Ancient trees and modern wildfires: Declining resilience to wildfire in the highly fire-adapted giant sequoia

•Giant sequoias are a long-lived, fire-adapted species of limited distribution.•The extent of high severity fire in giant sequoia forests is rapidly increasing.•Large sequoias had 84% mortality in remote sensing-defined high severity areas.•Delayed mortality was predicted by percent crown damage and presence of a fire scar.•Forest treatments that reduce fire severity could help protect this unique species.