for National Geographic News
The U.S. West's old-growth forests may look quite a bit thinner in the future.
A new study suggests these forests could feature fewer and smaller trees, and global warming may be driving the change. (Learn about global warming.)
"[Tree] death rates have doubled over the last two decades in old-growth stands across the Western U.S.," said Phillip van Mantgem, a U.S. Geological Survey scientist and co-leader of the research team.
Forests are losing trees faster than new ones are able to grow, he said.
Though they contain trees of all ages, the forests van Mantgem and colleagues studied are mature stands established 200 to 500 years ago.
The team spotted the rising death rate trend across a wide range of ages and species in western forest landscapes from regions as diverse as California, Colorado, British Columbia, and Arizona.
Temperatures in the region have risen by about 1 degree Fahrenheit (0.55 degree Celsius) during the past 20 years.
That small but significant warming has helped reduce snowpack and lengthen summer droughts. The added warmth may also be aiding insects and diseases that negatively impact tree health.
The temperature changes may be subtle, but they are contributing to a doubling of tree mortality rates as fast as every 17 years in some areas, the study found. But the rate of tree loss compounds upon itself, so even small fluctuations can make a big impact.
"We may only be talking about an annual tree mortality rate changing from 1 percent a year to 2 percent a year, an extra tree here and there," said Mark Harmon, professor of forest ecology at Oregon State University, in a statement. "But over time a lot of small numbers can add up. The ultimate implications for our forests and environment are huge."
The new research, published in tomorrow's issue of the journal Science, employs data collected over the past 50 years.
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