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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.

Abstract. Changes in vegetation and fuels were evaluated from measurements taken before and after fuel reduction treatments (prescribed fire, mechanical treatments, and the combination of the two) at 12 Fire and Fire Surrogate (FFS) sites located in forests with a surface fire regime across the conterminous United States. To test the relative effectiveness of fuel reduction treatments and their effect on ecological parameters we used an informationtheoretic approach on a suite of 12 variables representing the overstory (basal area and live tree, sapling, and snag density), the understory (seedling density, shrub cover, and native and alien herbaceous species richness), and the most relevant fuel parameters for wildfire damage (height to live crown, total fuel bed mass, forest floor mass, and woody fuel mass).In the short term (one year after treatment), mechanical treatments were more effective at reducing overstory tree density and basal area and at increasing quadratic mean tree diameter. Prescribed fire treatments were more effective at creating snags, killing seedlings, elevating height to live crown, and reducing surface woody fuels. Overall, the response to fuel reduction treatments of the ecological variables presented in this paper was generally maximized by the combined mechanical plus burning treatment. If the management goal is to quickly produce stands with fewer and larger diameter trees, less surface fuel mass, and greater herbaceous species richness, the combined treatment gave the most desirable results. However, because mechanical plus burning treatments also favored alien species invasion at some sites, monitoring and control need to be part of the prescription when using this treatment.

Policies have been enacted to encourage carbon (C) sequestration through afforestation, reforestation, and other silvicultural practices; however, the effects of wildfires on forest C stocks are poorly understood. We present information from Sierran mixed-conifer forests regarding how control, mechanical, prescribed-fire, and mechanical followed by prescribed-fire treatments affected C pools. Secondly, we report CO2 emissions from machinery and burning associated with the treatments. Lastly, the effects of treatments on the potential for C loss to wildfire are presented. The amount of aboveground C in live trees was significantly reduced in mechanical-only and mechanical plus fire treatments; C contained in dead trees was not significantly different. There was no significant difference in aboveground live and dead tree C between the fire-only and control treatments. Fire-only and mechanical plus fire treatments emitted significantly more CO2 than the mechanical treatment and control. Modeling results for the control demonstrated 90% of the live tree C had a high (>75%) chance of being killed in a wildfire; in contrast, all three active treatments had low vulnerabilities to C loss. With wildfire severity increasing in most Sierran forests, management actions designed to increase fire resistance are justified for long-term C sequestration.

During the late fall of 2002 we administered three burns in mixed conifer forest sites in the north-central Sierra Nevada. Eight months later we measured fire-induced injury and mortality in 1300 trees. Using logistic regression, an array of crown scorch, stem damage, fuels, and fire-behavior variables were examined for their influence on tree mortality. In Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), white fir (Abies concolor (Gord. & Glend.) Lindl.), and incense cedar (Calocedrus decurrens (Torr.) Florin), smaller trees with greater total crown damage had higher mortality rates. Smaller stem diameters and denser canopies predicted mortality best in ponderosa pine (Pinus ponderosa Dougl. ex P. Laws. & C. Laws). Duff consumption and bark char severity increased model discrimination for white fir and incense cedar and California black oak (Quercus kelloggii Newberry), respectively. In tanoak (Lithocarpus densiflorus (Hook. & Arn.) Rehd.), greater total crown damage in shorter trees resulted in higher mortality rates. Along with tree diameter and consumption of large (>7.6 cm diameter at breast height, DBH) rotten downed woody debris, fire intensity was a significant predictor of overall tree mortality for all species. Mortality patterns for white fir in relation to crown damage were similar among sites, while those for incense cedar were not, which suggests that species in replicated sites responded differently to similar burns. Our results demonstrate actual fire-behavior data incorporated into mortality models, and can be used to design prescribed burns for targeted reduction of tree density in mixed conifer forests.Résumé : À la fin de l'automne 2002, nous avons réalisé trois brûlages dirigés sur des stations forestières mixtes de conifères dans la région centre-nord de la Sierra Nevada. Huit mois plus tard, nous avons mesuré les blessures et la mortalité induite par le feu sur 1300 arbres. Par le biais d'une régression logistique, nous avons étudié la mortalité des arbres en fonction d'une gamme de variables comprenant le roussissement de la cime, les dommages à la tige, les combustibles et le comportement du feu. Pour le douglas vert (Pseudotsuga menziesii (Mirb.) Franco), le sapin argenté (Abies concolor (Gord. & Glend.) Lindl.) et le cèdre à rayons (Calocedrus decurrens (Torr.) Florin), les plus petits arbres ayant la plus grande quantité de dommages cumulés à la cime étaient associés à une mortalité élevée. La mortalité du pin ponderosa (Pinus ponderosa Dougl. ex P. Laws. & C. Laws) était fortement associée aux petits diamètres et à des canopées denses. La consommation de litière et la sévérité de la carbonisation de l'écorce ont respectivement augmenté le pouvoir discriminant du modèle pour le sapin argenté, le cèdre à rayons et le chêne noir de Californie (Quercus kelloggii Newberry). Dans le cas du chêne à tan (Lithocarpus densiflorus (Hook. & Arn.) Rehd.), les plus forts taux de mortalité ont été observés sur les plus petits arbres ayant la plus grande quantité de dommages cumulés à la cime. Ave...

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Across the western United States, decades of fire exclusion combined with past management history have contributed to the current condition of extensive areas of high-density, shade-tolerant coniferous stands that are increasingly prone to high-severity fires. Here, we report the modeled effects of constructed defensible fuel profile zones and group selection treatments on crown fire potential, flame length, and conditional burn probabilities across 11 land allocation types for an 18 600 ha study area within the northern Sierra Nevada, California. Fire modeling was completed using FlamMap and FARSITE based on landscape files developed with high-resolution aerial (IKONOS) imagery, ground-based plot data, and integrated data from ARCFUELS and the Forest Vegetation Simulator. Under modeled 97th percentile weather conditions, average conditional burn probability was reduced between pre- and post-treatment landscapes. A more detailed simulation of a hypothetical fire burning under fairly severe fire weather, or “problem fire”, revealed a 39% reduction in final fire size for the treated landscape relative to the pre-treatment condition. To modify fire behavior at a landscape level, a combination of fuel treatment strategies that address topographic location, land use allocations, vegetation types, and fire regimes is needed.