Bark Beetles of the Southern California Forests

What Caused the Bark Beatle Outbreak ?

Bark beetles are naturally occurring species in the Southern California forests. Under normal conditions they occur at low population levels, going unnoticed by humans. In recent years conditions have become favorable for bark beetle population growth, which ultimately led to the current outbreak. Several factors have led to this favorable beetle environment. First, decades of fire suppression has resulted in over-crowded forests. The dense stands of mature trees compete for limited nutrients and water, reducing the health of the stands. Second, long term exposure to air pollution, particularly ozone, has reduced the health of conifers in the forest. Excessive ozone exposure causes premature loss of pine needles, reducing the trees ability to produce food and tissues. Third, Southern California experienced four continuous years of drought (1999-2002). This long term water stress further reduced the health of the conifers. The combination of these factors has resulted in a forest full of unhealthy vulnerable trees.

San Bernardino County Museum