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Ponderosa pine forests in the Sierra Nevada that were decimated by the western pine beetle during the 2012-2015 drought will not recover to pre-drought densities, depleting an important reservoir for atmospheric carbon.
“Forests store a lot of atmospheric carbon, so when western pine beetle infestations kill millions of trees, that carbon dioxide goes back into the atmosphere,” said Zachary Robbins, a postdoctoral fellow at Los Alamos National Laboratory.
Robbins is a corresponding author of a new paper published in the journal Frontiers in Environmental Science On carbon stored in living ponderosa pine in the Sierra Nevada of California.
“We also found that while many trees died during the Great Drought, the risk of another mass die-off this century is much lower because bark beetles will have fewer host trees,” he said.
The effects are a mixed bag
But it’s a mixed bag. Western pine beetle outbreaks driven by climate change will continue to occur, limiting forest regeneration after drought.
“Some of the carbon loss will not be recovered in trees, but fewer trees in the landscape reduce the severity of western pine beetle outbreaks,” Robbins said. “The forest seems to have reached an equilibrium at a certain point.”
A minimum density of beetle plants is required to support explosive population growth. “Our paper shows that once forests reach a certain threshold of ponderosa pine density, they become more likely to undergo western pine beetle-driven die-off,” Robbins said.
Complex interactions between climate change, weather, trees and insects are destroying ponderosa forests in California and elsewhere. During the 2012-2015 drought, an estimated 129 million trees of various species died in the Sierra Nevada.
Carbon sink or carbon source?
As a carbon sink, forests store (store) more than one-tenth of greenhouse gas emissions in the United States. Trees can use the increased carbon in the air to grow and thus fix more carbon in the form of wood.
“If there is no disturbance, such as a beetle outbreak, then carbon is stored, but when insects are exposed to higher temperatures from climate change, their populations increase more rapidly,” said Zhonggang Xu, a senior scientist at Los Alamos who co-authored the paper. “Warmer minimum temperatures cause fewer insects to die in winter.”
Add climate-driven drought and “it all contributes to more frequent beetle outbreaks,” Xu said.
“Ecosystems have fundamentally changed under the effects of climate change,” Xu said. “This means the forest cannot recover to pre-Megahara carbon levels.”
During an outbreak, millions of trees die and begin returning carbon to the atmosphere through microbial decay.
Xu said it is important for future carbon budgets to include impacts on forest regeneration due to beetle outbreaks and other disturbances such as drought and fire. Carbon budgeting involves understanding how to manage carbon emissions to prevent catastrophic warming.
Rising temperatures also exacerbate the effects of drought. “In the past, three years of drought would have had a substantial impact on tree mortality,” Xu said. “But in a warmer future, a two-year drought could have the same consequences.”
Robbins, Xu and their colleagues simulated forest and beetle dynamics at 31 sites dominated by ponderosa pine in the central and southern Sierra Nevada. To study tree growth, regeneration and background mortality, they modeled insect mortality and phenology (cyclic biological events), tree defense and insect attack.
They considered two time periods, 2006-2018 and 2018-2100. The team used data from observations and modeling to simulate each site’s climate for 2006-2018, and they used climate model data alone for the 2018-2100 projections.
Outbreaks in California and beyond
This work has implications for ponderosa forests outside of California.
“Many of the interactions we’ve described here are very similar to those across the West, where you have warmer temperatures that support beetle populations and you have dense forests of stressed trees, which will be stressed by future droughts,” Robbins said. “This will increase the risk of outbreaks. Our paper is specific to California, but this is a fundamental problem and we expect it to be reflected in different forest regions in the western United States.”
Xu and Robbins say their findings reflect what land managers have known for years: Overstocked pine stands are particularly susceptible to beetle outbreaks. As drought severity increases, managers may need to reduce forest density through active management to reduce increased risk.
“Our evidence shows that there is much to be gained by those active management processes,” Robbins said.
In a related 2022 paper, Robbins, Xu and colleagues published a new modeling framework to assess the risk of bark beetles in many forest ecosystems under climate change. Adding together the effects of compromised tree defenses (15% to 20%) and bark beetle population growth (20%), the team determined that 35% to 40% more ponderosa pines would die from beetle attack for each degree Celsius of warming.
Zachary J. Robbins et al, Carbon stored in live ponderosa pine in the Sierra Nevada will not return to pre-drought (2012) levels in the 21st century due to bark beetle outbreaks, Frontiers in Environmental Science (2023). DOI: 10.3389/fenvs.2023.1112756