Fire Regimes
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We found strong support for top-down and bottom-up spatial and temporal controls on fire patterns. Fire weather was a main driver of large fire occurrence, but area burned was moderated by ignition frequencies and by areas of limited fuels and fuel contagion (i.e., fire fences). Landscapes comprised of >40% area in fire fences rarely experienced large fire years. When large fires did occur during the simulation period, a recovery time of 100–300 years or more was generally required to recover pre-fire vegetation patterns.
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Wildfires burned more area on non-forested lands than forested lands over the past 20 years. This was true for all land ownerships in CONUS and the western US. Burned area increased over the 20-year time period for both non-forest and forest. Across CONUS, annual area burned was higher on non-forest than forests for 14 of the past 21 years (Fig. 1), and total area burned was almost 3,000,000 ha more in non-forest than in forest. For the western US, total burned area was almost 1,500,000 ha more in non-forest than in forest. From a federal agency perspective, approximately 74% of the burned area on Department of the Interior (DOI) lands occurred in non-forest and 78% of the burned area on US Forest Service (FS) lands occurred in the forest.
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We use a unique dataset derived from contemporary (∼2016) remeasurement of 440 historical quadrats (∼4m2) in the central Sierra Nevada, California, in which overstory trees, tree regeneration, and microsite conditions were measured and mapped both before and after logging in 1928–1929. Pine relative abundance changed little with logging and declined to 5% of the contemporary regeneration layer. In contrast, the relative abundance of incense-cedar regeneration (32%) already outpaced its representation in the overstory (17% by basal area) before logging and proceeded to dominate the contemporary understory (49%). We did not find strong evidence for the positive influence of gaps on pine regeneration in any time period. However, across species, post-logging skid trails were positively associated with regeneration and woody debris was negatively associated with regeneration in at least one time period. We discovered that the occurrence of advance regeneration (regeneration that preceded and survived logging) best predicted new contemporary trees across all species. For shade-tolerant species, post-logging regeneration that established up to ten years after logging was also associated with new contemporary trees. In contrast, the few pine that transitioned into the contemporary canopy during the study period all established prior to logging. Our work provides evidence that low pine abundance in the regeneration layer as early as 1928 contributed to low rates of pine in the overstory in 2016, showcasing that the decline of pine likely began before logging and official federal fire suppression policies. We suggest that fire exclusion before logging perpetuated shifts towards shade-tolerant and fire-intolerant species in the regeneration layer that were early and lasting.
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Presenter: Douglas J. Shinneman, Research Ecologist, USGS Forest and Rangeland Ecosystem Science Center
Description: Accurately assessing recent and historical wildfire activity is critical for numerous agencies who manage fire-prone landscapes. 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-selected regions of interest, time periods, and ecosystem types. For instance, users can determine whether the area burned by wildfire is increasing or decreasing over time for a specific ecoregion or for land ownership types of interest. The tool is available via a web application and generates a variety of maps, graphs, and tabular data that provide useful information for fire science and management objectives, as well as for the interested public.
Human population growth and accessibility from cities shape rangeland condition in the American West
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Human population growth contributes to the decline of sagebrush-steppe rangelands. More accessible rangelands from population centers have higher quality. Open space preservation provides opportunities for rangeland conservation in cities. Coordinated conservation strategies are necessary to protect rangeland ecosystems.
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This study documented a 246% rise in West-wide structure loss from wildfires between 1999–2009 and 2010–2020, driven strongly by events in 2017, 2018, and 2020. Increased structure loss was not due to increased area burned alone. Wildfires became significantly more destructive, with a 160% higher structure-loss rate (loss/kha burned) over the past decade. Structure loss was driven primarily by wildfires from unplanned human-related ignitions (e.g. backyard burning, power lines, etc.), which accounted for 76% of all structure loss and resulted in 10 times more structures destroyed per unit area burned compared with lightning-ignited fires. Annual structure loss was well explained by area burned from human-related ignitions, while decadal structure loss was explained by state-level structure abundance in flammable vegetation. Both predictors increased over recent decades and likely interacted with increased fuel aridity to drive structure-loss trends.
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Although fire is a fundamental ecological process in western North American forests, climate warming and accumulating forest fuels due to fire suppression have led to wildfires that burn at high severity across larger fractions of their footprint than were historically typical. These trends have spiked upwards in recent years and are particularly pronounced in the Sierra Nevada–Southern Cascades ecoregion of California, USA, and neighboring states. We assessed annual area burned (AAB) and percentage of area burned at high and low-to-moderate severity for seven major forest types in this region from 1984 to 2020. We compared values for this period against estimates for the pre-Euro-American settlement (EAS) period prior to 1850 and against a previous study of trends from 1984 to 2009.
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Here, we examine how the ventenata invasion alters simulated fire across forest-mosaic landscapes of the 7 million ha Blue Mountains Ecoregion using the large fire simulator (FSim) with custom fuel landscapes: present-day invaded versus historic uninvaded. Invasion increased simulated mean fire size, burn probability, and flame lengths throughout the ecoregion, and the strength of these impacts varied by location and scale. Changes at the ecoregion scale were relatively modest given that fine fuels increased in only 2.8% of the ecoregion where ventenata invaded historically fuel-limited vegetation types. However, strong localized changes were simulated
within invaded patches (primarily dwarf-shrublands) and where invasion facilitated fire spread into nearby forests.
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But over the past few decades, wildfires have worsened by almost every metric. It’s impossible to ignore this new consequence of environmental change. Fires are getting larger, more severe, more destructive and dangerous, and eliminating entire patches of forests, grasslands, and shrublands.
The combination of changing climate, extreme weather, land use, aggressive fire suppression policies, and wildland urban interface expansion have contributed to altered fire behavior regimes. And all of these past and current factors are converging in a big way in the western U.S. Today’s megafires pose an increasing threat to human health, infrastructure, natural resources, and ecosystem resilience.
