Responding to reports that a native bark beetle may be contributing to the death of ancient giant sequoias that were thought to be incredibly insect resistant, Save the Redwoods League is embarking on a study to better understand this beetle and its potential threat to the giant trees.

Visitors to the Sierra Nevada in recent years have been stunned to see whole mountainsides full of brown, dead trees, mostly pines and firs. This extensive tree mortality was the result of a combination of bark beetles and drought. These beetles have evolved with the forest, where they have always preyed on weak trees, creating dead ‘snags’ that are important features of the forest. But the recent, severe drought has weakened the trees and made them much more susceptible. The extent of this devastation has been jaw-dropping: more than 129 million trees have been killed since 2016.

As this crisis was unfolding across the forests of the Sierra, one bright spot was the imperviousness of the giant sequoia to this type of threat. Its thick bark is rich with tannins that repel most pests, helping make them resistant to insect damage. That changed in 2018, when researchers identified several dead giant sequoia monarchs that had significant signs of beetle infestations (see Appendix A of this study). These dead monarchs were observed at the tail-end of the historic 2014-2016 drought and occurred in areas that had sustained recent low severity fires, where the trees were injured by the fire, but unlikely killed by fire damage alone. The hypothesis is that perhaps the combination of drought stress and fire injury weakened the trees enough for beetles to have a significant, and potentially lethal, impact. Ironically, this phenomena was concentrated in wetter areas, perhaps because the trees in these areas had the least extensive root systems prior to the drought. Once the severe drought hit, they may have had poor access to deeper, more reliable water. We still have a lot to learn about the degree to which the beetle is contributing to individual tree death.