Table 2: Primary contributing factors leading to forest disturbances in the San Bernardino National Forest.

Contributing factor to forest disturbanceContext Consequence

DroughtDriest physical state forests had been in over a century.Trees died from water stress; those that survived had increased vulnerability to bark beetles.

Forest management practicesShift from forest cutting to forest protection; managing for “park like” conditions; homeowner laws and public perceptions that limited or forbid direct forest management.Tree densities increased, as did the age class of trees, contributing to a landscape with more mature trees, which bark beetles preferentially target, providing sufficient host material for the outbreak to occur.

FiresStrict fire prevention practices and policies implemented; removal of natural fire regime across the landscape. Lack of fire contributed to dense forests with older age classes. This created a continuous canopy that contributed to uncontrollable crown fires.

BeetlesEndemic beetle populations had access to increased host material due to drought and more mature trees on the landscape. Controls that typically hold populations at low levels were lost. Populations exploded, killing trees at unprecedented levels.