Long‐term fuel and understorey vegetation response to fuel treatments in oak and chaparral stands of northern California

Aims Fuel reduction treatments are broadly implemented to reduce the risk of extreme wildfire, but research on the long‐term effectiveness and impacts of these treatments is lacking. In this study, we examined short‐ and long‐term (two and 15 years) changes in fuels and understorey vegetation after treatment in chaparral and oak‐dominated stands. Location Whiskeytown National Recreation Area, California, USA. Methods Treatments, including hand thinning, mastication only, burn only, and mastication + burning, were applied randomly to one or two units within each of 10 blocks across two vegetation types (chaparral and oak‐dominated). Vegetation data were initially measured two years after treatment and remeasured 15 years later with additional sampling of dead surface fuels and shrubs. Fuel and vegetation data were analyzed to examine the effect of treatment, time, and vegetation type. Results In chaparral stands, shrub height and cover in the hand‐thinning, mastication, and mastication + burning treatments were still lower than in the control after 15 years. However, only the hand‐thinning treatment reduced fine woody fuels. Hand thinning and mastication + burning increased native species richness after two years, but this was also associated with an increase in exotic species richness and cover that persisted after 15 years. In oak‐dominated stands, treatments had varied and relatively fewer changes to fuels and vegetation. Shrub height was reduced in both the hand‐thinning and Burn‐only treatments, but only mastication reduced shrub cover. Species richness and vegetative cover were largely unaffected by treatment in oak‐dominated stands, except for lower native plant cover in mastication and mastication + burning treatments. Conclusion Treatments varied in their level of effectiveness and most involved trade‐offs between their impacts on fuels and vegetation responses that differed by vegetation type and time. Our findings provide insights for managers interested in balancing these trade‐offs when making fuel treatment decisions and emphasize the importance of examining longer‐term effectiveness of fuels treatments. Treatments varied in effectiveness and most involved trade‐offs between their impacts on fuels and vegetation in chaparral and oak‐dominated stands. In chaparral stands, shrubs stayed below control levels 15 years following most treatments but only the mastication treatment did not increase exotic species. In oak stands, effectiveness in treat