Mega-disturbances cause rapid decline of mature conifer forest habitat in California
Federal wildland firefighters perform hazardous tasks in hazardous environments.
Historically, frequent, low to moderate intensity fire ignited by lightning or Indigenous peoples maintained mature forests, which are characterized by large fire-resistant trees. Fire suppression and exclusion in the 20th and 21st centuries allowed for build-up of understory vegetation, including dense small fire-sensitive trees and shrubs, that result in excess fuels, fuel ladders, and increased competition for water.
Additionally, historic logging of large trees, which are also valuable for timber, has limited the contemporary extent of mature forests in the region. These changes to forest structure plus a warming climate pose a threat to remnant mature forests and associated wildlife.
Between 2011-2020, 30 percent of conifer forests in California’s southern Sierra Nevada temporarily or permanently changed to non-forest vegetation. Of the remnant mature forests, 50 percent transitioned to lower density forest or non-forest vegetation types. These changes are attributable to the combination of widespread and severe wildfires, drought, and drought-associated beetle epidemics. Between 2011 and 2020, canopy cover across 30 percent of conifer forests of the southern Sierra Nevada declined, changing to either sparse woodland/savanna or in many cases to non-treed vegetation. This change was attributed to compounding megafires and a historic drought.
Different types of mature forest showed different changes. Mature forest habitat declined by 50 percent, transitioning to lower density forest or through complete conversion to non-forest vegetation. Changes were particularly pronounced among dense mature forests, which declined by 85 percent. Spotted owl Protected Activity Centers (PACs), also saw dramatic declines with a loss of 49 percent of their 2011 canopy cover, whereas non-PAC areas declines by an average of 42 percent. Some forests that persisted despite low-moderate severity fire may have experienced some restoration of resilience to future disturbances. However, much of the region’s conifer forests remain susceptible to severe disturbance without active management for beneficial disturbance dynamics.