Collapse and restoration of mature forest habitat in California

Mature and old-growth forests provide critically important ecosystems services and wildlife habitats, but they are being lost at a rapid rate to uncharacteristic mega-disturbances. We developed a simulation system to project time-to-extinction for mature and old-growth forest habitat in the Sierra Nevada, California, USA. The simulation parameters were derived from a 1985–2022 empirical time-series of habitat for the southern Sierra Nevada fisher (Pekania pennanti), an endangered native mammal and old-forest obligate that has seen a 50 % decline in its habitat over the past decade from a combination of extensive drought-related tree mortality and high severity fire. We found that under a continuation of the status quo (extensive tree mortality and severe fire, minimal management intervention), fisher habitat had a 64 % chance of complete loss in the next 50 years (by 2075), a risk that increased to 99 % by 2100. However, under a simulated management regime focused on forest restoration, including expanded use of beneficial fire and mechanical thinning aimed at large tree retention and recruitment, the chances of complete loss were reduced to 2–5 % by 2075 and 2–20 % by 2100, and in some cases resulted in a trend reversal and future habitat increases. Our work suggests that fuels reduction and forest restoration interventions, implemented rapidly and at sufficient scale, could conserve remnant mature and old-growth forest habitat in the southern Sierra Nevada and facilitate its recovery over the next century. Our study serves as an exploratory analysis that aims to help quantify coarse levels of risk and helps to focus future studies and forest restoration investments.