Oregon Public Broadcasting

Trees are dying in undisturbed Western forests at twice the rate observed in the 1980’s.  Scientists finger climate change as their prime suspect in a study to be published Friday.  Correspondent Tom Banse has details.

A team of U.S. Geological Survey and university researchers based their findings on data that stretches back 50 years.  It comes from 76 different patches of protected old growth forests scattered across the West. 

An old-growth sugar pine in the Sierra Nevada Range in California dies after a bark beetle attack. Warmer temperatures that reduce snow pack, prolong drought and favor insects could be affecting trees.

Every few years, a student or scientist visited each study plot to count all the living and dead trees. 

Lead study author Phil Van Mantgem says crunching all the data together reveals a worrisome trend. Tree death rates have more than doubled over the last few decades across the Western U.S.

Phil Van Mantgem: “This trend was pervasive across a wide variety of forest types, across all elevations and trees of all sizes and among major species.  That includes pines, firs and hemlocks and other kinds of trees.”

The researchers naturally asked, why is this happening?  The science team ruled out air pollution, tree overcrowding, or fire control as primary causes. 

They conclude the most plausible explanation for the widespread jump in mortality is rising temperatures.  On average, the West has warmed by about one degree Fahrenheit over the last couple decades.

USGS scientist Nate Stephenson says that seemingly small change can hurt trees in multiple ways.

Researchers ride in the gondola of the 28-story Wind River Canopy Crane being used to study, among other things, tree mortality in an old growth forest in the Gifford Pinchot National Forest in southwest Washington State.

Nate Stephenson: “Summers are getting longer.  The summer drought in the western United States is getting longer.  So one possible cause is that trees are under more drought stress. Another possible cause is that warmer temperatures are better for insects and pathogens that like to eat on trees.”

If current trends hold, Stephenson says forests will become “sparser” over time.  That also means future forests could store less carbon.

Phil Van Mantgem: “Further speeding up the pace of global warming.”

The study authors say we should prepare ourselves for higher wildfire risk as a consequence of more dead wood lying around.

Ecologists from the University of Washington, Oregon State University, and University of British Columbia joined the federal scientists working on this peer-reviewed study.  The results appear in the latest issue of the journal Science.