A new study published in the journal Frontiers in Environmental Science has found that the Ponderosa pine forests in California’s Sierra Nevada that were destroyed by western pine beetles during the 2012-2015 megadrought will not recover to pre-drought densities. This reduction in the forest’s ability to store carbon is concerning as forests play an important role in sequestering atmospheric carbon.
“When western pine beetle infestations kill off millions of trees, that carbon dioxide goes back into the atmosphere,” explained study lead author Zachary Robbins, a postdoctoral researcher at Los Alamos National Laboratory.
The research showed that there is much less risk of another huge die-off this century because the bark beetles will have fewer host trees. However, western pine beetle outbreaks driven by climate change will continue to occur, limiting forest regeneration after the drought.
“Some carbon loss won’t be resequestered in trees, but fewer trees on the landscape dampens the severity of western pine beetle outbreaks,” Robbins said. “The forest seems to reach an equilibrium at a certain point.”
During the 2012-2015 drought, an estimated 129 million trees of various species died in the Sierra Nevada, leading to complex interactions among climate change, weather, trees and beetles, which are decimating ponderosa forests in California and elsewhere.
As carbon sinks, forests sequester, or store, more than a tenth of the greenhouse-gas emissions in the United States. Trees can use the increasing carbon in the air to grow and thus fix more carbon in the form of wood.
“If there’s no disturbance, such as a beetle outbreak, then the carbon gets stored, but when the beetle comes with the higher temperatures of climate change, their populations develop more rapidly. Fewer beetles die in the winter because of warmer minimum temperatures,” said study co-author Chonggang Xu, a senior scientist at Los Alamos. “Furthermore, climate-driven drought contributes to more frequent beetle outbreaks.”
“The ecosystem has been fundamentally changed by the effects of climate change. That means the forest can’t recover to the pre-megadrought carbon level.” During a beetle outbreak, millions of trees die and begin releasing carbon back to the atmosphere through microbial decay.
“It’s critical for future carbon budgeting to include the effects on forest regeneration caused by beetle outbreaks and other disturbances such as drought and fire,” said Xu. Carbon budgeting aims to manage carbon emissions and prevent catastrophic warming.
Xu also noted that rising temperatures worsen the impact of drought on trees. “In the past, a three-year drought would have substantial impact on tree mortality,” Xu said. “But in a warmer future, a two-year drought could have similar consequences.”
The researchers simulated forests and beetle dynamics in 31 sites dominated by ponderosa pines in the central and southern Sierra Nevada. They used modeling to study tree growth, regeneration, and background mortality, as well as insect mortality, phenology, tree defense, and insect attacks.
The team considered two periods, 2006-2018 and 2018-2100, and used climate data from observations and modeling for the former and climate model data alone for the latter. The research has implications beyond California.
“Many of the interactions we describe here are very similar to those across the West, where you have warming temperatures helping beetle populations and you have dense forests of stressed trees, which will continue to be stressed by future drought,” said Robbins.
The experts emphasized that overstocked pine stands are particularly vulnerable to beetle outbreaks and that proactive management processes can mitigate the increased risk posed by drought. “Our evidence shows there’s a lot to be gained by those proactive management processes,” Robbins said.
In a related 2022 paper, Robbins, Xu, and their collaborators published a new modeling framework to assess the risk bark beetles pose in many forest ecosystems under climate change. The team found that for each degree Celsius of warming, 35 to 40 percent more ponderosa pines would die from beetle attacks, factoring in the effects of compromised tree defenses and increased bark beetle populations.
The study’s findings highlight the importance of protecting forests and taking action to mitigate climate change, as these actions are critical for preserving the carbon-sequestration potential of forests.
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