Use of prescribed fire to reduce fuel buildup can be problematic because of air quality concerns, problems with weed invasions after fire, and fire induced loss of ecosystem carbon and soil nutrients. The problem is even greater at the forest-urban interface where structures are interspersed with dense thickets of shrubs with high fuel loadings—a common condition in lower elevations of the commercial forest zone. Combining high population density and property values, droughty conditions, highly flammable fuels, and a mosaic of mixed ownerships spells the highest risk of catastrophic wildfire in the forested West. In 2001 we initiated a series of experiments to determine the efficacy of conventional and less-conventional mechanical fuel reduction treatments on reducing fuels, conserving site organic matter, and improving soil quality and forest health. Research findings from studying fuel reduction alternatives at the forest-urban interface can help guide managerial decisions about fuel treatments.
A feasibility study was established in spring 2000 as part of a fuel break operation near Shaver Lake on the Sierra National Forest. Initial findings justified expansion to other sites. National Fire Plan funding afforded establishing a new, 5-treatment experiment at sites in Siskiyou Co. (Mt. Shasta), Shasta Co. (Whitmore), Yuba Co. (Challenge), and Sierra Co. (Sierraville). All sites represent open pine forests with a dense understory of perennial shrubs and they are arrayed along a climatic gradient of cool and dry (Sierraville) through warm and moist (Challenge).
Research objectives are to:
- Compare the relative costs and effectiveness of alternative methods of mechanically reducing understory fuels
- Determine how the alternative treatments affect the health of overstory trees
- Examine how alternative treatments affect the nature of returning vegetation
- Understand how fuel treatments affect the cycling and storage of organic carbon above and below ground
- Evaluate the impact of alternative treatments on soil quality and ultimate forest health.
Treatments are applied to 0.4-ha plots replicated four times at each installation in a randomized block design. They are:
- Untreated Control
- Manual Removal of all understory woody vegetation
- Mechanical Mastication of all understory woody vegetation
- Mechanical Mastication followed by Incorporation into the surface soil
- Mechanical Mastication followed by Prescribed Fire
The Mechanical Mastication followed by Prescribed Fire treatment is scheduled for two of the installations (Challenge and Whitmore) in fall 2004 or spring 2005. Prescribed Fire will be scheduled for Mt. Shasta and Sierraville in fall 2005 or spring 2006. At Challenge and Whitmore, Mastication and Mastication + Incorporation treatments have been split, with one half of each treatment plot receiving the herbicides hexazinone, glyphosate, and imazapyr and the other half left to develop naturally. This split-plot supplemental experiment allows us to follow the pathway of herbicides in the soil and soil solution.
Each plot at each location has been sampled intensively for overstory tree sizes, fuel loadings and soil physical and chemical properties prior to treatment. Follow-up measurements will examine the quality and quantity of revegetation, fuel reestablishment, modeled fire behavior, and overstory tree responses. Measurements also will be made of soil physical and chemical characteristics, particularly in respect to organic carbon storage and release, nitrogen availability, and available soil water. Plots have been instrumented for measuring soil moisture and temperature. Special soil solution lysimeters were established within and beneath the fine root zone in spring 2003 to track the fate of herbicides and effects of vegetation control on the mobility of soil nutrients. Collaborating scientists and graduate students at several western universities are helping study how these treatments affect the carbon and nitrogen cycles. These installations have great potential for demonstration to local communities and regional fire-safe councils.