Fire Regimes

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Climate change and western fires: A 3-part series from Ecological Applications

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Fire exclusion caused profound changes in many western North American forested landscapes, leaving them vulnerable to seasonal increases in drought and wildfire. As climate warms, the likelihood of severe, large-scale disturbance increases. There is generally strong agreement that wildfires, insects and disease are rapidly changing western landscapes and that the pace and scale of adaptive management is insufficient. However, confusion persists regarding the need for proactive management. In three articles, this Invited Feature evaluates the strength of scientific evidence regarding changing forest conditions, fire regimes, and science-based strategies for adapting western forests to climate change and future wildfires.

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76-year decline and recovery of aspen mediated by contrasting fire regimes

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Our study area in northeastern California on the Lassen, Modoc and Plumas National Forests has experienced recent large mixed-severity wildfires where aspen was present, providing an opportunity to study the re-introduction of fire. We observed two time periods; a 52-year absence of fire from 1941 to 1993 preceding a 24-year period of wildfire activity from 1993 to 2017. We utilized aerial photos and satellite imagery to delineate aspen stands and assess conifer cover percent. We chose aspen stands in areas where wildfires overlapped (twice-burned), where only a single wildfire burned, and areas that did not burn within the recent 24-year period. We observed these same stands within the first period of fire exclusion for comparison (i.e., 1941–1993). In the absence of fire, all aspen stand areas declined and all stands experienced increases in conifer composition. After wildfire, stands that burned experienced a release from conifer competition and increased in stand area. Stands that burned twice or at high severity experienced a larger removal of conifer competition than stands that burned once at low severity, promoting expansion of aspen stand area. Stands with less edge:area ratio also expanded in area more with fire present. Across both time periods, stand movement, where aspen stand footprints were mostly in new areas compared to footprints of previous years, was highest in smaller stands. In the fire exclusion period, smaller stands exhibited greater loss of area and changes in location (movement) than in the return of fire period, highlighting their vulnerability to loss via succession to conifers in the absence of disturbances that provide adequate growing space for aspen over time.

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Fireshed Registry: Fireshed and project area boundaries for the continental US

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The Fireshed Registry is a geospatial dashboard and decision tool built to organize information about wildfire transmission to communities and monitor progress towards risk reduction for communities from management investments. The concept behind the Fireshed Registry is to identify and map the source of risk rather than what is at risk across all lands in the continental United States. While the Fireshed Registry was organized around mapping the source of fire risk to communities, the framework does not preclude the assessment of other resource management priorities and trends such as water, fish and aquatic or wildlife habitat, or recreation. The Fireshed Registry is also a multi-scale decision tool for quantifying, prioritizing, and geospatially displaying wildfire transmission to buildings in adjacent or nearby communities.

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Contrasting the role of human- and lightning-caused wildfires on future fire regimes on a Central Oregon landscape

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No significant change was projected for the number of human-caused fire ignitions, but we projected a 14% reduction in lightning-caused ignitions under future conditions. Mean fire sizes were 31% and 22% larger under future conditions (2031–2060) for human and lightning-caused ignitions, respectively. All but one climate model projected increased frequency of record-breaking events relative to the contemporary period, with the largest future fires being about twice the size of those of the contemporary period. This work contributes to understanding the role of lightning- and human-caused fires on future fire regimes and can help inform successful adaptation strategies in this landscape.

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Recent fire regimes of the bi-national Madrean Sky Islands: Implications for collaborative, transboundary fire management

Webinar recording.

Overview: This webinar shares results of a recent study of contemporary fire regimes over a 32-year period (1985-2017) in the Madrean Sky Islands of the U.S. and México. During the study period 335 fires burned approximately 28% of the study area, with re-burns occurring on over 25% of the burned areas. The greatest variation in fire regimes, including fire size, frequency, and severity was observed in places with the most diverse human activities and land uses – particularly in the mountain ranges adjacent to the U.S.- México border. Average severity of recent fires was low despite some extreme outliers in cooler, wetter environments. Fire frequency was also higher than historical expectations in these cool and wet environments that support forest types such as Spruce-Fir, indicating threats to these systems possibly attributable to drought and other factors. In cooler and wetter environments in more remote areas of México, pine-oak forests burned with fire frequencies close to historical. In contrast, fires were absent or infrequent across large expanses of lower elevation Woodlands and Grasslands due possibly to overgrazing, which reduces abundance and continuity of fine fuels needed to carry fire. Our findings provide a new depiction of fire regimes in the Sky Islands that can help inform fire management, restoration, and regional conservation planning, fostered by local and traditional knowledge and collaboration among landowners and managers.

Presenter: Dr. Miguel Villarreal
Co-authors: José M. Iniguez, Aaron D. Flesch, Jamie S. Sanderlin, Citlali Cortés Montaño, Caroline R. Conrad, Sandra L. Haire


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Quantifying pyrodiversity and its drivers

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This study presents a generalizable functional diversity approach for measuring pyrodiversity, which incorporates multiple fire regime traits and can be applied across scales. Further, we tested the socioecological drivers of pyrodiversity among forests of the western United States. Largely mediated by burn activity, pyrodiversity was positively associated with actual evapotranspiration, climate water deficit, wilderness designation, elevation and topographic roughness but negatively with human population density. These results indicate pyrodiversity is highest in productive areas with pronounced annual dry periods and minimal fire suppression. This work can facilitate future pyrodiversity studies including whether and how it begets biodiversity among taxa, regions and fire regimes.

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Fire science and management in an uncertain future: Tools and approaches for managing fire in future climates in the SW

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Workshop purpose: Identify fire science and management needs and discuss tools and approaches to natural resource assessments and adaptation strategies for fire dynamics in future climates in Southwest (DOI Regions 8 & 10 [CA, NV, AZ]) bioregions.

Provide awareness of tools needed for decision-making in an uncertain future
Generate a list of new science actions to meet fire needs for practitioners/planners in future, non-analog landscapes and communities
Suggest how we might address and accomplish these identified needs going forward
Exchange Information
Make connections

This four-hour, virtual Summit was an abbreviated, rescheduled version of ‘Building Bridges and Solutions: Partners in Facing Fire-Science Challenges’ that was cancelled in April due to COVID-19. We assembled scientists and fire practitioners/leaders in an interagency effort to identify fire science and management needs and to discuss decision-making tools and approaches that address resource assessments and adaptation strategies for fire dynamics in future climates in the Southwest (Department of Interior [DOI] Regions 8 and 10 [CA, NV, AZ]). This overriding goal threaded together the Summit’s talks, Q&A, and break-out sessions. Speakers from various agencies, institutes, and academia focused on fire management and planning in future non-analog landscapes and climate-fire-ecosystem impact relationships in western forest (e.g., mixed-conifer, subalpine), desert (hot and cold, grassland, pinyon-juniper, sage-steppe), and Mediterranean/chaparral bioregions. Syntheses from talks, Mentimeter-conducted discussions, and break-out groups on management and actionable-science needs will be summarized in a white paper and posted on the Southwest, Great Basin, and California Fire Science Exchange websites. Let’s work together to address fire science and management in an uncertain future!

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Weather impacts on fire thresholds: Recipe for big fire

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Because three key thresholds must be crossed all at once for a wildfire to start, avoiding just one of these thresholds─ ignitions, drought, or continuous fuels (Fig.1)─ could significantly reduce the likelihood of wildfire. As climate change makes fire weather more common everywhere, managing ignitions where wind is problematic and managing fuels where drought is problematic will help to keep stochastic, out-of-regime fires contained. Where fire management tools won’t help, a fire danger zone should be designated to reduce human activity and development, much like volcano or flooding zone designations.

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Effect of seeding treatments and climate on fire regimes in Wyoming sagebrush

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Sites drill seeded before the most recent fire had fewer, less frequent fires with longer fire return intervals (15–20 years) than aerially seeded sites (intervals of 5–8 years). The response of fire regime variables at unseeded sites fell between those of aerial and drill seeding. Increased moisture availability resulted in decreased fire frequency between 1994 and 2014 and the total number of fires since 1955 on sites with unseeded and aerially pre-fire seeding, but fire regimes did not change when drill seeded. Greater annual grass biomass likely contributed to frequent fires in the arid region. In Wyoming big sagebrush steppe, drill seeding treatments reduced wildfire risk relative to aerial seeded or unseeded sites.

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Disturbance and sustainability in forests of the western US

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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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