Warming temperatures and the 2012-2015 megadrought fueled an outbreak of western pine beetles that decimated forests in the Sierra Nevada. Future conditions may limit both the scale of infestations and regrowth of timber to pre-megadrought levels. Credit: Zachary Robbins/Los Alamos National Laboratory
Key points:
- Ponderosa pine forests in Sierra Nevada, which suffered drought and beetle infestation, may never come back.
- The forests are being affected by climate change, weather, trees and beetles.
- Research indicates overstocked pine stands are susceptible to beetle outbreaks and must be actively managed.
The Ponderosa pine forests in Sierra Nevada took a big hit from western pine beetles from 2012-2015, and simultaneously suffered from a severe drought. A new study says the important trees may never recover to pre-drought density, which could greatly reduce important storehouses for atmospheric carbon.
“Forests store huge amounts of atmospheric carbon, so when western pine beetle infestations kill off millions of trees, that carbon dioxide goes back into the atmosphere,” said corresponding author Zachary Robbins, a postdoctoral researcher at Los Alamos National Laboratory.
For the study, published in Frontiers, Robbins and his team simulated forests and beetle dynamics at 31 sites where ponderosa pines predominate in Sierra Nevada. They modeled insect mortality and phenology, tree defense and insect attacks to study tree growth, regeneration and background mortality.
The team used data from observations and modeling to simulate the climate for each site for 2006-2018, and then used climate model data alone for the 2018-2100 projections.
Robbins said the results reflect what land managers have known for many years: Overstocked pine stands are particularly susceptible to beetle outbreaks. As droughts increase in severity, managers may have to scale back forest density through active management to mitigate the increased risk.
“Our evidence shows there’s a lot to be gained by those proactive management processes,” Robbins said.
If there’s a bright side, the researchers say that because so many trees died during the megadrought, there’s much less risk of another huge die-off this century because the bark beetles will have fewer host trees.