No abstract

No abstract

No abstract

Fire exclusion in mixed conifer forests has increased the risk of fire due to decades of fuel accumulation. Restoration of fire into altered forests is a challenge because of a poor understanding of the spatial and temporal dynamics of fire regimes. In this study the spatial and temporal characteristics of fire regimes and forest age structure are reconstructed in a 2325-ha mixed conifer forest in the Klamath Mountains. Forests were multiaged and burned frequently at low and moderate severity, but forest age structure did not vary with aspect, elevation, or topographic position. Recently there has been an increase in forest density and a forest compositional shift to shade-tolerant species. Median fire return intervals (FRI) ranged from 11.5 to 16.5 yr and varied with aspect but not with forest composition or elevation. The median area burned was 106 ha, and the pre-Euro-American fire rotation of 19 yr increased to 238 yr after 1905. Intra-annual position of fire scars in the tree rings indicates that 93% of fires occurred during the dry midsummer through fall period. Spatial patterns of sites with similar fire dates were spatially coherent and separated from others by topographic features that influence fire spread. Thus, patterns of fire occurrence tended to be fixed in space with timing of fires varying among groups of sites. Spatial and temporal patterns of fire occurrence suggest that managers using physical features to contain prescribed fire will create burn patterns consistent with historical fires in the Klamath Mountains.

No 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.

Shaded fuelbreaks and larger landscape fuel treatments, such as prescribed ®re, are receiving renewed interest as forest protection strategies in the western United States. The effectiveness of fuelbreaks remains a subject of debate because of differing fuelbreak objectives, prescriptions for creation and maintenance, and their placement in landscapes with differing ®re regimes. A well-designed fuelbreak will alter the behavior of wildland ®re entering the fuel-altered zone. Both surface and crown ®re behavior may be reduced. Shaded fuelbreaks must be created in the context of the landscape within which they are placed. No absolute standards for fuelbreak width or fuel reduction are possible, although recent proposals for forested fuelbreaks suggest 400 m wide bands where surface fuels are reduced and crown fuels are thinned. Landscape-level treatments such as prescribed ®re can use shaded fuelbreaks as anchor points, and extend the zone of altered ®re behavior to larger proportions of the landscape. Coupling fuelbreaks with area-wide fuel treatments can reduce the size, intensity, and effects of wildland ®res. #

Conifer forests in northwestern Mexico have not experienced systematic fire suppression or logging, making them unique in western North America. Fire regimes of Pinus jeffreyi Grev. & Balf. mixed conifer forests in the Sierra San Pedro Martir, Baja California, Mexico, were determined by identifying 105 fire dates from 1034 fire scars in 105 specimens. Fires were recorded between 1521 and 1980 and median fire return intervals were less than 15 years at all compositing scales. Significant differences in mean fire return intervals were detected from 1700 to 1800, 1800 to 1900, and 1900 to 1997, most often at intermediate spatial compositing scales, and the proportion of trees scarred in the fires of the 1700s was significantly different from the fires of either the 1800s, the 1900s, or the combined post-1800 period. Superposed epoch analysis determined that moderate and large spatial scale fires occurred on significantly dry years during the length of the record, but before 1800, these fires were preceded by significantly higher precipitation 1 year before the fire. The dominance of earlywood fires in the Sierra San Pedro Martir is similar to the seasonality found in the southwest United States and is different from the western slope of the Sierra Nevada and Klamath Mountains of California.