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Colorado aspen woes linked with extreme summer heat

Climate change trouble ahead for high country forests

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Global warming spells trouble for Colorado’s aspen groves. Bob Berwyn photo.

By Summit Voice

FRISCO — In a sign of what global warming may have in store for Colorado forests, a team of scientists say they’ve found convincing evidence that the 2002 drought led directly to a massive aspen die-off that affected up to 17 percent of the state’s famed trees.

The 2002-2003 drought subjected the aspens to the most extreme growing season water stress of the past century, the scientists said, adding that summer high temperatures seem to be the main factor in aspen mortality.

The heat stress didn’t kill the trees directly, but impaired their ability to provide water to the leaves, leading to a decline in growth and increased mortality that has continued for a decade after the drought. Many aspen stands have recovered since then, but Colorado’s forests once again experienced extreme temperatures in the summer of 2012.

The team 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. Isotopes are atoms with the same number of protons, but different numbers of neutrons and their ratios are signatures of where and when water originates, among other features.

“Mother nature provides us with natural fingerprints in the ratio of oxygen isotopes,” said researcher Leander Anderegg. “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.”

Until recently, there has been little attention paid to what drought characteristics (seasonal differences, severity, or durations) actually cause trees to die. Scientists additionally have lacked a sufficient understanding of the processes that lead to die-offs, which inhibits the ability to predict how climate change can affect different ecosystems.

The scientists examined the isotopes in the aspen sap during natural and experimental drought in an area in Colorado that had heavy tree casualties. It turns out that aspens generally use shallow soil moisture, which evaporated quickly with increased temperatures during the summer drought of 2002. They then looked at climate data finding that these high temperatures were part of a long-term increasing trend, likely linked with climate change, a unique feature of this drought that separates it from earlier less damaging droughts.

“Forests store about 45 percent of the carbon found on land,” said William Anderegg. “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.”

In a previous study the brothers, with colleagues, looked at two competing theories for how forest trees die during a drought. One hypothesis was that the trees starved due to decreased photosynthesis. Another was that the system for transporting water within a tree was damaged beyond repair. They looked at both carbon starvation and water-transportation stress and found no evidence of significantly decreased carbon reserves. They did find a notable depressed function in the trees’ water-transport systems, especially in the roots—some 70 percent loss of water conductivity.

This study pinpoints the trigger of this loss—summer temperature was the most important climate variable for explaining aspen death by drying out surface soil and stressing the trees’ water-transport system. Joe Berry, a co-author and Carnegie staff scientist, noted that understanding how and where the trees get their water was 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,” said Chris Field, director of the Carnegie department. This study is a milestone in linking plant-level physiology measurements with large-scale climate to predict vulnerability to climate change in these forests.

The research is published on-line in Global Change Biology. Another related study appeared earlier this year in the same journal.

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