank order), and turnover or species appearance or disappearance. Total perennial herbaceous, forb, and grass density increased over time in sites grazed and ungrazed by cattle, though species varied in the direction of their response to contemporary cattle grazing. Community composition metrics indicated convergence over time including increasing evenness, decreasing Bray–Curtis dissimilarity, decreasing shifts in species relative abundance (rank order), and lower rates of species turnover (and gain and loss). Contemporary cattle grazing was not associated with convergence or divergence in composition. Precipitation anomalies for the current or prior water year were only occasionally significant in herbaceous density and community composition change models. Our results indicate similar long‐term recovery trajectories occurred in sites with moderate cattle grazing or removal of all livestock following cessation of intense sheep grazing. Management planning and resource assessment focused on herbaceous perennial communities in sagebrush steppe should seek to separate the impacts of historic from contemporary livestock grazing practices. [ABSTRACT FROM AUTHOR]

We assessed plant community succession following prescribed fire on ungrazed Wyoming big sagebrush steppe, eastern Oregon. Treatments were burned (Burn; September and October, 2002) and unburned (Control) sagebrush steppe. Herbaceous yield, vegetation canopy cover and density were compared between treatments after fire (2003–18). Herbaceous yield in the Burn treatment was about double the Control for most of the study period. Prior to fire, native perennials comprised 90–95% of herbaceous yield. After fire, native perennials represented 78% (range 67–93%) and exotic annuals 22% (range 7–33%) of total yield. Exotic annuals increased after fire and responded in two stages. In the first 8 years after fire, desert alyssum dominated the annual plant composition. In the last half of the study, cheatgrass co-dominated the annual component with alyssum. Sagebrush recovery was slow and we estimated sagebrush cover would return to pre-burn levels, at the earliest, in 115 years. Burning Wyoming big sagebrush steppe would be detrimental to sagebrush-obligate wildlife for an extended time period, because of lost cover and structure provided by sagebrush. The additional forage provided on burned areas may give livestock manager's greater flexibility to rest or defer unburned habitat for wildlife species of critical concern. We measured plant community succession, spanning 16 years, following fire on Wyoming big sagebrush steppe, eastern Oregon. Treatments were burned and unburned sagebrush steppe. Early succession was dominated by native herbaceous perennials, though non-native exotics also increased. It was estimated that cover of sagebrush would return to pre-burn levels in 115 years. [ABSTRACT FROM AUTHOR]

Woody vegetation has increased on rangelands worldwide for the past 100–200 years, often because of reduced fire frequency. However, there is a general aversion to reintroducing fire, and therefore, fire surrogates are often used in its place to reverse woody plant encroachment. Determining the conservation effectiveness of reintroducing fire compared with fire surrogates over different time scales is needed to improve conservation efforts. We evaluated the conservation effectiveness of reintroducing fire with a fire surrogate (cutting) applied over the last ~30 years to control juniper (Juniperus occidentalis Hook.) encroachment on 77 sagebrush‐steppe sites. Critical to conservation of this imperiled ecosystem is to limit juniper, not encourage exotic annual grasses, and promote sagebrush dominance of the overstory. Reintroducing fire was more effective than cutting at reducing juniper abundance and extending the period of time that juniper was not dominating the plant community. Sagebrush was reduced more with burning than cutting. Sagebrush, however, was predicted to be a substantial component of the overstory longer in burned than cut areas because of more effective juniper control. Variation in exotic annual grass cover was explained by environmental variables and perennial grass abundance, but not treatment, with annual grasses being problematic on hotter and drier sites with less perennial grass. This suggests that ecological memory varies along an environmental gradient. Reintroducing fire was more effective than cutting at conserving sagebrush‐steppe encroached by juniper over extended time frames; however, cutting was more effective for short‐term conservation. This suggests fire and fire surrogates both have critical roles in conservation of imperiled ecosystems. [ABSTRACT FROM AUTHOR]

Restoration of shrubs in arid and semi-arid rangelands is hampered by low success rates. Planting shrub seedlings is a method used to improve success in these rangelands; however, it is expensive and labor intensive. The efficiency of shrub restoration could be improved by identifying microsites where shrub survival is greater. Bitterbrush ( Purshia tridentata Pursh DC) is an important shrub to wildlife that has declined because of conifer encroachment, excessive defoliation, wildfires, and low recruitment. We investigated planting bitterbrush seedlings in western juniper ( Juniperus occidentalis ssp. occidentalis Hook) encroached shrublands after prescribed fire was used to control trees. Bitterbrush seedlings were planted in under (canopy) and between (interspace) former juniper canopies at five blocks and evaluated for three growing seasons. Bitterbrush survival was greater than 50% in the former canopy, but only 5% in the interspace microsite by the third growing season. Growth of bitterbrush was also greater in the former canopy compared with the interspace, potentially due to markedly less perennial vegetation in this microsite. Exotic annual grasses and annual forbs became prevalent in the former canopy in the second and third growing season, suggesting that soil resource availability was greater in this microsite. These results suggest that restoration success will vary by specific locations within a burned landscape and that this variability can be used to improve restoration efficiency. In this situation, bitterbrush restoration can be improved by planting seedlings in former canopy compared with interspace microsites. [ABSTRACT FROM AUTHOR]

Fire, herbivory and their interaction influence plant community dynamics. However, little is known about the influence of prefire herbivory on postfire plant community response, particularly long-term resistance to postfire exotic plant invasion in areas that historically experienced limited large herbivore pressure and infrequent, periodic fires., We investigated the long-term postfire effects of prefire herbivory by cattle, an exotic herbivore, in Artemisia (sagebrush) plant communities in the northern Great Basin, USA. Study areas were moderately grazed or not grazed by cattle since 1936 and then were burned in 1993. Plant community response was measured the 19th through the 22nd year postfire. Prior to burning exotic annual grass presence was minimal (<0.5% foliar cover) and plant community characteristics were similar between grazed and ungrazed treatments, with the exception of litter biomass being two times greater in the ungrazed treatment., Two decades postfire, Bromus tectorum L., an exotic annual grass, dominated the ungrazed treatment. Native bunchgrasses, species richness, and soil biological crusts were greater in prefire grazed areas compared to ungrazed areas., These results suggest that moderate prefire herbivory by cattle increased the resistance of the plant community to postfire invasion and dominance by B. tectorum. We presume that herbivory reduced mortality of large perennial bunchgrasses during the fire by reducing fine fuel (litter) and subsequently burn temperatures., Synthesis: This research demonstrates that a moderate disturbance (herbivory) may mediate the effects of a subsequent disturbance (fire). The effects of disturbances are not independent; therefore quantifying these interactions is critical to preventing oversimplification of complex plant community dynamics and guiding the conservation of endangered ecosystems. [ABSTRACT FROM AUTHOR]

Sagebrush ecosystems of western North America are experiencing widespread loss and degradation by invasive annual grasses. Positive feedbacks between fire and annual grasses are often invoked to explain the rapid pace of these changes, yet annual grasses also appear capable of achieving dominance among vegetation communities that have not burned for many decades. Using a dynamic, remotely-sensed vegetation dataset in tandem with remotely-sensed fire perimeter and burn severity datasets, we examine the role of fire in transitions to and persistence of annual grass dominance in the U.S. Great Basin over the past 3 decades. Although annual grasses and wildfire are so tightly associated that one is rarely mentioned without the other, our findings reveal surprisingly widespread transformation of sagebrush ecosystems by invasive annual grasses in the absence of fire. These findings are discussed in the context of strategic management; we argue a pivot from predominantly reactive management (e.g., aggressive fire suppression and post-fire restoration in heavily-infested areas) to more proactive management (e.g., enhancing resistance and managing propagule pressure in minimally-invaded areas) is urgently needed to halt the loss of Great Basin sagebrush ecosystems. • Invasive annual grasses are displacing shrublands and grasslands in the Great Basin. • Management is focused on suppressing wildfire and post-fire restoration. • Most transitions to annual grass dominance have occurred without the catalyst of fire. • We must pivot toward a greater emphasis on proactive protection of intact ecosystems. [ABSTRACT FROM AUTHOR]

Exotic annual grasses dominate millions of hectares and increase fire frequency in the sagebrush ecosystem of North America. This devastating invasion is so costly and challenging to revegetate with perennial vegetation that restoration efforts need to be prioritized and strategically implemented. Management needs to break the annual grass-fire cycle and prevent invasion of new areas, while research is needed to improve restoration success. Under current land management and climate regimes, extensive areas will remain annual grasslands, because of their expansiveness and the low probability of transition to perennial dominance. We propose referring to these communities as Intermountain West Annual Grasslands, recognizing that they are a stable state and require different management goals and objectives than perennial-dominated systems. We need to learn to live with annual grasslands, reducing their costs and increasing benefits derived from them, at the same time maintaining landscape-level plant diversity that could allow transition to perennial dominance under future scenarios. To accomplish this task, we propose a framework and research to improve our ability to live with exotic annual grasses in the sagebrush biome. [ABSTRACT FROM AUTHOR]