Invasive Species
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Exotic annual grasses and forbs were each reduced by herbicides and by seeding perennial grasses. The combination of herbicide and seeding reduced annuals and led to the largest increases in perennials. Although these outcomes support the intended effects of the treatments, there was high variability in outcomes among studies. Combined use of pre-emergent herbicides and seeding can increase the success of restoration interventions that are aimed at reducing the invasion of exotic annual grasses and increasing perennials after fire in sagebrush steppe. Our analysis revealed, however, that the available literature was not suited to answering more specific questions, in spite of the massive amount of post-fire herbicide and seedings that have been applied in burned sagebrush steppe. Specifically, there were too few topically relevant studies with adequate scientific reporting to properly evaluate differences among specific treatment methodologies, including specific herbicides, that affect restoration success.
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We applied indaziflam in fall 2019 to replicate plots within two sagebrush-steppe sites in the Northern Great Basin, USA: 1) a relatively intact, uninvaded, unburned “core” site and 2) a partially invaded site that burned in the 2015 Soda Wildfire. Vegetation cover, density, and growth responses of native perennials were measured annually to 2024. We asked whether our treatments “defended” and “grew” core sagebrush areas. EAG cover remained <15% in indaziflam-treated plots while increasing to >30% in control plots by the fifth year after treatment at the unburned site but did not differ with treatment at the burned site. Native perennial grasses, forbs, and big sagebrush cover and growth did not differ with indaziflam treatment at either site. Moss cover was temporarily lower in indaziflam-treated plots at the unburned site, and cover of a native annual forb was significantly lower in indaziflam-treated plots throughout the study across both sites. Despite posttreatment drought and apparent patchiness in treatment implementation, our treatments “defended the core” by preventing crossing of the 20% EAG invasion threshold in the unburned site but not did not “grow the core.” Our results provide an example of a case in which proactive protection may be easier to accomplish than reactive restoration. Herbicide treatment effects may be sensitive to weather and application details. Implementation monitoring could help explain variability and improve success.
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To better understand the social acceptability of core features of ISM in the United States, we conducted an online experiment with vignettes describing hypothetical but realistic ISM scenarios, varying targeted taxon (insect or plant), control method (mechanical, chemical and biological), risk severity (low and high) and type of non-target risk (to humans or native species). Not surprisingly, management with low risk was most acceptable, particularly for mechanical control. In high-risk scenarios, only mechanical control was acceptable, but only by a slim majority of respondents. Overall, chemical and biological controls showed low levels of acceptability. Surprisingly, there was no significant difference in how respondents ranked risks to people and risks to native species. Beyond differences in acceptability between management factors, we also find that the acceptability of management and attitudes towards risk were associated with respondents’ demographic characteristics. Overall, our findings indicate that widespread acceptability of ISM should not be assumed. While management activities representing low-risk scenarios find some support in the public, our results highlight a disconnect between the effectiveness of common management methods and their social acceptability. Our findings highlight the need for evidence-guided ISM, which includes evidence of harmful impacts of introduced species, as well as risks and benefits of management activities, as one potential way to increase the social acceptability of non-native species management.
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We asked how the soil-microbiome responded to the bioherbicide Pseudomonas fluorescens strain ACK55 in comparison to the separate and combined effects of a conventional pre-emergent chemical herbicide, imazapic, in two cheatgrass-invaded sagebrush-steppe sites. First-year microbial responses were evaluated using targeted sequencing of the 16S and LSU rRNA genes for bacteria+archaea and fungi, respectively, and were related to plant-community responses. A strong cheatgrass reduction with imazapic at one site was accompanied by a small shift in bacteria+archaea (16S) community composition with no effect on microbial alpha diversity, and this shift was small in comparison to natural microbiome variation between sites. ACK55 was not detected in soil a year after application, and it caused only transient and marginally significant reductions in annual grass cover accompanied by small reductions in soil fungi species richness. Full-length sequencing of the ACK55 16S rRNA gene and phylogenetic analyses revealed that ACK55 is more likely P. salmonii than P. fluorescens. Knowledge gaps remain on the duration and consequences of microbial-community shifts with imazapic and why molecular analyses showed ACK55 did not persist in soils. Confusion regarding microbial biopesticides can result where isolation, effectiveness testing, commercial release, and regulation are not guided by molecular taxonomic analyses.
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In a factorial design replicated across four common garden locations in Idaho and Wyoming, USA, we tested for the effect of current environment (i.e., density treatment, temperature treatment, and common garden location), source environment (i.e., genotype source climate), and their interaction on each plant’s flowering phenology. Flowering timing was strongly influenced by a plant’s current environment, with plants that experienced warmer current climates and higher densities flowering earlier than those that experienced cooler current climates and lower densities. Genotypes from hot and dry source climates flowered consistently earlier than those from cool and wet source climates, even after accounting for genotype relatedness, suggesting that this genetically based climate cline is a product of natural selection. We found minimal evidence of interactions between current and source environments or genotype-by-environment interactions. Phenology was more sensitive to variation in the current climate than to variation in source climate. These results indicate that cheatgrass phenology reflects high levels of plasticity as well as rapid local adaptation. Both processes likely contribute to its current success as a biological invader and its capacity to respond to future environmental change.
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We investigated the spread of “Snowstorm” forage kochia at 11 invasive annual grass-dominated plant communities in southeastern Oregon a decade after they had been seeded. The seeding boundary was permanently marked at the time of seeding to ensure accurate measurements of forage kochia spread. Forage kochia established beyond the seeding boundary at two of the 11 seeded sites. The maximum distance that forage kochia spread from the seeding boundary was 65 cm. These results suggest that forage kochia does not readily spread in annual grass-dominated sagebrush steppe and what little spread it exhibited was over a relatively short distance. The risk of forage kochia spreading and dominating annual grass-invaded rangelands, at least in this region, appears minimal. Additional investigations of forage kochia spread in different environments and across various plant communities is warranted to inform land managers of any potential risks. Multiple decades evaluations of its potential to spread would also be important. Even though additional research would be valuable, our results that forage kochia does not readily spread suggest it may be a relatively safe option to seed to diversify nonnative grasslands.
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We measured soil organic carbon (SOC) and its particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) constituents in the surface soils of sites that had sagebrush canopies but differed in whether their understories had been invaded by cheatgrass or not, in both warm and relatively colder ecoregions of the western USA. MAOC stocks were 36.1% less in the 0–10 cm depth and 46.1% less in the 10–20 cm depth in the cheatgrass-invaded stands compared to the uninvaded stands of the warmer Colorado Plateau, but not in the cooler and more carbon-rich Wyoming Basin ecoregion. In plots where cheatgrass increased SOC, it was via unstable POC. These findings indicate that cheatgrass effects on the distribution of soil carbon among POC and MAOC fractions may vary among ecoregions, and that cheatgrass can reduce forms of carbon that are otherwise considered stable and ‘secure’, i.e. sequestered.
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We studied the impact of increased fire frequencies on the composition and abundance of herbaceous and woody species in the Interior Coast Range of northern California. Our study area is one of the most frequently burned areas in California, which allowed us to investigate higher fire frequencies than previously published in the scientific literature for California. We surveyed fifty-four 250-m2 plots to assess changes in plant community composition and postfire regeneration of chaparral shrubs across a wide range of fire frequencies, including plots that have burned up to six times in the past 30 years. Our findings reveal that short-interval fires significantly reduced postfire native woody regeneration, with obligate seeding species experiencing a 99% reduction and facultative species showing an 83% reduction in regeneration in the most frequently burned plots. Moreover, the overall marginal effect of one additional fire since 1985 decreased the proportion of native species cover by 12% and both richness and Shannon diversity by 4%. Consequently, areas with higher fire recurrence supported a more structurally and botanically homogeneous landscape dominated by a homogeneous group of non-native species.
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The rate of change in invasive annual grass cover describes the trajectory of invasion. This information can be used to fine-tune priority locations and strategies for invasive species treatments. We identified locations with positive, neutral, negative, and variable rates of change. Although rates of change have accelerated, there were many locations with a consistent neutral rate of change in cover. High positive rates of change frequently preceded high invasive annual grass cover, and locations that had low cover rarely had a history of high positive rates of change. We identified potential management opportunities by combining rates of change in cover and percent cover data, illustrating both invasion severity and trajectory. We applied these potential opportunities to a map of the sagebrush biome using example thresholds. This map identifies locations that could be prioritized for different management goals and shows how those areas align with the Sagebrush Conservation Design management
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This study aimed to determine the flammability of cheatgrass compared to two native perennial grasses (Columbia needlegrass [Achnatherum nelsonii] and bluebunch wheatgrass [Pseudoroegneria spicata]) across a range of fuel moistures. All three grass species had decreased flammability with increasing fuel moisture. Columbia needlegrass averaged 11% lower mass consumption than cheatgrass, and bluebunch wheatgrass had longer flaming duration and higher maximum temperatures than cheatgrass and Columbia needlegrass. The addition of cheatgrass to each perennial grass increased combined mass consumption, flaming duration, and flame heights. For these three attributes, the impact differed by the amount of cheatgrass in the mixture. Maximum and mean temperatures during perennial grass combustion were similar with and without cheatgrass addition. Some attributes of Columbia needlegrass flammability when burned with cheatgrass were higher than expected based on the flammability of each species, suggesting that Columbia needlegrass may be susceptible to pre-heating from combustion of cheatgrass. Conversely, the flammability of bluebunch wheatgrass and cheatgrass together had both positive and negative interactive effects, suggesting the impact on joint flammability from cheatgrass differs by perennial grass species.