Western U.S. Tree Mortality Rates have Increased Rapidly

The mortality rate of trees in old-growth forests of the western United States has more than doubled over the last few decades and the most probable cause is regional warming, according to researchers from the United States Geological Survey and other participating Universities.

A burned out redwood tree.

Tree death rates have increased across a wide variety of forest types, at all elevations, in trees of all sizes, and in pines, firs, hemlocks, and other kinds of trees., according to the U.S.G.S. report.

The research team also noted that increasing tree mortality rates mean that western forests could become net sources of carbon dioxide to the atmosphere, further speeding up the pace of global warming.

A number of other potential causes were ruled out before the researchers concluded that increasing regional temperature was correlated with tree deaths. The causes that were ruled out were air pollution, long-term effects from fire supression and normal forest dynamics.

The greater than one degree F. increase in temperature over the western U.S. over the past few decades has been enough to reduce winter snowpack, cause earlier snowmelt, and lengthen the summer drought, according to Phil van Mantgem, a USGS scientist. Drought is also stressing the trees and the warming has allowed diseases and insects such as tree-killing bark beetles to thrive.

This study was posted in this month's online edition of Science.

You can also find supporting online material here.

There is also a Science podcast interview with Phil van Mantgem.