It is evident that drought can create massive problems for ecosystems, especially for trees and plants but no research has been done to determine which drought characteristics actually cause trees to die-off. It has been difficult for scientists to understand how seasonal differences, severity, and duration of droughts have affected tree mortality and even more difficult to predict how climate change can affect different ecosystems.
However, a team of scientists, led by researchers at Carnegie's Department of Global Ecology, has determined that decreased precipitation exacerbated by high summer temperatures has caused the widespread die-off of Colorado trembling aspen trees. The die-off, triggered by the drought from 2000-2003, is estimated to have affected up to 17% of Colorado aspen forests.
To study this die-off, a team led by brothers Leander and William Anderegg, looked at the dynamics of water availability to the trees by examining the ratio of oxygen isotopes in the sap contained in the tree "veins" that transport water.
The ratio provides insight into the type of water that is available to the trees (i.e. is the water from summer rain or winter snow?) Scientists then use these markers to figure out where and when the water found in tree veins was taken up, which consequently determines drought impacts. Drought affects the ability of trees to provide water to their leaves, leading to a decline in growth and increased mortality that can continue years after the initial drought.
The scientists focused on aspen sap during natural and experimental drought in an area in Colorado that had heavy tree casualties. Aspens generally use shallow soil moisture, which evaporated quickly with increased temperatures during the summer drought of 2002. Scientists looked at climate data and found that these high temperatures were part of a long-term increasing trend, likely linked with climate change, and therefore explains why this drought was more damaging that previous ones.
William Anderegg stated: "Widespread tree death can radically transform ecosystems, affecting biodiversity, posing fire risks, and even harming local economies. Rapid shifts in ecosystems, particularly through vegetation die-offs could be among the most striking impacts of increased drought and climate change around the globe."
This study confirms that summer temperature was the most important climate variable for explaining aspen death by drying out surface soil and stressing the trees' water-transport system.