A new study led by the U.S. Geological Survey and involving the University of Colorado at Boulder and Oregon State University as well as other research institutes indicates tree deaths in the West’s old-growth forests have more than doubled in recent decades, likely from regional warming and related drought conditions.
The study, published in the Jan. 23 issue of Science, documented tree deaths in all tree sizes in the West located at varying elevations, including tree types such as pine, fir and hemlock. Significant die-offs also were documented in the interior West — including Colorado and Arizona — as well as Northwest regions like northern California, Oregon, Washington and southern British Columbia.
The researchers speculated higher tree deaths could lead to substantial ecological changes in the West, including cascading effects affecting wildlife populations. The tree deaths also could lead to possible increases in atmospheric carbon dioxide levels contributing to warming, which could stem from lower CO2 uptake and storage by smaller trees and increased CO2 emissions from more dead trees on the forest floors.
The study shows the establishment of new, replacement trees is not keeping pace with climbing tree mortality in the study plots, said CU-Boulder geography Professor Thomas Veblen, study co-author. The new study is the largest research project based on long-term forest plots ever published on North American forests, said Veblen.
USGS researchers Phil van Mantgem and Nathan Stephenson led the study. Co-authors included Veblen and Jeremy Smith of CU-Boulder, John Byrne of the U.S. Forest Service Rocky Mountain Research Station, Lori Daniels of the University of British Columbia, Jerry Franklin and Andrew Larson of the University of Washington, Peter Fule of Northern Arizona University and Mark Harmon of Oregon State University.
“This regional warming has contributed to widespread hydrologic changes, such as a declining fraction of precipitation falling as snow, declining water snowpack content, earlier spring snowmelt and runoff, and a consequent lengthening of the summer drought,” wrote the researchers in Science.
“The increase in tree mortality rates documented in the study is further compelling evidence of ecosystem responses to recent climate warming,” said Veblen. “The findings are consistent with other well documented, climate-induced ecological changes, including increased wildfire activity since the mid-1980s and bark beetle outbreaks that are occurring at unprecedented levels in western North America forests, including Alaska.”