In a forestry aspect, smog incurs a cost on the existence value of trees and wild plants. In Los Angeles, smog was attributed to the deaths of 50% of trees in nearby areas (ibid.). Similarly, ground-level ozone from the Central Valley and San Francisco-Oakland metropolitan areas was responsible for increasing stress and vulnerability on the ponderosa and Jeffrey pines in the Sierra Nevada (ibid.). An attack by western pine beetles subsequently diminished the number of these trees.