Using fire return interval departure (FRID) analysis to map spatial and temporal changes in fire frequency on national forest lands in California

In California, fire regimes and related ecosystem processes have been altered by land use practices associated with Euro-American settlement, and climate warming is exacerbating the magnitude and effects of these changes. Because of changing environmental baselines, restoration of narrowly defined historical conditions may no longer be an attainable or sustainable long-term management goal, but comparisons between historical and current fire regimes can assist managers in prioritizing areas for ecological restoration and other management actions. Fire return interval departure (FRID) analysis quantifies the difference between current and presettlement fire frequencies. We assessed FRID variability along geographic, climatic, and vegetation gradients in California on lands managed by the U.S. Department of Agriculture Forest Service and three forest-dominated national parks, using two types of FRID metrics: percent FRID (PFRID), and the NPS-FRID index. Much of northern California north of the Tehachapi Mountains has missed multiple fire cycles owing to fire suppression, while southern California is characterized by large areas burning at higher frequencies than under presettlement conditions. PFRID exhibited a unimodal (hump-shaped) relationship to elevation across our study area. PFRID showed little relationship to precipitation in northwest California or the Sierra Nevada region, but it decreased with precipitation in southern California. PFRID trends with temperature were unimodal, reaching a maximum at temperatures that approximate the elevation of the mean freezing line in winter storms, which also marks the transition between moist mixed conifer and red fir in most of northern California. Low- and middle-elevation vegetation types supported the greatest departures from presettlement fire frequencies, with oak woodlands, yellow pine, and mixed-conifer forests missing the most fire cycles, and coastal fir, coastal sage scrub, and chaparral tending to experience shorter FRIs than under presettlement conditions. Our results help refine our understanding of departures from presettlement fire regimes across California, and provide a spatial basis for resource management and planning focused on ecological restoration and adaptation to climate change in a fire-prone region.