USA: Tree death in forests dramatically increase due to climate change

Tree death rates have more than doubled over the last few decades in
old-growth forests of the western United States, and the most probable
cause of the worrisome trend is regional warming, according to a U.S.
Geological Survey-led (USGS) study published in Science on January 23.
The study 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. Regardless of the cause, higher
tree death rates ultimately could lead to substantial changes in
western forests, said Phil van Mantgem, a USGS scientist and co-leader
of the research team.

Such changes, the team noted, can have
cascading effects, such as by changing forest suitability for wildlife
species. 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.” The
result is that forests have begun to lose trees faster than they’re
gaining them, said van Mantgem. The study’s authors ruled out a number
of possible sources of the increasing tree deaths, including air
pollution, long-term effects of fire suppression, and normal forest
dynamics. In contrast, increasing regional temperature was correlated
with tree deaths. ”

Average temperature in the West rose by more than
1° F over the last few decades,” said van Mantgem. “While this may
not sound like much, it has been enough to reduce winter snowpack,
cause earlier snowmelt, and lengthen the summer drought.” The
lengthening summer drought could be stressing trees, leading to higher
death rates, he said. Warmer temperatures also might favor insects
and diseases that attack trees. Some recent outbreaks of tree-killing
bark beetles in the West have already been linked to warming
temperatures. “Tree death rates are like interest on a bank account –
the effects compound over time,” said Nate Stephenson, also with the
U.S. Geological Survey and research team co-leader. “A doubling of
death rates eventually could reduce average tree age in a forest by
half, thus reducing average tree size.”
http://www.usgs.gov/newsroom/article.asp?ID=211