USGS: large tree population declining in Yosemite

FRESNO, Calif. — Scientists with the U.S. Geological Survey said Wednesday there are fewer large-diameter trees growing in Yosemite National Park than in years past, most likely because of climate change.

Warmer temperatures and smaller snow packs are creating conditions where fewer Ponderosa and sugar pines and other heartier trees can flourish, said Jim Lutz, a researcher at the University of Washington who co-wrote the study.

“Most of the water that becomes available in the Sierra Nevada comes from the snow pack,” Lutz said. “Higher temperatures might increase populations of insects or make fungi more aggressive … which all could increasingly contribute to tree mortality.”

Lutz explained that when smaller snow packs collect in April and May, the trees have less water to sustain them in the dry summer months that follow. Warmer temperatures also can increase the severity of wildfires, which can kill off trees, he said.

Another factor in the decline may be that parts of Yosemite haven’t experienced wildfires for 100 years. That could have allowed other species that compete with the bigger trees to survive and suck up the water large-diameter trees need to keep growing in girth, Lutz said.

Overall, researchers concluded that the density of large-diameter trees inside the park dropped by 24 percent from 1932 to 1999, according to a detailed analysis of data from two groups of tree surveys taken throughout the park. Trees in the park’s higher elevations were among the most affected, the USGS found.

Yosemite’s protected status as a national park — and rangers’ and ecologists’ good stewardship of its forests — allowed researchers a unique opportunity to study trees’ health over time, Lutz said.

Future studies could examine the decline’s impact on animals like the spotted owl and fisher, which need the habitat large trees provide to live, as do some vegetation like mosses, orchids and lichens, researchers said.

“What we are really concerned with, is can all of the species that make up the ecosystem continue to persist with characteristic abundance,” Lutz said.

Yosemite spokesman Scott Gediman said that the information gathered will be valuable for future resource management.

“A healthy forest is something we strive to maintain as a national park,” Gediman said. “The fact that they looked at these trees and talked about a smaller number of larger-diameter trees means something’s going on.”