At this New Mexico site, piñon pines were damaged by 2002 (left) and dead by 2004 (right) in a scorching drought. At sites across four states, a team headed by David Breshears of the University of Arizona in Tucson found that 40 to 80 percent of the pine nut–producing trees died during the drought and its plague of bark beetles.
Fewer piñons perished during a similarly prolonged drought decades earlier. The recent drought "wasn't any drier than the 1950s drought, but it was hotter," Breshears says. Heat reduces ground moisture, dehydrates trees, and encourages beetle infestations. Tree-killing droughts could increase in frequency as the world's climate warms, the researchers warn in the Oct. 18 Proceedings of the National Academy of Sciences.