Fire and Grazing Influence Site Resistance to <Emphasis Type="Italic">Bromus tectorum</Emphasis> Through Their Effects on Shrub,Bunchgrass and Biocrust Communities in the Great Basin (USA)

Shrubs, bunchgrasses and biological soil crusts (biocrusts) are believed to contribute to site resistance to plant invasions in the presence of cattle grazing. Although fire is a concomitant disturbance with grazing, little is known regarding their combined impacts on invasion resistance. We are the first to date to test the idea that biotic communities mediate the effects of disturbance on site resistance. We assessed cover of Bromus tectorum, shrubs, native bunchgrasses, lichens and mosses in 99 burned and unburned plots located on similar soils where fires occurred between 12 and 23 years before sampling. Structural equation modeling was used to test hypothesized relationships between environmental and disturbance characteristics, the biotic community and resistance to B. tectorum cover. Characteristics of fire and grazing did not directly relate to cover of B. tectorum. Relationships were mediated through shrub, bunchgrass and biocrust communities. Increased site resistance following fire was associated with higher bunchgrass cover and recovery of bunchgrasses and mosses with time since fire. Evidence of grazing was more pronounced on burned sites and was positively correlated with the cover of B. tectorum, indicating an interaction between fire and grazing that decreases site resistance. Lichen cover showed a weak, negative relationship with cover of B. tectorum. Fire reduced near-term site resistance to B. tectorum on actively grazed rangelands. Independent of fire, grazing impacts resulted in reduced site resistance to B. tectorum, suggesting that grazing management that enhances plant and biocrust communities will also enhance site resistance.     

Drivers of <Emphasis Type="Italic">Bromus tectorum</Emphasis> Abundance in the Western North American Sagebrush Steppe

Bromus tectorum can transform ecosystems causing negative impacts on the ecological and economic values of sagebrush steppe of the western USA. Although our knowledge of the drivers of the regional distribution of B. tectorum has improved, we have yet to determine the relative importance of climate and local factors causing B. tectorum abundance and impact. To address this, we sampled 555 sites distributed geographically and ecologically throughout the sagebrush steppe. We recorded the canopy cover of B. tectorum, as well as local substrate and vegetation characteristics. Boosted regression tree modeling revealed that climate strongly limits the transformative ability of B. tectorum to a portion of the sagebrush steppe with dry summers (that is, July precipitation <10 mm and the driest annual quarter associated with a mean temperature >15°C) and low native grass canopy cover. This portion includes the Bonneville, Columbia, Lahontan, and lower Snake River basins. These areas are likely to require extreme efforts to reverse B. tectorum transformation. Our predictions, using future climate conditions, suggest that the transformative ability of B. tectorum may not expand geographically and could remain within the same climatically suitable basins. We found B. tectorum in locally disturbed areas within or adjacent to all of our sample sites, but not necessarily within sagebrush steppe vegetation. Conversion of the sagebrush steppe by B. tectorum, therefore, is more likely to occur outside the confines of its current climatically optimal region because of site-specific disturbances, including invasive species control efforts and sagebrush steppe mismanagement, rather than climate change.     

Fire effects on the mobilization and uptake of nitrogen by cheatgrass (Bromus tectorum L.)

Cheatgrass (Bromus tectorum L.), an invasive annual grass, is displacing native species and causing increased fire frequency in the Great Basin of the southwestern United States. Growth and nitrogen uptake patterns by cheatgrass were examined in a greenhouse study using soils from sites with the same soil type but different fire histories: 1) an area that burned in 1999 that is now completely invaded with cheatgrass (CG); 2) an area that has not burned recently and is now dominated by Wyoming big sagebrush (Artemisia tridentatassp.wyomingensis Beetle and Young) and Sandberg’s bluegrass (Poa secunda J. Presl) (WBS); and 3) a Wyoming big sagebrush area that burned in August of 2008 just prior to soil collection (NB). Cheatgrass seedlings had higher leaf numbers, height and mass in the NB soil. Ammonium-N mobilized by fire in the NB soil had significantly enriched ¹⁵N than soils from CG or WBS sites and this pattern was reflected in the isotopic signatures of the plants. Fire-mobilized mineral N accounted for only 58% of N taken up by cheatgrass in the NB soil, suggesting fire enhanced the ability of cheatgrass to assimilate more recalcitrant soil organic N.     

Conversion of sagebrush shrublands to exotic annual grasslands negatively impacts small mammal communities

Aim The exotic annual cheatgrass (Bromus tectorum) is fast replacing sagebrush (Artemisia tridentata) communities throughout the Great Basin Desert and nearby regions in the Western United States, impacting native plant communities and altering fire regimes, which contributes to the long‐term persistence of this weedy species. The effect of this conversion on native faunal communities remains largely unexamined. We assess the impact of conversion from native perennial to exotic annual plant communities on desert rodent communities. Location Wyoming big sagebrush shrublands and nearby sites previously converted to cheatgrass‐dominated annual grasslands in the Great Basin Desert, Utah, USA. Methods At two sites in Tooele County, Utah, USA, we investigated with Sherman live trapping whether intact sagebrush vegetation and nearby converted Bromus tectorum‐dominated vegetation differed in rodent abundance, diversity and community composition. Results Rodent abundance and species richness were considerably greater in sagebrush plots than in cheatgrass‐dominated plots. Nine species were captured in sagebrush plots; five of these were also trapped in cheatgrass plots, all at lower abundances than in the sagebrush. In contrast, cheatgrass‐dominated plots had no species that were not found in sagebrush. In addition, the site that had been converted to cheatgrass longer had lower abundances of rodents than the site more recently converted to cheatgrass‐dominated plots. Despite large differences in abundances and species richness, Simpson’s D diversity and Shannon‐Wiener diversity and Brillouin evenness indices did not differ between sagebrush and cheatgrass‐dominated plots. Main conclusions This survey of rodent communities in native sagebrush and in converted cheatgrass‐dominated vegetation suggests that the abundances and community composition of rodents may be shifting, potentially at the larger spatial scale of the entire Great Basin, where cheatgrass continues to invade and dominate more landscape at a rapid rate.     

Facilitation and interference of seedling establishment by a native legume before and after wildfire

In semi-arid ecosystems, heterogeneous resources can lead to variable seedling recruitment. Existing vegetation can influence seedling establishment by modifying the resource and physical environment. We asked how a native legume, Lupinus argenteus, modifies microenvironments in unburned and burned sagebrush steppe, and if L. argenteus presence facilitates seedling establishment of native species and the non-native annual grass, Bromus tectorum. Field treatments examined mechanisms by which L. argenteus likely influences establishment: (1) live L. argenteus; (2) dead L. argenteus; (3) no L. argenteus; (4) no L. argenteus with L. argenteus litter; (5) no L. argenteus with inert litter; and (6) mock L. argenteus. Response variables included soil nitrogen, moisture, temperature, solar radiation, and seedling establishment of the natives Elymus multisetus and Eriogonum umbellatum, and non-native B. tectorum. In both unburned and burned communities, there was higher spring soil moisture, increased shade and reduced maximum temperatures under L. argenteus canopies. Adult L. argenteus resulted in greater amounts of soil nitrogen (N) only in burned sagebrush steppe, but L. argenteus litter increased soil N under both unburned and burned conditions. Although L. argenteus negatively affected emergence and survival of B. tectorum overall, its presence increased B. tectorum biomass and reproduction in unburned plots. However, L. argenteus had positive facilitative effects on size and survival of E. multisetus in both unburned and burned plots. Our study indicates that L. argenteus can facilitate seedling establishment in semi-arid systems, but net effects depend on the species examined, traits measured, and level of abiotic stress.     

Variation in the impact of exotic grasses on native plant composition in relation to fire across an elevation gradient in Hawaii

The impact that an exotic species can have on the composition of the community it enters is a function of its abundance, its particular species traits and characteristics of the recipient community. In this study we examined species composition in 14 sites burned in fires fuelled by non‐indigenous C4 grasses in Hawaii Volcanoes National Park, Hawaii. We considered fire intensity, time since fire, climatic zone of site, unburned grass cover, unburned native cover and identity of the most abundant exotic grass in the adjacent unburned site as potential predictor variables of the impact of fire upon native species. We found that climatic zone was the single best variable for explaining variation in native cover among burned sites and between burned and unburned pairs. Fire in the eastern coastal lowlands had a very small effect on native plant cover and often stimulated native species regeneration, whereas fire in the seasonal submontane zone consistently caused a decline in native species cover and almost no species were fire tolerant. The dominant shrub, Styphelia tameiameia, in particular was fire intolerant. The number of years since fire, fire intensity and native cover in reference sites were not significantly correlated with native species cover in burned sites. The particular species of grass that carried the fire did however, have a significant effect on native species recovery. Where the African grass Melinis minutiflora was a dominant or codominant species, fire impacts were more severe than where it was absent regardless of climate zone. Overall, the impacts of exotic grass‐fuelled fires on native species composition and cover in seasonally dry Hawaiian ecosystems was context specific. This specificity is best explained by differences between the climatic zones in which fire occurred. Elevation was the main physical variable that differed among the climatic zones and it alone could explain a large percentage of the variation in native cover among sites. Rainfall, by contrast, did not vary systematically with elevation. Elevation is associated with differences in composition of the native species assemblages. In the coastal lowlands, the native grass Heteropogon contortus, was largely responsible for positive changes in native cover after fire although other native species also increased. Like the exotic grasses, this species is a perennial C4 grass. It is lacking in the submontane zone and there are no comparable native species there and almost all native species in the submontane zone were reduced by fire. The lack of fire tolerant species in the submontane zone thus clearly contributes to the devastating impact of fire upon native cover there.     

Effects of invasive plants on fire regimes and postfire vegetation diversity in an arid ecosystem

We assessed the impacts of co‐occurring invasive plant species on fire regimes and postfire native communities in the Mojave Desert, western USA. We analyzed the distribution and co‐occurrence patterns of three invasive annual grasses (Bromus rubens, Bromus tectorum, and Schismus spp.) known to alter fuel conditions and community structure, and an invasive forb (Erodium cicutarium) which dominates postfire sites. We developed species distribution models (SDMs) for each of the four taxa and analyzed field plot data to assess the relationship between invasives and fire frequency, years postfire, and the impacts on postfire native herbaceous diversity. Most of the Mojave Desert is highly suitable for at least one of the four invasive species, and 76% of the ecoregion is predicted to have high or very high suitability for the joint occurrence of B. rubens and B. tectorum and 42% high or very high suitability for the joint occurrence of the two Bromus species and E. cicutarium. Analysis of cover from plot data indicated two or more of the species occurred in 77% of the plots, with their cover doubling with each additional species. We found invasive cover in burned plots increased for the first 20 years postfire and recorded two to five times more cover in burned than unburned plots. Analysis also indicated that native species diversity and evenness as negatively associated with higher levels of relative cover of the four invasive taxa. Our findings revealed overlapping distributions of the four invasives; a strong relationship between the invasives and fire frequency; and significant negative impacts of invasives on native herbaceous diversity in the Mojave. This suggests predicting the distributions of co‐occurring invasive species, especially transformer species, will provide a better understanding of where native‐dominated communities are most vulnerable to transformations following fire or other disturbances.     

Strong genetic differentiation in the invasive annual grass <Emphasis Type="Italic">Bromus tectorum</Emphasis> across the Mojave–Great Basin ecological transition zone

Bromus tectorum, an inbreeding annual grass, is a dominant invader in sagebrush steppe habitat in North America. It is also common in warm and salt deserts, displaying a larger environmental tolerance than most native species. We tested the hypothesis that a suite of habitat-specific B. tectorum lineages dominates warm desert habitats. We sampled 30 B. tectorum Mojave Desert and desert fringe populations and genotyped 10–26 individuals per population using 69 single nucleotide polymorphic (SNP) markers. We compared these populations to 11 Great Basin steppe and salt desert populations. Populations from warm desert habitats were dominated by members of two haplogroups (87 % of individuals) that were distinct from haplogroups common in Great Basin habitats. We conducted common garden studies comparing adaptive traits and field performance among haplogroups typically found in different habitats. In contrast to the haplogroup abundant in sagebrush steppe, warm desert haplogroups generally lacked a vernalization requirement for flowering. The most widespread warm desert haplogroup (Warm Desert 1) also had larger seeds and a higher root:shoot ratio than other haplogroups. In the field, performance of warm desert haplogroups was dramatically lower than the sagebrush steppe haplogroup at one steppe site, but one warm desert haplogroup performed as well as the steppe haplogroup under drought conditions at the other site. Our results suggest that B. tectorum succeeds in widely disparate environments through ecotypic variation displayed by distinct lineages of plants. Accounting for this ecotypic variation is essential in modeling its future distribution in response to climate change.     

Duff Distribution Influences Fire Severity and Post-Fire Vegetation Recovery in Sagebrush Steppe

Woody plant expansion is a global phenomenon that alters the spatial distribution of nutrients, biomass, and fuels in affected ecosystems. Altered fuel patterns across the landscape influences ecological processes including fire behavior, fire effects, and can impact post-fire plant germination and establishment. The purpose of this study was to determine how accumulations of ground fuels beneath western juniper (Juniperus occidentalis ssp. occidentalis) canopies, composed of litter and duff, affect post-fire species response in sagebrush steppe and to quantify fuel loading patterns. Field sampling and analysis was conducted across environmental gradients following the 2007 Tongue-Crutcher Wildfire in southwestern Idaho to determine conditions that were most influential in post-fire vegetation recovery patterns. Duff depth and fire severity were determined to be the most influential factors affecting post-fire vegetation response. Decreasing species richness and native perennial grass cover was represented along the increasing duff depth gradient. Species response grouped by fire severity revealed significant presence of cheatgrass (Bromus tectorum) in low severity sites and a dominance of snowbrush ceanothus (Ceanothus velutinus) in higher severity sites. Determining sub-crown surface fuel characteristics offers the potential to predict future patterns and processes as they relate to burn severity and vegetation recovery components in developing woodlands.     

Fire promotes downy brome (Bromus tectorum L.) seed dispersal

Particularly well-known among the many impacts of the invasive annual grass downy brome (Bromus tectorum, Poaceae) is its ability to alter fire cycles and increase in abundance after fire. However, little is known about how fire influences B. tectorum dispersal. We quantified fire effects on B. tectorum dispersal using three recently burned areas in the western region of the Colorado Rocky Mountains by marking diaspores (seeds) with fluorescent powder, and then recovering them at night using ultraviolet lights. Diaspores were of two types: with and without sterile florets attached. We also characterized vegetation cover and near-surface wind speed in burned and unburned areas. Diaspores travelled much farther in burned areas than in nearby unburned areas (mean ± standard error at the end of the experiment: 209 ± 16 cm and 38 ± 1 cm, respectively; maximal distance at the end of the experiment: 2,274 cm and 150 cm, respectively), indicating an increase in dispersal distance after fire. Diaspores with sterile florets attached dispersed longer distances than those without sterile florets (mean ± standard error at the end of the experiment: 141 ± 14 cm and 88 ± 7 cm, respectively). Vegetation cover was lower and wind speeds were higher in the burned areas. Our results indicate that at least one of the mechanisms by which the spread of B. tectorum is promoted by fire is through increased seed dispersal distance. Preventing movement of seeds from nearby infestations into burned areas may help avoid the rapid population expansion often observed.     

Barriers to shrub reestablishment following fire in the seasonal submontane zone of Hawai'i

Introduced grass species have invaded extensive areas of Hawaii Volcanoes National Park and increased the size and frequency of fire. Following fire, grass cover is enhanced while native shrub cover is reduced; the reduction in most shrubs persists for at least 20 years even in the absence of fire. Shrub seedlings were planted in burned and unburned plots with and without grass cover. Biomass of 14 month old shrub seedlings was generally highest in recently burned/grass removed plots, intermediate in old burn/grass removed plots, and lowest in unburned/grass removed plots. In contrast, shrub biomass in plots with grass cover was low and did not differ significantly among burn treatments. Light competition is likely to be responsible for differences in shrub growth rates; grass cover reduced light to 1–10% of background levels. In addition, pool sizes of available soil N were highest in recently burned, intermediate in old burn, and lowest in unburned areas.     

Restoration of Native Perennials in a California Annual Grassland after Prescribed Spring Burning and Solarization

Grasslands dominated by exotic annual grasses have replaced native perennial vegetation types in vast areas of California. Prescribed spring fires can cause a temporary replacement of exotic annual grasses by native and non‐native forbs, but generally do not lead to recovery of native perennials, especially where these have been entirely displaced for many years. Successful reintroduction of perennial species after fire depends on establishment in the postfire environment. We studied the effects of vegetation changes after an April fire on competition for soil moisture, a key factor in exotic annual grass dominance. As an alternative to fire, solarization effectively kills seeds of most plant species but with a high labor investment per area. We compared the burn to solarization in a study of establishment and growth of seeds and transplants of the native perennial grass Purple needlegrass (Nassella pulchra) and coastal sage species California sagebrush (Artemisia californica). After the fire, initial seed bank and seedling densities and regular percent cover and soil moisture (0–20 cm) data were collected in burned and unburned areas. Burned areas had 96% fewer viable seeds of the dominant annual grass, Ripgut brome (Bromus diandrus), leading to replacement by forbs from the seed bank, especially non‐native Black mustard (Brassica nigra). In the early growing season, B. diandrus dominating unburned areas consistently depleted soil moisture to a greater extent between rains than forbs in burned areas. However, B. diandrus senesced early, leaving more moisture available in unburned areas after late‐season rains. Nassella pulchra and A. californica established better on plots treated with fire and/or solarization than on untreated plots. We conclude that both spring burns and solarization can produce conditions where native perennials can establish in annual grasslands. However, the relative contribution of these treatments to restoration appears to depend on the native species being reintroduced, and the long‐term success of these initial restoration experiments remains to be determined.     

Soil water exploitation after fire: competition between Bromus tectorum (cheatgrass) and two native species

Summary Causes for the widespread abundance of the alien grass Bromus tectorum (cheatgrass) after fire in semiarid areas of western North America may include: (1) utilization of resources freed by the removal of fireintolerant plants; and (2) successful competition between B. tectorum and individual plants that survive fire. On a site in northwestern Nevada (USA), measurements of soil water content, plant water potential, aboveground biomass production, water use efficiency, and B. tectorum tiller density were used to determine if B. tectorum competes with either of two native species (Stipa comata and Chrysothamnus viscidiflorus) or simply uses unclaimed resources. Soil water content around native species occurring with B. tectorum was significantly lower (P<0.05) than around individuals without B. tectorum nearby. Native species had significantly more negative plant water potential when they occurred with B. tectorum. Aboveground biomass was significantly higher for native species without B. tectorum. However, the carbon isotope ratio of leaves for native species with B. tectorum was not significantly different from individuals without B. tectorum. Thus, B. tectorum competes with native species for soil water and negatively affects their wate status and productivity, but the competition for water does not affect water use efficiency of the native species. These adverse effects of B. tectorum competition on the productivity and water status of native species are also evident at 12 years after a fire. This competitive ability of B. tectorum greatly enhances its capability to exploit soil resources after fire and to enhance its status in the community.     

Fire, native species, and soil resource interactions influence the spatio-temporal invasion pattern of Bromus tectorum

Bromus tectorum (cheatgrass) is an invasive annual that occupies perennial grass and shrub communities throughout the western United States. Bronus tectorum exhibits an intriguing spatio‐temporal pattern of invasion in low elevation ponderosa pine Pinus ponderosa/bunchgrass communities in western Montana where it forms dense rings beneath solitary pines following fire. This pattern provides a unique opportunity to investigate several indirect effects of native vegetation that influence the invasion pattern of B. tectorum, and specifically how native species, disturbance, and soil resources interact to influence the spatio‐temporal pattern of invasion. We established four replicate field sites, each containing burned‐tree, burned‐grass, unburned‐tree, and unburned‐grass sampling locations, and initiated a series of field sampling and greenhouse experiments utilizing these locations. The objective of our first greenhouse experiment was to identify whether belowground factors contributed to the pattern of B. tectorum biomass observed in these field locations. This experiment generated a B. tectorum biomass response that was nearly identical to the invasion pattern observed in the field, suggesting further investigation of belowground factors was necessary. We measured resin‐sorbed NH₄⁺ and NO₃⁻ during one generation of B. tectorum, and measured a suite of P fractions through a sequential extraction procedure from these soils. These data revealed that a resource island of high N and P exists beneath pine trees. Through a second greenhouse experiment, we determined that N limited B. tectorum biomass in tree soil, whereas P limited biomass in bunchgrass soil. Finally, through a germination experiment we determined that pine litter strongly inhibited B. tectorum germination. These data suggest B. tectorum is regulated by P in bunchgrass soil, and by N and inhibition by pine litter beneath trees, effects that are likely alleviated by fire. These data demonstrate the combined role of direct and indirect interactions between native and invasive species in regulating biological invasions.     

Net carbon exchange and evapotranspiration in postfire and intact sagebrush communities in the Great Basin

Invasion of non-native annuals across the Intermountain West is causing a widespread transition from perennial sagebrush communities to fire-prone annual herbaceous communities and grasslands. To determine how this invasion affects ecosystem function, carbon and water fluxes were quantified in three, paired sagebrush and adjacent postfire communities in the northern Great Basin using a 1-m³ gas exchange chamber. Most of the plant cover in the postfire communities was invasive species including Bromus tectorum L., Agropyron cristatum (L.) Gaertn and Sisymbrium altissimum L. Instantaneous morning net carbon exchange (NCE) and evapotranspiration (ET) in native shrub plots were greater than either intershrub or postfire plots. Native sagebrush communities were net carbon sinks (mean NCE 0.2–4.3 μmol m⁻² s⁻¹) throughout the growing season. The magnitude and seasonal variation of NCE in the postfire communities were controlled by the dominant species and availability of soil moisture. Net C exchange in postfire communities dominated by perennial bunchgrasses was similar to sagebrush. However, communities dominated by annuals (cheatgrass and mustard) had significantly lower NCE than sagebrush and became net sources of carbon to the atmosphere (NCE declined to −0.5 μmol m⁻² s⁻¹) with increased severity of the summer drought. Differences in the patterns of ET led to lower surface soil moisture content and increased soil temperatures during summer in the cheatgrass-dominated community compared to the adjacent sagebrush community. Intensive measurements at one site revealed that temporal and spatial patterns of NCE and ET were correlated most closely with changes in leaf area in each community. By altering the patterns of carbon and water exchange, conversion of native sagebrush to postfire invasive communities may disrupt surface-atmosphere exchange and degrade the carbon storage capacity of these systems.     

Grass-Shrub Associations over a Precipitation Gradient and Their Implications for Restoration in the Great Basin,USA

As environmental stress increases positive (facilitative) plant interactions often predominate. Plant-plant associations (or lack thereof) can indicate whether certain plant species favor particular types of microsites (e.g., shrub canopies or plant-free interspaces) and can provide valuable insights into whether “nurse plants” will contribute to seeding or planting success during ecological restoration. It can be difficult, however, to anticipate how relationships between nurse plants and plants used for restoration may change over large-ranging, regional stress gradients. We investigated associations between the shrub, Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis), and three common native grasses (Poa secunda, Elymus elymoides, and Pseudoroegneria spicata), representing short-, medium-, and deep-rooted growth forms, respectively, across an annual rainfall gradient (220–350 mm) in the Great Basin, USA. We hypothesized that positive shrub-grass relationships would become more frequent at lower rainfall levels, as indicated by greater cover of grasses in shrub canopies than vegetation-free interspaces. We sampled aerial cover, density, height, basal width, grazing status, and reproductive status of perennial grasses in canopies and interspaces of 25–33 sagebrush individuals at 32 sites along a rainfall gradient. We found that aerial cover of the shallow rooted grass, P. secunda, was higher in sagebrush canopy than interspace microsites at lower levels of rainfall. Cover and density of the medium-rooted grass, E. elymoides were higher in sagebrush canopies than interspaces at all but the highest rainfall levels. Neither annual rainfall nor sagebrush canopy microsite significantly affected P. spicata cover. E. elymoides and P. spicata plants were taller, narrower, and less likely to be grazed in shrub canopy microsites than interspaces. Our results suggest that exploring sagebrush canopy microsites for restoration of native perennial grasses might improve plant establishment, growth, or survival (or some combination thereof), particularly in drier areas. We suggest that land managers consider the nurse plant approach as a way to increase perennial grass abundance in the Great Basin. Controlled experimentation will provide further insights into the life stage-specific effectiveness and practicality of a nurse plant approach for ecological restoration in this region.     

Prefire grazing by cattle increases postfire resistance to exotic annual grass (Bromus tectorum) invasion and dominance for decades

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Introduced annual grass increases regional fire activity across the arid western USA (1980–2009)

Non‐native, invasive grasses have been linked to altered grass‐fire cycles worldwide. Although a few studies have quantified resulting changes in fire activity at local scales, and many have speculated about larger scales, regional alterations to fire regimes remain poorly documented. We assessed the influence of large‐scale Bromus tectorum (hereafter cheatgrass) invasion on fire size, duration, spread rate, and interannual variability in comparison to other prominent land cover classes across the Great Basin, USA. We compared regional land cover maps to burned area measured using the Moderate Resolution Imaging Spectroradiometer (MODIS) for 2000–2009 and to fire extents recorded by the USGS registry of fires from 1980 to 2009. Cheatgrass dominates at least 6% of the central Great Basin (650 000 km²). MODIS records show that 13% of these cheatgrass‐dominated lands burned, resulting in a fire return interval of 78 years for any given location within cheatgrass. This proportion was more than double the amount burned across all other vegetation types (range: 0.5–6% burned). During the 1990s, this difference was even more extreme, with cheatgrass burning nearly four times more frequently than any native vegetation type (16% of cheatgrass burned compared to 1–5% of native vegetation). Cheatgrass was also disproportionately represented in the largest fires, comprising 24% of the land area of the 50 largest fires recorded by MODIS during the 2000s. Furthermore, multi‐date fires that burned across multiple vegetation types were significantly more likely to have started in cheatgrass. Finally, cheatgrass fires showed a strong interannual response to wet years, a trend only weakly observed in native vegetation types. These results demonstrate that cheatgrass invasion has substantially altered the regional fire regime. Although this result has been suspected by managers for decades, this study is the first to document recent cheatgrass‐driven fire regimes at a regional scale.     

Consequences of Seed Origin and Biological Invasion for Early Establishment in Restoration of a North American Grass Species

Local, wild-collected seeds of native plants are recommended for use in ecological restoration to maintain patterns of adaptive variation. However, some environments are so drastically altered by exotic, invasive weeds that original environmental conditions may no longer exist. Under these circumstances, cultivated varieties selected for improved germination and vigor may have a competitive advantage at highly disturbed sites. This study investigated differences in early establishment and seedling performance between wild and cultivated seed sources of the native grass, Poa secunda, both with and without competition from the invasive exotic grass, Bromus tectorum. We measured seedling survival and above-ground biomass at two experimental sites in western Montana, and found that the source of seeds selected for restoration can influence establishment at the restoration site. Cultivars had an overall advantage when compared with local genotypes, supporting evidence of greater vigor among cultivated varieties of native species. This advantage, however, declined rapidly in the presence of B. tectorum and most accessions were not significantly different for growth and survival in competition plots. Only one cultivar had a consistent advantage despite a strong decline in its performance when competing with invasive plants. As a result, cultivated varieties did not meet expectations for greater establishment and persistence relative to local genotypes in the presence of invasive, exotic species. We recommend the use of representative local or regional wild seed sources in restoration to minimize commercial selection, and a mix of individual accessions (wild, or cultivated when necessary) in highly invaded settings to capture vigorous genotypes and increase the odds native plants will establish at restoration sites.     

The potential importance of unburned islands as refugia for the persistence of wildlife species in fire‐prone ecosystems

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