The vast conifer forests of California’s Sierra Nevada range inspire awe and create lasting memories. The size and longevity of these trees make them seem both otherworldly and everlasting. Indeed, their grandeur is such that visitors may not appreciate how these forests are connected to the larger landscape, and so there is little understanding that something as common as a drought could lead to the death of so many trees and trigger massive and irreversible changes in the nature of these forests (Box 1: Reflections on the California Drought). Although tree death is a natural and essential process, human activity is increasing the mortality rate to such an extent that it threatens the persistence of the "big tree" forests we have come to associate with the Sierra Nevada. Here, we briefly explain how forests change, with a focus on the role of tree mortality. We then describe the contemporary risks and outline a way toward restoring the resilience of Sierran forests.