Fire treatment effects on vegetation structure, fuels, and potential fire severity in western U.S. forests

Abstract

Abstract. Forest structure and species composition in many western U.S. coniferous forests have been altered through fire exclusion, past and ongoing harvesting practices, and livestock grazing over the 20th century. The effects of these activities have been most pronounced in seasonally dry, low and mid-elevation coniferous forests that once experienced frequent, low to moderate intensity, fire regimes. In this paper, we report the effects of Fire and Fire Surrogate (FFS) forest stand treatments on fuel load profiles, potential fire behavior, and fire severity under three weather scenarios from six western U.S. FFS sites. This replicated, multisite experiment provides a framework for drawing broad generalizations about the effectiveness of prescribed fire and mechanical treatments on surface fuel loads, forest structure, and potential fire severity. Mechanical treatments without fire resulted in combined 1-, 10-, and 100-hour surface fuel loads that were significantly greater than controls at three of five FFS sites. Canopy cover was significantly lower than controls at three of five FFS sites with mechanical-only treatments and at all five FFS sites with the mechanical plus burning treatment; fire-only treatments reduced canopy cover at only one site. For the combined treatment of mechanical plus fire, all five FFS sites with this treatment had a substantially lower likelihood of passive crown fire as indicated by the very high torching indices. FFS sites that experienced significant increases in 1-, 10-, and 100-hour combined surface fuel loads utilized harvest systems that left all activity fuels within experimental units. When mechanical treatments were followed by prescribed burning or pile burning, they were the most effective treatment for reducing crown fire potential and predicted tree mortality because of low surface fuel loads and increased vertical and horizontal canopy separation. Results indicate that mechanical plus fire, fire-only, and mechanical-only treatments using whole-tree harvest systems were all effective at reducing potential fire severity under severe fire weather conditions. Retaining the largest trees within stands also increased fire resistance.

Fuel Classes in Conifer Forests of Southwest Sichuan, China, and Their Implications for Fire Susceptibility

“…FT6 would benefit from the removal of some "ladder fuels" to reduce the canopy density. This could be effective in stopping passive crown fires and preventing the start of new fires, especially if it was combined with patchy canopy and vertical layer management [13,14,29].…”

“…The environmental dataset included tree age, height, elevation, slope, aspect, density, canopy closure, and diameter at breast height (DBH) in each plot. The environmental data were measured in the field except aspect need to be converted by digitizing in the calculation process [13]. Additionally, the fire type in each plot (under extreme burning conditions) was considered the "species" in the data matrix.…”

“…The term "fuel" does not stand for a single object but is a complex multi-layered system, which includes the following layers from surface to canopy: a semi-decomposed layer, an active/fine fuel layer, a large fuel layer, a herb layer, a shrub layer and a tree layer [12]. The spatial characteristics of fuel include the quantity, size, and continuity, which mainly affect behavior of the fire, such as the rate of spread and fire intensity [13,14]. It would be prohibitively difficult to maintain an inventory of all fuel characteristics because fuel is structurally complex and varies widely in its physical attributes.…”

Quantifying Ladder Fuels: A New Approach Using LiDAR

“…Fuel reduction treatments are often used to mitigate fire hazards in strategic areas to help decrease the risk of harming these assets [3]. These treatments have been shown to be effective in ameliorating fire behavior and reducing tree mortality through both modeled simulations [4,5] and when encountered by real wildfires [3,[6][7][8][9][10][11][12][13]. Recent studies suggest the duration for reduction in potential fire behavior and effects, or treatment longevity, can exist for up to 8-15 years in the Sierra Nevada [14,15].…”

“…Fuel reduction treatments in the Sierra Nevada have multiple effects on fuel structure, reducing surface fuels, ladder fuels, and canopy continuity while maintaining large, fire-resistant trees [3,4]. Treatments utilize different methods to accomplish these changes: prescribed burning, hand-thinning (generally targets trees under 30 cm in diameter at breast height (dbh)), mastication (targets understory trees and shrubs), mechanical harvesting (thin from below, targeting trees up to 76 cm dbh), and sometimes a combination of these.…”