Tuesday, 18 November 2003: 9:30 AM
Long-term surface fuel accumulation in burned and unburned mixed-conifer forests of the central and southern Sierra Nevada, CA (USA)
MaryBeth Keifer, National Park Service, Three Rivers, CA; and J. van Wagtendonk
After nearly a century of fire exclusion in many central and southern Sierra Nevada mixed-conifer forests, dead and down surface fuels have reached high levels in the absence of recurring fire to consume the accumulated organic matter. A long-term monitoring program has tracked changes in fuel load and vegetation as a result of prescribed fire treatments in Yosemite, Sequoia, and Kings Canyon National Parks over the last several decades. Prior to prescribed fire application, and after many decades without fire, the mean total fuel load was 20.4 kg/m2 (91.0 tons/acre) for giant sequoia-mixed conifer, 16.3 kg/m2 (72.7 tons/acre) for white fir-mixed conifer, and 16.6 kg/m2 (74.0 tons/acre) for ponderosa pine-dominated forest types. Immediately following initial treatment with prescribed fire, these fuel loads were reduced by an average of 73-96%. Ten years following prescribed fire treatments, the mean total fuel loads accumulated to 66% of pre-burn levels in giant sequoia-mixed conifer, 83% in white fir-mixed conifer, and 22% of pre-burn levels after five years in ponderosa pine-dominated forests. This post-burn fuel complex generally has more woody fuel and less duff compared to the pre-burn fuel complex. Most of these areas are either scheduled for a second treatment with prescribed fire within about ten years or they have not yet reached the next monitoring phase (20-years post burn), therefore it is difficult to know how fuel accumulation rates continue over time. In several early giant sequoia plots that were treated twice with prescribed fire, mean total fuel load increased only slightly from 10 to 20 years after the second fire treatment. Additionally, some ponderosa pine plots that were last burned 30 years ago were recently re-measured.
Over the course of the monitoring program, some plots were installed in areas that remained unburned. While the initial fuel load in these areas was lower than that in the areas treated with prescribed fire, the mean total fuel load only increased slightly in the giant sequoia- and white fir-mixed conifer forest types over ten years, and increased by 3.6 kg/m2 (16.1 tons/acre) in the ponderosa pine-dominated forest type over 5 years.
In untreated areas, fuel accumulation appears to be relatively slow in giant sequoia- and white fir-mixed conifer forest types. These results may indicate that untreated surface fuel loads are currently at a threshold where decay rates are similar to accumulation rates. Also, most fuel accumulation appears to occur within the first decade after prescribed fire treatment. This information may have important implications for fire management planning, including scheduling fuel hazard reduction and maintenance treatments.