Fire Regimes
This study found that wildfires burned more area of non-forest lands than forest lands at the scale of the conterminous and western U.S. and the Department of Interior (DOI). In an agency comparison, 74% of DOI burned area occurred on non-forest lands and 78% of U.S. Forest Service burned area occurred on forested lands. Landscape metrics revealed key differences between forest and non-forest fire patterns and trends in total burned area, burned patch size, distribution, and aggregation over time across the western U.S. Opposite fire patterns emerged between non-forest and forest burns when analyzed at the scale of federal agency jurisdictions. In addition, a fire regime departure analysis comparing current large fire probability with historic fire trends identified certain vegetation types and locations experiencing more fire than historically. These patterns were especially pronounced for cold desert shrublands, such as sagebrush where increases in annual area burned, and fire frequency, size, and juxtaposition have resulted in substantial losses over a twenty-year period.
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Human-dominated pyromes (85% mean anthropogenic ignitions), with moderate fire size, area burned, and intensity, covered 59% of CONUS, primarily in the East and East Central. Physically dominated pyromes (47% mean anthropogenic ignitions) characterized by relatively large (average 439 mean annual ha per 50 km pixel) and intense (average 75 mean annual megawatts/pixel) fires occurred in 14% of CONUS, primarily in the West and West Central. The percent of anthropogenic ignitions increased over time in all pyromes (0.5–1.7% annually). Higher fire frequency was related to smaller events and lower FRP, and these relationships were moderated by vegetation, climate, and ignition type. Notably, a spatial mismatch between our derived modern pyromes and both ecoregions and historical fire regimes suggests other major drivers for modern U.S. fire patterns than vegetation-based classification systems. This effort to delineate modern U.S. pyromes based on fire observations provides a national-scale framework of contemporary fire regions and may help elucidate patterns of change in an uncertain future.
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The Wildland Fire Trends Tool (WFTT) is a data visualization and analysis tool that calculates and displays wildfire trends and patterns for the western U.S. based on user-defined regions of interest, time periods, and ecosystem types. Users can use the tool to easily generate a variety of maps, graphs, and tabular data products that are informative for all levels of expertise. The WFTT provides information that can be used for a wide range of purposes, from helping to set agency fire management objectives to providing useful information to scientists, interested public, and the media.
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Reports from the early 1900s, along with paleo- and dendro-ecological records, indicate similar and potentially even larger wildfires over the past millennium, many of which shared similar seasonality (late August/early September), weather conditions, and even geographic locations. Consistent with the largest historical fires, strong east winds and anomalously dry conditions drove the rapid spread of high-severity wildfire in 2020. We found minimal difference in burn severity among stand structural types related to previous management in the 2020 fires. Adaptation strategies for similar fires in the future could benefit by focusing on ignition prevention, fire suppression, and community preparedness, as opposed to fuel treatments that are unlikely to mitigate fire severity during extreme weather. While scientific uncertainties remain regarding the nature of infrequent, high-severity fires in westside forests, particularly under climate change, adapting to their future occurrence will require different strategies than those in interior, dry forests.
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This study used a coupled ecohydrologic and fire regime model to examine how climate change and CO2 scenarios influence fire regimes. In this semiarid watershed, we found an increase in burned area and burn probability in the mid-21st century (2040s) as the CO2 fertilization effect on vegetation productivity outstripped the effects of climate change-induced fuel decreases, resulting in greater fuel loading. However, by the late-21st century (2070s), climatic warming dominated over CO2 fertilization, thus reducing fuel loading and burned area. Fire regimes were shown to shift from flammability- to fuel-limited or become increasingly fuel-limited in response to climate change. We identified a metric to identify when fire regimes shift from flammability- to fuel-limited: the ratio of the change in fuel loading to the change in its aridity. The threshold value for which this metric indicates a flammability versus fuel-limited regime differed between grasses and woody species but remained stationary over time. Our results suggest that identifying these thresholds in other systems requires narrowing uncertainty in exogenous drivers, such as future precipitation patterns and CO2 effects on vegetation.
2020 California fire season: A year like no other, a return to the past, or harbinger of the future?
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The 2020 fires were part of an accelerating decades-long trend of increasing burned area, fire size, fire severity and socio-ecological costs in California. In fire-prone forests, the management emphasis on reducing burned area should be replaced by a focus on reducing the severity of burning and restoring key ecosystem functions after fire. There have been positive developments in California vis-à-vis collaborative action and increased pace and scale of fuel management and pre- and postfire restoration, but the warming climate and other factors are rapidly constraining our options.
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Study results suggest increasing the frequency of disturbances (a lower disturbance return interval [DRI]) would reduce the percentage of high-severity fire on landscape but not the total amount of wildfire in general. However, a higher DRI reduced carbon storage and sequestration, particularly in management strategies that emphasized prescribed fire over hand or mechanical fuel treatments.
Webinar recording.
Presented by: Dan Neary
Wildfires can produce significant hydrological and ecological impacts on forest, woodland, and grassland ecosystems depending on fire size, severity, duration, timing, fuel loads, and weather conditions. In the past several decades, wildfire conditions have changed from previous ones in the 20th Century. Wildfires are now burning larger areas in hotter, windier, and drier weather. In addition, the timeframe for these fires has expanded by four months in some regions to 12 months in fire-prone states like California. These large fires, known as megafires (greater than 40,000 acres) are burning more wildland areas every year. Some reach the giga-fire classification (405,000+ acres) with increasing frequency. These trends are contributing to increased desertification of forest lands. This presentation examines the role of these large fires in producing desertification of wildland ecosystems.
Webinar recording.
In this deep dive webinar, Dr. Becky Kerns and collaborating scientists will present and synthesize results from a Joint Fire Science funded project aimed at understanding the current and future Ventenata dubia (ventenata) invasion in the Blue Mountains Ecoregion. Wildfires in 2014 and 2015 in the ecoregion reportedly spread in an unusual fashion owing to this invasive annual grass. Concern was raised that ventenata might be a “game-changer” for wildfire. Results from our studies show that ventenata has ecosystem transformation potential and influences landscape-scale fire across the ecoregion. In this webinar we report these findings and the management implications, as well as place our results in the context of other plant invasion research. The webinar includes 90 minutes of scientific presentations with short Q&A, and ends with a 30-minute wrap up and panel discussion. Talks will adhere to the following agenda to allow attendees to join and leave the meeting for specific talks, if desired.
Webinar recording.
Description: Changing wildfire regimes are causing rapid shifts in forests worldwide. In particular, forested landscapes that burn repeatedly in relatively quick succession may be at risk of conversion when pre-fire vegetation cannot recover between fires. Fire refugia (areas that burn less frequently or severely than the surrounding landscape) support post-fire ecosystem recovery and the persistence of vulnerable species in fire-prone landscapes. Observed and projected fire-induced forest losses highlight the need to understand where and why forests persist in refugia through multiple fires. This research need is particularly acute in the Klamath-Siskiyou ecoregion of southwest Oregon and northwest California, USA, where expected increases in fire activity and climate warming may result in the loss of up to one-third of the region’s conifer forests, which are the most diverse in western North America. We model the key controls on fire refugia occurrence and persistence through one, two, and three fire events over a 32-year period. Refugia that persisted through three fire events appeared to be partially entrained by landscape features that offered protection from fire, suggesting that topographic variability may be an important stabilizing factor as forests pass through successive fire filters. Results from this study could inform management strategies designed to protect fire-resistant portions of biologically and topographically diverse landscapes.
Presenter: Meg Krawchuk, Oregon State University