However, in the face of the kind of sustained "superdrought" some climatologists predict will occur toward the end of this century, both species would be heavily affected, he said.
"If we had a significant, prolonged drought in the future, we might actually lose both of them," he said. "We could see a lot of the areas that are forests today become grasslands."
Although both carbon starvation and hydraulic failure will kill a tree eventually, it is often some other agent, such as a parasite, that delivers the killing blow, he said. Biotic agents like bark beetles, and the blue-stain fungus the beetle introduces to the tree, act like a third wave of assault, attacking weakened trees and overwhelming their defenses.
Healthy trees can often fight off beetle attacks, drowning invaders in secretions of sap. But some evidence suggests that carbon and water are both necessary ingredients in the production of sap, indicating that the depletion of either can leave a tree less able to defend itself.
"These mechanisms all interact," McDowell said. "Whether the tree succumbs to carbon starvation, water loss or biota, it's just a matter of different mechanisms in different places."
In many parts of the West, a spate of uncommonly dry years has already led to the worst bark beetle outbreak in historical record.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500