Fire Regimes
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This analysis revealed that persistent unburned islands are not randomly distributed across the landscape. While the topography and vegetation fuel type that underlie persistent unburned islands differ from burned areas, these differences are dependent upon fire regime group and are less pronounced than what other studies have found. The topographic features that differed the most between persistent unburned islands and burned areas were terrain ruggedness, slope, and transformed aspect. We also found that, as unburned islands increased in persistence (i.e., remained unburned for an increasing number of overlapping fires), they decreased in size and shape complexity.
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Despite late twentieth‐century increases in area burned, we show that Pacific Northwest forests have experienced an order of magnitude less fire over 32 yr than expected under historic fire regimes. Within fires that have burned, severity distributions are disconnected from historical references. From 1984 to 2015, 1.6 M ha burned; this is 13.3–18.9 M ha less than expected. Deficits were greatest in dry forest ecosystems adapted to frequent, low‐severity fire, where 7.2–10.3 M ha of low‐severity fire was missing, compared to a 0.2–1.1 M ha deficit of high‐severity fire. When these dry forests do burn, we observed that 36% burned with high‐severity compared to 6–9% historically. We found smaller fire deficits, 0.3–0.6 M ha, within forest ecosystems adapted to infrequent, high‐severity fire. However, we also acknowledge inherent limitations in evaluating contemporary fire regimes in ecosystems which historically burned infrequently and for which fires were highly episodic. The magnitude of contemporary fire deficits and disconnect in burn severity compared to historic fire regimes have important implications for climate change adaptation. Within forests characterized by low‐ and mixed‐severity historic fire regimes, simply increasing wildfire extent while maintaining current trends in burn severity threatens ecosystem resilience and will potentially drive undesirable ecosystem transformations. Restoring natural fire regimes requires management that facilitates much more low‐ and moderate‐severity fire.
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Historical fires burned every 9–10 years on average up until 1879, when fires ceased contemporaneous with introduction of Euro-American livestock grazing and timber harvest in upland forests. Abundant tree regeneration occurred after fire exclusion, with tree density averaging 45 trees ha−1 in reconstructed 1880 forests versus 106 trees ha−1 today. Intervals between recent (since 1988) wildfires and prescribed fires in these same stands ranged from 7 to 13 years, similar to historical fire timing. Depending on whether plots had burned from zero to three times in recent fires, we found significant differences in canopy base heights (increased), duff and litter depths (decreased), and percent cover of grass and forbs (increased), but not tree density, tree basal area, shrub height, shrub cover, or woody fuels. Combined effects of recent fires on overstory and understory structure resulted in a significant difference in likelihood of crown fire occurrence, declining from a mean of 58% in plots with no fire since 1879 to 13% in plots with three fires since 1988. Significant effects were generally seen after two or three fires, suggesting it is the reintroduction of the fire regime and not just individual fire events that restore resiliency. Overall, effects of recent fires are building on the latent resiliency of ponderosa pine forests at Zion National Park, although questions remain about extent and future dynamics of oak and manzanita shrubfields that occupy similar environmental settings, along with a general lack of ponderosa pine regeneration across all plots.
This webinar seeks to inform participants about what to expect from LANDFIRE Remap products, and what has and has not changed from previous product offerings. We will discuss what we have learned since February 2019 when the products were made available to users in the Northwest, and how LANDFIRE resources can address specific fire and land management issues.
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Here, we provide a brief overview of examples where anthropogenically driven changes in fire frequency, fire pattern, fuels consumed and fire intensity constitute perturbations that greatly disrupt natural disturbance cycles and put ecosystems on a different trajectory resulting in type conversion. These changes are not due to fire per se but rather anthropogenic perturbations in the natural disturbance regime.
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Estimates of historical fire regime parameters in mountain big sagebrush communities can be compared with current fire regimes and trends to establish general guidelines for ecological restoration. A synthesis of information on historical patterns and contemporary changes in fuels and fire regimes in mountain big sagebrush communities is available in the Fire Effects Information System (FEIS). This research brief summarizes information from that FEIS Fire Regime Synthesis.
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Humans are the source of 84 percent of wildfires, and not all are intentional. Often they come from vehicle accidents in dry landscapes. In fact, over the last 20 years, 11 of the 50 largest wildfires in the U.S. have occurred in the Great Basin. From 2000 to 2018, approximately 15 million acres of sagebrush burned primarily in the Great Basin, and approximately 9 million of those acres burned from 2014 to 2018 alone, said Michele Crist, a landscape ecologist with the Bureau of Land Management at the National Interagency Fire Center in Boise, Idaho.
In this webinar, Dr. Cameron Naficy presents findings from a synthesis of multiple projects conducted in 3 unique mixed-severity ecosystems from the Rocky Mountains of the U.S. and Canada. He briefly reviews a new reconstruction method his research group developed that combines extensive dendroecological plot networks and detailed forest structure mapping from high-resolution historical aerial imagery. Using these data, he describes the spatial and temporal patterns of fire frequency and severity for each study ecosystem, the fire-mediated stand dynamics and vegetation conditions that characterized each ecosystem, and some novel resilience mechanisms and ecological surprises associated with the mixed-severity fire regime model. This project demonstrates how historical data can be used to move beyond simple summaries of historical fire regime attributes and landscape condition by using historical data to reveal fundamental fire regime processes, drivers, and ecological outcomes.
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These results highlight a high sensitivity of statistical projections to changing threshold relationships and data uncertainty, implying that projections of future ecosystem change in threshold‐governed ecosystems will be accompanied by notable uncertainty. This work also suggests that ecological responses to climate change will exhibit high spatio‐temporal variability as different regions approach and surpass climatic thresholds over the 21st century.
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This study reviews published studies on reburns in fire-adapted ecosystems of the world, including temperate forests of North America, semi-arid forests and rangelands, tropical and subtropical forests, grasslands and savannas, and Mediterranean ecosystems. To date, research on reburns is unevenly distributed across the world with a relative abundance of literature in Australia, Europe and North America and a scarcity of studies in Africa, Asia and South America. This review highlights the complex role of repeated fires in modifying vegetation and fuels, and patterns of subsequent wildfires. In fire-prone ecosystems, the return of fire is inevitable, and legacies of past fires, or their absence, often dictate the characteristics of subsequent fires.