Status and trends of fire activity in southern California yellow pine and mixed conifer forests

Frequent, low to moderate severity fire in mixed conifer and yellow pine forests of California played an integral role in maintaining these ecosystems historically. Fire suppression starting in the early 20th century has led to altered fire regimes that affect forest composition, structure and risk of vegetation type conversion following disturbance. Several studies have found evidence of increasingly large proportions and patches of high severity fire in fire-deprived conifer forests of northern California, but few studies have investigated the impacts of fire suppression on the isolated forests of southern California. In this study, spatial data were used to compare the current fire return interval (FRI) in yellow pine and mixed conifer forests of southern California to historical conditions. Remotely sensed burn severity and fire perimeter data were analyzed to assess changes in burn severity and fire size patterns over the last 32-100 years. Half of the yellow pine and mixed conifer forest in this study has missed multiple burn opportunities and has not experienced fire in the 109-year fire record. The average proportion of conifer forests burned in high severity fire ( > 90% tree basal area loss), average high severity patch size, and maximum high severity patch size all increased significantly from 1984 to 2016. The average fire proportion burned at high severity from 2000 to 2016 is 1.5 times higher than predicted for the natural range of variation (NRV). Additionally, the years after 2000 had high severity patches larger than 25 ha, whereas no fires before 2000 had patches this large, indicating a deviation from NRV since the turn of the century. Fire size in conifer forests significantly increased from 1910 to 2016, owed to a substantial increase in the occurrence of large fires (larger than the natural range of variation) after 2000. This analysis indicates that southern California conifer forests are like their northern counterparts in that they have burned very infrequently since the early 1900s, resulting in large and homogenous areas of stand replacing burns. This is likely exacerbated even further by recent fire-conducive weather conditions and extended periods of drought. In southern California, recovery from large, high severity burns is likely to be impeded by the small and disparate nature of mixed conifer forests and limited seed dispersal capabilities of remaining trees. Therefore, preemptive forest treatments and careful fire management is needed to return natural structure and function to these forests.