Rising temperatures and below average precipitation could increase the risks of trees weakening and even dying, says a University of Arizona professor.

The increased warmth allows insects, not native to Arizona, to weather the cold in winter, said Gregg Garfin, a researcher in the UA's Climate Assessment for the Southwest project.

“Speculation is that the increase in temperature has enabled them to survive over winter and be able to reproduce,” Garfin said.

He compared tree mortality to a boxing match, with trees on one team and a one-two combination of drought and insect outbreaks on the other.

“So the drought hits the trees, and they’re kind of wobbly because they’re stressed for moisture," he said. "They start using their carbon reserves and all their available extra energy, and they’re kind of wobbly. Then the insects come in and knock them out.”

Among areas hit by the combination is Mount Graham in southeastern Arizona, Garfin said. There, in the Coronado National Forest, a large number of trees died in the last decade because of insect outbreaks.

Yoohyun Jung is a University of Arizona journalism student and intern at Arizona Public Media.