Alan McStravick for redOrbit.com – Your Universe Online
Two brothers at the Carnegie Institution for Science have undertaken the study of the widespread die-off of Colorado aspen trees. Their most recent study, published online in the journal Global Change Biology, follows on the heels of a previous study they did.
Their last study explored two competing theories for how forest trees tend to die off in drought conditions. The first theory suggested that trees actually starved to death due to a decrease in photosynthetic activity. The other theory explored how the trees´ internal system for transporting water might possibly be damaged beyond repair.
Their study looked at both carbon starvation and water-transportation stress and found that there was no evidence of significantly decreased carbon reserves. What they did find, however, was a significant depression in the function in the trees´ water-transport systems. Most notably, the root structure was most affected, accounting for upwards of 70 percent to the total loss of water conductivity within the tree.
Together with colleagues from the Carnegie Institution for Science´s Department of Global Ecology, the most recent study focuses on the dramatic die-off of Colorado trembling aspen trees as a direct result of decreased precipitation exacerbated by elevated summer temperatures.
They looked at the die-off triggered by the drought from 2000-2003. From their findings, they estimated that some 17 percent of Colorado aspen forests had been directly affected. In fact, the drought of 2002 is known to have subjected the trees to the most extreme growing season water stress of the past century.
It may not have been the drought and its after-effects that actually killed the trees, however, the drought is responsible for damaging the ability of the tree to provide water to its leaves, thereby leading to an overall decline in growth and an increase in mortality that continued for a full decade after the drought.
UNCHARTED TERRITORY
Brothers Leander and William Anderegg are among the first researchers to pioneer a study into which drought characteristics, among seasonal differences, severity and durations, are the most likely cause of tree mortality. Previous studies in this field were inhibited by the science community´s lack of a sufficient understanding of the processes that lead to die-offs. As there was little previous research, the ability to predict how climate change might affect different ecosystems was largely uncharted territory.
At the time of the research, William Anderegg was a PhD student, and his brother Leander was an undergraduate. The duo observed 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. Isotopes are atoms with the same number of protons, but different numbers of neutrons and their ratios serve as signatures of where and when water originates.
“Mother nature provides us with natural fingerprints in the ratio of oxygen isotopes,” explained Leander. “They tell us about the type of water available to the trees. For instance, summer rain has different isotopic ratios than winter snow. So we can use these markers to figure out where and when the water found in tree veins was taken up, which in turn helps us determine drought impacts.”
In looking at an area of Colorado that experienced heavy tree casualties, the scientists examined the isotopes in the aspen sap during both natural and simulated droughts. What they learned was that aspens generally take moisture from shallow soil, which tends to evaporate very quickly with increased temperatures. The drought of 2002 was one such example, with increased summer temperatures responsible for the loss of water in the top soil. Climate data the team looked over showed the elevated temperatures were part of a long-term increasing trend they believe was likely linked with climate change. They claim this link is a unique feature of this drought that separates it from previously recorded, less damaging droughts.
“Forests store about 45 percent of the carbon found on land,” explained William. “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.”
The brothers Anderegg and their study effectively identified the trigger of the aspen die-offs as being related to summer temperatures, this being the most important climate variable. The elevated temperatures dry out the surface soil and overburden the trees´ water-transport system. Co-author and Carnegie staff scientist Joe Berry pointed out the importance of the knowledge the team gained in understanding how and where the trees get their water as well as how that information provided the key to unraveling cause and effect in this study.
“Since there is a very strong upward trend in Colorado summer temperatures, they could link tree death to climate change,” added Chris Field, director of Carnegie´s Department of Global Ecology. Experts say that this study is a milestone in linking plant-level physiology measurements to the large-scale climate, and that this link will help future studies gauge the vulnerability of such forests to climate change.
While the brothers Anderegg were compiling their data and completing their study, this region underwent another hot summer drought in the summer of 2012. If their study is correct, researchers say we could expect to see another major die-off of the Colorado trembling aspen in the near future.
Established in 2002, the Department for Global Ecology was founded to help build the scientific foundations for a sustainable future. The department is located on the campus of Stanford University. However, it remains an independent research organization funded solely by the Carnegie Institution. Researchers and scientists for the Institution conduct research on a wide range of large-scale environmental issues. Among them are climate change, ocean acidification, biological invasions and changes in biodiversity.
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