1. The Effect of Fuel Treatments on the Invasion of Nonnative Plants Kyle E. Merriam 1, Jon E. Keeley 1, and Jan L. Beyers 2. [1] USGS Western Ecological Research Center, Sequoia-Kings Canyon Field Station, Three Rivers, CA 93271, kmerriam@usgs.gov, (559) 565-4266. [2] US Forest Service Pacific Southwest Research Station, Riverside Fire Laboratory, 4955 Canyon Crest Dr., Riverside, CA 92507 This study is funded by the Joint Fire Science Program BACKGROUND Fuel reduction treatments are designed to break up landscape fuels and improve access for fire suppression. They vary in design from linear features such as fuel breaks, to large, thinned areas with some remaining overstory canopy cover, sometimes referred to as shaded fuel breaks. Fuel breaks are an increasingly important component of state and federal fuels management programs, particularly at the wildland and urban interface. However, fuel breaks may promote the establishment of nonnative invasive plants by creating soil disturbance, increasing light levels, and changing soil nutrients and hydrology. Maintenance activities, such as thinning and prescribed burning, also may provide disturbance regimes that favor invasive species. The presence of invasive plants in fuel breaks could establish an exotic seed source in close proximity to remote wildland areas. These areas might then be more susceptible to invasion, particularly following wide spread disturbances such as natural or prescribed fires. A shaded fuel break maintained by a prescribed burn, Plumas National Forest, California Fuel break in Casper’s Wilderness Park, Orange County, California METHODS We sampled along belt transects oriented perpendicular to the fuel break. Transects extend 40 meters (m) from the edge of the fuel break into the surrounding vegetation on both sides. Along each transect two 1 x 1 m plots are established within the fuel break, and four plots are located at 5, 10, 20 and 40 m from the edge of the fuel break into the surrounding wildlands. A minimum of 8 transects are placed at up to 1000 m intervals along each fuel break. Fuel Break Road 250-1000 M (Not to scale) 40 M Transect 1X1 M Plots Within each 1 x 1 m quadrat, data are collected on plant species composition, cover, and density. Litter depth, duff depth, litter cover, bare ground, and overstory canopy cover also are recorded. Additional data collected include: slope, aspect, elevation; fuel break age and width; construction and maintenance method; distance to roads, other fuel breaks, and urban interface; and land use and fire history. EffectCoefficient Standard Error Standardized Coefficient tP Constant12.4550.91613.5990.001 Elevation-0.0040.001-0.311-7.3490.001 Slope-0.0800.032-0.103-2.4860.013 Aspect * Litter Depth-0.0340.016-0.096-2.0990.036 Duff Depth-0.0370.017-0.099-2.1160.035 Canopy Cover * Bare Ground-0.2160.118-0.081-1.8330.068 Distance from Fuel Break -0.7670.135-0.239-5.6820.001 Table 1. Multiple regression analysis of environmental variables on nonnative plant cover. Elevation and distance from fuel break were most strongly correlated with nonnative plant cover. *Variables with P>0.15 were removed from the model. N=459, R 2 =0.237, adjusted multiple R 2 =0.227. STUDY SITES/ COOPERATORS We have surveyed 24 fuel breaks across California in a range of habitat types including chapparal, oak woodland, and coniferous forest. Our cooperators include: Los Angeles County Fire Department California Department of Forestry and Fire Protection Orange County Parks Department United States Forest Service National Park Service Bureau of Land Management RESEARCH QUESTIONS Do nonnative plants become established within fuel breaks? Are some types of fuel breaks less likely to support nonnative species? Do fuel breaks promote the invasion of nonnative plants into adjacent wildland areas after disturbances such as fire ? Figure 1. Relative nonnative plant species, cover, density and richness was significantly higher on the fuel break than in the adjacent wildland, ANOVA, p<0.001. Figure 2. Relative nonnative plant cover on the fuel break varied greatly between sites. Figure 4. Relative nonnative plant cover was higher in the wildland area adjacent to fuel breaks at sites that had experienced several fires, compared to sites that had not experienced any fires. This effect was most evident at distances less than 40 meters from the fuel break. SUMMARY Fuel breaks support greater cover, density and numbers of nonnative plants than adjacent wildland areas (Figure 1); Some fuel breaks support a higher abundance of nonnative species than others (Figure 2); Nonnative plant cover was most strongly influenced by elevation and distance from the fuel break (Table 1); and Nonnative plants are found in greater abundance in wildlands adjacent to fuel breaks in areas that have experienced multiple fires (Figure 4). We will continue our data analysis to identify which characteristics of fuel breaks are most strongly correlated with the presence of nonnative plants, and to look at landscape level variables such as distance to urban areas and roads. We hope our results will provide our cooperators with information that will allow them to better design and implement fuel reduction treatments. Figure 3. Relative nonnative plant cover was significantly higher on fuel breaks constructed by bulldozers than those constructed by hand, ANOVA, p<0.001.