Forest structure predictive of fisher (Pekania pennanti) dens exists in recently burned forest in Yosemite, California, USA

Abstract

One challenge that land managers face in the southern Sierra Nevada is how to balance conservation of fisher (Pekania pennanti) habitat with the reintroduction of fire. The fisher population in the southern Sierra Nevada is of high conservation priority, due to its small population size, genetic isolation, and the risk of habitat loss due to wildfire and fuel reduction activities. It is unknown whether contemporary forests altered by fire suppression can support fisher habitat following the reintroduction of fire. We examined whether patch-scale forest habitat conditions used by fishers in a landscape with limited recent fire (Dinkey study area) also exist in a landscape with restored fire (Yosemite study area). We developed random forest and logistic regression models using lidar-derived forest structure metrics to distinguish reproductive den presence (n = 261) from randomly-generated “available” points (n = 261) within an estimate of the female population home range. The full logistic regression model correctly classified (under cross-validation) 69.5% of observations and the random forest model correctly classified 74.3%. The parsimonious logistic regression model we selected had comparable accuracy to the full model (correctly classified 68.8% of observations) and included the following variables: cover >2 m, 95th percentile height, and 25th percentile height. Partial dependence plots suggest thresholds at which predicted probability of reproductive den presence exceeds 50%: cover >2 m greater than 60%, 95th percentile height of at least 32 m, and 25th percentile height between 4 m and 14 m. We found that comparable thresholds of forest cover and tree height exist in burned areas in Yosemite; 43.0% of burned pixels had similar lidar-derived forest structural characteristics to those predictive of reproductive dens in Dinkey. Areas with similar forest structures occurred within a range of fire severities and years since the most recent fire, and particularly in low-severity fire conditions (mean differenced normalized burn ratio [dNBR] value: 128.4). These results are promising for land managers who face the challenge of simultaneously reducing the risk of high-severity fire and conserving fisher habitat, however more research is needed to conclude whether suitable fisher habitat can exist in burned areas at all scales of selection and for all activities of the fisher population.