Bark beetles are small insects that attack the trunk of trees and eventually end up killing them. Zachary Robbins, a researcher in the Dynamic Ecosystems and Landscapes Lab at North Carolina State University, was shocked upon seeing the large numbers of withered pines that dotted the Sierra Nevada as he drove by in 2019.
He said that most of the trees probably died in 2016 because of the drought and the growing population of bark beetles, which kill giant trees.
The population of Tiny Bark Beetles is Booming: A Bad News for the Pine Trees in the Sierra Nevada.
Climate Change Contributes to Bark Beetles Infestation
According to Wired, the western pine beetle dominates the Sierra Nevada among the 600 species of bark beetles. They chew away its bark to create "galleries" that look like long-legged centipedes and live within the phloem of ponderosa pines, which transports nutrients and eventually kill them. Dead trees are dangerous to residents because they are susceptible to wildfires.
The US Forest Service said that about 150 million trees died during the five-year drought in California that ran from 2012 to 2016. Drought and lack of water can weaken and kill trees, making them an easier target for bark beetles. According to a 2019 study, an estimated 90% of the ponderosa pines were attacked by bark beetles after the drought in the state.
Due to this, Robbins had an idea that the die-off might be associated with climate change because the warmer the climate is, the more beetles there would be to kill trees. Researchers used a computer model to test his theory that shows how drought and warming temperature could affect the Sierra Nevada.
In their study, titled "Warming Increased Bark Beetle-Induced Tree Mortality by 30% During an Extreme Drought in California," published in Global Change Biology, researchers wrote that there is a 20% increase in the number of dead trees for every rise of 1 degree Celsius. During the drought season, the model showed a worse scenario that increased the number of trees killed by 35% to 40%.
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How Do Bark Beetles Kill Trees?
Bark beetles easily destroy trees during a drier and warmer climate because trees are more stressed at this time, Wired reported. They would not be able to defend themselves by releasing waxy resin and chemicals that push the beetles out because they would close the pores in their leaves, preventing photosynthesis from creating carbon dioxide needed for them to live.
Instead, the tree would devote its resources to tissue maintenance and make less of its resins and other defense chemicals, opening an opportunity for bark beetles to invade.
Ectotherms like bark beetles function depending on external temperatures. Their life cycle will also speed up because they reach their reproductive age sooner and produce more offspring to allow populations to grow to larger and more explosive sizes. Once they start to invade, they release pheromones that signal other beetles to invade.
The drought years between 2012 to 2016 are particularly harmful due to their long duration. But experts said that 2018, 2020, and 2021 have also been very dry, which increased bark beetle activity in western pine bark and other trees were recorded across California. Now, they are growing concerned about the huge amount of dry fuel that is just waiting to burn.
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