Supplementary file 1_Deforestation and reforestation in a world hotspot of fire-driven forest loss: trends in California conifer forests 1991–2023.docx

North America is a continental leader in fire-driven deforestation, with US western and Mediterranean-climate forests being major centers of forest loss. 25% of the average annual reforestation need in the western US is wholly in California, where background mortality rates are high, and recent droughts, insect outbreaks, and severe wildfires have led to increasing rates of forest loss. Our objective was to use a “ground up” approach to document fire-driven trends in California in deforestation severity and reforestation priority (treated synonymously here: DS/RP). Using field data on postfire conifer regeneration from across the State, we employed the POSCRPT (Postfire Spatial Conifer Regeneration Prediction Tool) platform to estimate postfire conifer regeneration probabilities at 900-m2 spatial resolution from 1991 to 2023. We then overlaid our maps of natural regeneration probabilities with reforestation data from US Forest Service (USFS) records to estimate reforestation deficit. Using three definitions of DS/RP (“moderate,” “high,” and “acute”), we found that trajectories in all three classes were best explained by exponential growth with no significant evidence of attenuation toward saturation. By 2021–2023, fire-driven deforestation reached nearly 150,000 ha per year under our moderate DS/RP definition, and 86,000 ha under our high DS/RP definition. Mixed conifer, yellow pine, and Douglas-fir/hardwood forests experienced the most fire-driven deforestation since 1991, but acceleration in the rate of deforestation was highest in high-elevation forests. Private and USFS lands experienced the highest amounts and rates of increase in DS/RP during the study period, accounting for >93% of all reforestation need. On USFS lands, postfire reforestation efforts in California began to fall behind fire-driven reforestation needs after 2006; between 2016 and 2023 < 1.2% of deforested hectares were replanted. California is one of the world leaders in fire-driven forest loss (deforested hectares minus reforested hectares), with estimated 2001–2023 mean annual losses of 0.25 to 0.47% (high and moderate DS/RP classes, respectively) in the forest types we assessed. Successful reversal of these trends will likely require major reform to the reforestation pipeline and attenuation of long-term declines in reforestation funding, staffing, and capacity.