Over the past 10 years, the death of forest trees due to drought and increased temperatures has been documented on all continents except Antarctica. This 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. New research led by Carnegie researcher and Stanford University PhD student William Anderegg offers evidence for the physiological mechanism governing tree death in a drought. The work is published the week of December 12 by the Proceedings of the National Academy of Sciences. The study concentrated on the aspen die-off, called Sudden Aspen Decline or SAD which began after severe droughts between 2000 and 2004 and affects about 17 percent of aspen forests in Colorado, as well as parts of the western United States and Canada. SAD continued through 2010, when the research was conducted.The aspens are all native to cold regions with cool summers, in the north of the Northern Hemisphere, extending south at high altitudes in the mountains. They are all medium-sized deciduous trees reaching (50—100 feet tall.
All of the aspens typically grow in large clonal colonies derived from a single seedling, and spreading by means of root suckers; new stems in the colony may appear at up to 100 feet from the parent tree. Each individual tree can live for 40—150 years above ground, but the root system of the colony is long-lived. In some cases, this is for thousands of years, sending up new trunks as the older trunks die off above ground. For this reason it is considered to be an indicator of ancient woodlands.
Aspens do not thrive very well in the shade, and it is difficult for aspen seedlings to grow in an already mature aspen stand. Fire indirectly benefits aspen
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.
The team focused their efforts on climate-induced die offs of trembling aspen trees in North America. They looked directly at both carbon starvation and water-transportation stress on ongoing forest deaths.
"Large scale mortality events, such as we see with aspens, are the dynamite in ecosystem responses to climate change. We know that when they occur, they make a huge difference. But we are at the early stages of being able to predict occurrence," said Field, director of Carnegie’s Department of Global Ecology and professor of biology and of environmental Earth systems science at Stanford.
The team found no evidence of significantly decreased carbon reserves in SAD-affected aspens. This undercuts the starvation theory, although it is possible that carbon starvation had occurred and already been rectified.
By contrast they did find notable losses of function in the tree’s water-transportation systems, especially in the roots. SAD-affected trees showed about a 70 percent loss of water conductivity. Potted trees exposed to a summer’s-worth of drought exhibited significant root mortality.
"Our study provides a snapshot of what future droughts could hold for the emblematic tree of the American West. Our results indicate an impaired ability to transport water due to drought damage plays an important role in the recent die-off of aspens," William Anderegg said.
For further information: http://carnegiescience.edu/news/case_dying_aspens