The Case of the Dying Trembling Aspens
Bill Anderegg preparing to make measurements of aspen canopy photosynthesis
Over the past 10 years, the death of forest trees due to drought and increased temperatures has been documented on all continents except Antarctica. The death of forests, which may be caused by global warming, can in turn drive global warming by reducing the amount of carbon dioxide removed from the atmosphere by trees and by releasing carbon locked up in their wood. Bill Anderegg — a Stanford PhD student and Carnegie Institution researcher supported by the Bill Lane Center for the American West — has now discovered evidence for the physiological mechanism governing tree death in a drought in the aspen forests of the American West. His work was published the week of December 12 by the Proceedings of the National Academy of Sciences.
Forests store about 45 percent of the carbon found on land. Their mortality can radically transform ecosystems, affect biodiversity, harm local economies, and pose fire risks, as well as increase global warming. These risks have become alarming apparent in the American West in recent years.
Scientists had two competing theories for how forest trees die during a drought. One hypothesis proposed that the trees starved due to decreased photosynthetic activity. The other proposed that the system for transporting water within a tree was damaged beyond repair due to the stresses of the drought.
Without knowing which theory was correct, it was difficult for researchers to build models and make projections about the larger impact of drought-induced forest mortality — and what fate may lie in store for forests around the world as well here at home in the American West.
Anderegg's team focused their efforts on climate-induced die offs of trembling aspen trees in the Rocky Mountains. They looked directly at both carbon starvation and water-transportation stress on ongoing forest deaths.
What is causing the widespread death of this emblematic tree of the American West?