Tree deaths double across western US

• 19:00 22 January 2009 by Catherine Brahic
• New Scientist, For similar stories, visit the Endangered Species and Climate Change Topic Guides
• http://www.newscientist.com/article/dn16469-tree-deaths-double-across-western-us.html

The majestic old trees of the western US are disappearing twice as fast as they were three decades ago, and climate change is most likely to blame, say scientists.

Philip van Mantgem of the US Geological Survey and colleagues collected data from 76 plots on the west coast – from California up to British Columbia, Canada – and in Idaho, Arizona and Colorado. These are plots without any direct human management, so any tree loss is not due to logging.

The team focused on old forests, where many of the trees were at least 200 years old, and sometimes as much as 1000 years old. In 87% of the plots, trees are disappearing faster than new trees are springing up. Death rates varied, but the trend held whether the trees were old or relatively young, big or small, high up in the mountains or down in valleys.

The Pacific Northwest, including the pine trees of British Columbia, were the worst affected – death rates there are doubling every 17 years.

Increased vulnerability

"We may only be talking about an annual tree mortality changing from 1% a year to 2% a year, an extra tree here and there," says Mark Harmon, a forest ecologist at Oregon State University in Corvallis, who participated in the study. "But over time a lot of small numbers can add up."

For 100 trees, 1% mortality per year means the plot would lose 40 trees in 50 years. But 2% mortality means the plot would lose 64 trees over the same time span.

"The ultimate implications for our forests and environment are huge," says Harmon. For instance, it puts forests at a greater risk of sudden and extensive die-backs, because of pest invasions or a spell of drought.

Beetles and fungi

The team believes the rise in average temperatures across the region is the main driver of things that are killing off the trees faster.

"Warming can cause a lot of changes," says van Mantgem. "It could [increase evaporation] in these stands and effectively dry them out, and it could make things that chew on trees much happier – things like bark beetles and fungi."

Van Mantgem and colleagues also considered the possibility that other factors are responsible for the die-offs. For instance, a popular theory is that human activities that aim to suppress forest fires ultimately increase tree mortality rates. But the surveys showed that trees are dying faster now even on plots that have not burned for 200 years.

Mark Ashton, a forest ecologist at Yale University, says the findings are interesting but preliminary. He adds that it is difficult to say if trees elsewhere are also dying faster because old, unmanaged forests are rarer on the east coast and in Europe.

Journal reference: Science (DOI: 10.1126/science.1165000, in press)