Diseases, insects, soil moisture, wind, fire, and snow combine with human activities to create tree hazards, especially in natural areas like Yosemite. The National Park Service defines tree hazard as any tree, either alive or dead, which due to outwardly visible defects could fall down (in part or entirety) and strike a person or property within any designated portion of a development zone.
Forestry workers in Yosemite balance protecting people and property with conserving unimpaired the natural and cultural resources of the park. Crews accomplish this through regularly scheduled surveillance and special surveys after severe storms, fires, or other disturbances. At that time, trees requiring hazard abatement are identified. Crews then post warnings or site closures, or proceed with hazard mitigation by pruning or tree removal. Tree debris is left on the ground in places where practicable, but often must be removed. Forestry workers remove a tree hazard in Yosemite National Park.
Unfortunately, trees without apparent defects also fail, and tree hazards cannot always be immediately identified and abated or mitigated. Catastrophic tree failures in Yosemite have killed eight people since 1963, seriously injured many more, and caused $1 million worth of property damage.