The inside of the bark from a Ponderosa pine infested by the western pine beetle near Mariposa, California, on June 29, 2016. Michael Macor/The Chronicle

New threat to California forests: climate-supercharged beetles

Dec 29, 2021 at 1:35pm

Kurtis Alexander

There’s new thinking on why extraordinary numbers of trees have died in the Sierra Nevada over the past decade, leaving vast swaths of evergreen forest a dingy orange and brown.

Some scientists believe that on top of drought conditions putting stress on the trees, which allowed bark beetles to move in and chew up the forest, the beetles were reproducing faster because of the warming climate.

A recent study published in the journal Global Change Biology suggests that 30% more ponderosa pines died in the Sierra during last decade’s drought as result of the hastened rate of beetle development. According to the paper, the western pine beetle matured more quickly amid higher temperatures, shortening the time it took to produce offspring and spawn new generations - a population boom that amounted to unprecedented carnage in the trees.

The finding, which quantifies earlier notions that climate change has supercharged the bark beetle, bodes poorly for the future of California’s forests. Since 2010, at least 163 million trees have died statewide due to a combination of drought and beetles, according to the U.S. Forest Service. More insect infestation would only bring greater hardship.

In some places, scientists say, tree conditions have gotten so bad that forests aren’t likely to recover and, as a result, woodlands will be increasingly replaced by shrubs and grassland. The threat is clear in the southern Sierra, where tree mortality was greatest during the 2012-2016 drought and new trees have struggled to gain a foothold.

“Having that extra generation allows more beetles to go out and kill more trees,” said Zachary Robbins, a researcher at Los Alamos National Laboratory in New Mexico and graduate student at North Carolina State University, who was lead author of the recent study. “There’s been a lot of concern about how climate might be at work here, but one thing that seemed to be lacking was coming up with … how the beetles are benefiting from the warmer climate.”

To answer that question, Robbins and the study’s co-authors, which include more than a dozen scientists from nine universities and research institutions across the country, developed a computer model showing how climate affects western pine beetles during periods of drought. The data for their model came from prior research on the beetles. The modeling was validated by comparing the results with field studies.

While drought itself kills trees, the weakened state of the trees in dry times makes them less able to ward off the one-eighth-of-an-inch insects and puts them at heightened risk of dying from the attacks. Many species of beetles, including the western pine beetle, are native to California and are usually around but are only a problem during droughts.

Continue reading the article from the San Fransisco Chronicle here.