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This report assesses recent forest disturbance in the Western United States and discusses implications for sustainability. Individual chapters focus on fire, drought, insects, disease, invasive plants, and socioeconomic impacts. Disturbance data came from a variety of sources, including the Forest Inventory and Analysis program, Forest Health Protection, and the National Interagency Fire Center. Disturbance trends with the potential to affect forest sustainability include alterations in fire regimes, periods of drought in some parts of the region, and increases in invasive plants, insects, and disease. Climate affects most disturbance processes, particularly drought, fire, and biotic disturbances, and climate change is expected to continue to affect disturbance processes in various ways and degrees.
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Based on our review of the scientific evidence, a range of proactive management actions are justified and necessary to keep pace with changing climatic and wildfire regimes and declining forest heterogeneity after severe wildfires. Science-based adaptation options include the use of managed wildfire, prescribed burning, and coupled mechanical thinning and prescribed burning as is consistent with land management allocations and forest conditions. Although some current models of fire management in wNA are averse to short-term risks and uncertainties, the long-term environmental, social, and cultural consequences of wildfire management primarily grounded in fire suppression are well documented, highlighting an urgency to invest in intentional forest management and restoration of active fire regimes.
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Relative to unburned sites, we found that burned sites had lower stem density and had lower proportions of recently dead trees (for stems ≤47.5 cm dbh) that presumably died during the drought. Differences in recent tree mortality among burned and unburned sites held for both fir (white fir and red fir) and pine (sugar pine and ponderosa pine) species. Unlike earlier results, models of individual tree mortality probability supported an interaction between plot burn status and tree size, suggesting the effect of prescribed fire was limited to small trees. We consider differences with other recent results and discuss potential management implications including trade-offs between large tree mortality following prescribed fire and increased drought resistance.
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This study demonstrates the importance of episodic periods of favorable weather for long-term plant population recovery following disturbance. Management strategies that increase opportunities for seed availability to coincide with favorable weather conditions, such as retaining unburned patches or repeated seeding treatments, can improve restoration outcomes in high-priority areas.
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During the first years after removal of perennial grasses and forbs, there was an increase in soil water availability in spring at 13–30 cm soil depth that was associated with sagebrush establishment, particularly at upper elevations. In subsequent years, sagebrush continued to dominate even though little difference in soil water availability existed between disturbed and undisturbed plots. This indicates that quickly establishing sagebrush preempted resources and reduced perennial herb recovery. Resource preemption after disturbance will likely be a major driver of plant succession in the future as in the past. Species that establish best under future warmer and drier conditions are most likely to dominate after disturbance. A negative correlation (r2 = 0.34) between the standard deviation of annual spring soil water availability and perennial vegetation cover, which helps resist annual grass invasion, supports the hypothesis that greater resource fluctuation is associated with greater plant community invasibility. Current responses to fire and loss of native plant cover across elevational gradients can indicate future responses under a warmer and drier climate.
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Invasive forb and grass species are likely to expand their ranges and continued increases in temperature, aridity and area burned will increase invasion risk. Monitoring species presence and absence and mapping known and potential ranges with a focus on presence detection, as in our methodology, will aid in identifying new invasions and prioritizing prevention and control.
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Treatments in cooler and moister woodland sites had more positive effects on sagebrush recruitment and perennial grass cover, less negative effects on sagebrush intraspecific interactions, and smaller increases in annual grass cover indicating potential increases in resilience to fire. In warmer and drier invasion sites, reductions in woody fuels resulted in lack of sagebrush recruitment, disruption of sagebrush intraspecific interactions, and progressive increases in annual grass indicating reduced resilience to fire and resistance to invaders.
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Seeded native and introduced bunchgrasses both increased bunchgrass abundance and cover, even though precipitation was below average the first year post-seeding. Seeding introduced wheatgrasses, however, increased bunchgrass cover and abundance more than seeding native bunchgrasses. Seeding introduced wheatgrasses also limited exotic annual grass abundance and cover, but seeding locally sourced native bunchgrasses did not. Native bunchgrasses are slow growing, thus may limit exotic annual grasses in time. Alternatively, additional treatments, such as exotic annual grass control, may be needed to improve their success. The establishment of seeded native bunchgrasses in Wyoming big sagebrush in a below-average precipitation year is a promising result and suggests further research to improve seeded native vegetation success is warranted. The greater establishment of introduced wheatgrasses and their ability to limit exotic annual grasses suggests that successful introduced species may serve as a model for guiding trait selection in native species.
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The encroachment of pinyon-juniper woodlands into sagebrush habitat in the Great Basin Ecoregion of the western USA, represents a potential source of habitat degradation for sagebrush-associated wildlife species. To restore sagebrush habitat, managers are conducting large-scale conifer removal efforts within the Great Basin, particularly within Greater Sage-Grouse (Centrocercus urophasianus) priority areas for conservation. Such largescale habitat modification efforts may result in unintended ecological trade-offs for wildlife. To investigate these trade-offs, we used community science data to develop species distribution models for two sagebrush and three pinyon-juniper associated bird species of conservation concern in the Great Basin. We evaluated the predictive performance of our models with an independent dataset of presence locations derived from systematic monitoring programs. We then simulated conifer removal across the Great Basin and mapped habitat gains and losses for our study species. Despite differing land cover associations, 31%-51% of suitable habitat for our study species coincided with Greater Sage-Grouse priority areas for conservation. Our conifer removal scenario increased suitable habitat by 6%-17% for sagebrush associates and reduced habitat by 11%-41% for pinyon-juniper associates. We identified areas of the Great Basin where conifer removal expanded habitat for sagebrush associates without concurrent habitat loss for pinyon-juniper associates. Our results provide guidance for conducting vegetation management in the Great Basin while addressing the habitat needs for multiple focal species. Our methods, which use freely available community science data and geospatial layers, can easily be transferred to other species and ecoregions.
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This study describes an approach for identifying and monitoring the types of resource benefits and tradeoffs considered in National Forest planning in the United States under the 2012 Planning Rule and demonstrates the use of tools for conceptualizing the production of ecosystem services and benefits from alternative land management strategies. Efforts to apply these tools through workshops and engagement exercises provide opportunities to explore and highlight measures, indicators, and data sources for characterizing benefits and tradeoffs in collaborative environments involving interdisciplinary planning teams. Conceptual modeling tools are applied to a case study examining the social and economic benefits of recreation on the Ashley National Forest.