Regional warming is likely to blame for a steady increase in tree death rates in old-growth forests of the western United States, according to 2009 study in Science.
The study, led by the U.S. Geological Survey (USGS), found that the increase in dying trees has been pervasive. 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. Researchers concluded that the most probable cause is regional warming due to global climate change.
Regardless of the cause, however, the study’s results foreshadow dire consequences for the forests, and the wildlife that calls them home. Additionally, 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.
“The same way that in any group of people a small number will die each year, in any forest a small number of trees die each year,” said van Mantgem. “But our long-term monitoring shows that tree mortality has been climbing, while the establishment of replacement trees has not.”
“Average temperature in the West rose by more than 1° F over the last few decades,” van Mantgem continued. “While this may not sound like much, it has been enough to reduce winter snowpack, cause earlier snowmelt, and lengthen the summer drought.”
Longer, drier summers mean trees become stressed and unhealthy. Drought conditions also provide favorable conditions for insects and diseases. Some recent outbreaks of tree-killing bark beetles in the West have already been linked to warming temperatures.
In some cases, increasing tree deaths could indicate forests vulnerable to sudden, extensive die-back, similar to forest die-back seen over the last few years in parts of the southwestern states, Colorado, and British Columbia. “That may be our biggest concern,” said Nate Stephenson of the USGS. “Is the trend we’re seeing a prelude to bigger, more abrupt changes to our forests?”
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Photo credit:John E. Kraminski