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UA: Drought, higher temps combo kills trees faster

Citizen Staff Writer



A slight increase in temperature could see drought-stricken trees die five times faster than they do now, a University of Arizona researcher says.

Scientists were able for the first time to isolate the impact of increased heat on mature piñon mortality by using the controlled environment in UA’s Biosphere 2, said Henry Adams, lead author of a paper that will appear this week in Proceedings of the National Academy of Sciences.

Drought killed at a faster rate piñon trees kept in an environment warmer than normal ambient temperature by 4 degrees centigrade – about 7 degrees Fahrenheit.

“The cooler trees lasted 28 percent longer,” said Adams, a doctoral student in ecology and evolutionary biology at UA. “All the warmer trees died before any of the cooler trees.”

Trees subjected to hotter drought conditions died in 18 weeks compared with 25 weeks for trees living in cooler drought conditions, he said.

A control group of trees watered normally survived at both temperatures, he said.

The effects of higher temperatures mean that lesser droughts, which occur with greater frequency than major droughts, will be deadly if temperatures increase, he said.

When researchers extrapolated the results using the region’s 100-year historical drought record, it showed that widespread piñon die-offs will occur five times faster than now based on anticipated temperature increases, Adams said.

Scientists from the Intergovernmental Panel on Climate Change have projected temperature increases of 3 to 5 degrees centigrade by 2100, he said.

Historical records from severe Southwestern droughts in the 1950s and earlier this decade showed that tree mortality was higher in a lesser drought with higher temperatures, said David Breshears, a UA professor of natural resources and the study co-investigator.

But researchers could not accurately say what impact higher temperatures had on tree mortality until this latest study led by Adams, Breshears said.

“What we’ve done is isolate the effect of temperature alone, and shown that this species is very sensitive to temperature,” Breshears said. “It is warmer in the future by 4 degrees centigrade you won’t need as long a drought to kill the trees.”

The study could show piñons are akin to canaries in coal mines, warning of deadly threats to a variety of species of trees and vegetation, Breshears said.

“I think it’s going to raise concerns of how big and vast the changes from increased temperature and drought are going to be,” he said. “In the Southwest, we’re going to have more drought and more frequent drought.”

Piñons try to protect themselves from drought by waiting it out, Adams said.

“They simply close their pores to not let water out so they are not losing water,” Adams said. “But they aren’t able to take in carbon dioxide from the atmosphere needed to photosynthesize.

“If the drought goes on long enough, they run out of stored energy from photosynthesis and die,” he said. “At higher temperatures, they run out of stored energy faster and die sooner.”

The trees used in the two-year study came from near Las Vegas, N.M., Adams said. They were about 6 feet tall and 20 to 30 years old, he said.

The next step in research has begun near Flagstaff, Adams said.

“We’ll try to re-create what we did inside outside,” he said. “We’ll get accurate mortality rates in field conditions.”

The pines have been transplanted on the north side of the San Francisco Peaks to get a look at how heat affects drought-stricken trees in a more natural environment.

Trees have been planted at two levels – one 400 meters higher in elevation – to give a temperature difference of 4 degrees centigrade, he said.

The ground surrounding test trees will be covered with tarps to ensure drought conditions, he said.

Plans are in the works, said Adams, to study the heat effect on drought mortality on different species of trees inside Biosphere 2 near Oracle.

UA: Drought-stricken pines may die five times faster as temps rise

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