Science: Tree-Death Rates, On the Rise Since 1955, May Be Linked to Climate Change

Sugar pine dying from bark beetle attack in Yosemite National Park

[Image courtesy Jerry Franklin]

Trees in the old forests of the western United States have been dying at ever greater rates in recent decades, likely because of regional climate warming, researchers say in the 23 January issue of Science.

Phillip van Mantgem, a research ecologist at the United States Geological Survey Western Ecological Research Center, and his colleagues analyzed records from undisturbed old-growth forests and report that tree mortality rates have more than doubled since 1955.

"Tree death rates are like interest on a bank account—the effects compound over time," said study coauthor Nate Stephenson, also with the U.S. Geological Survey. "A doubling of death rates eventually could reduce average tree age in a forest by half, thus reducing average tree size."

Changes in tree mortality can have long-term effects on a forest's overall structure and the resources it consumes, which in turn can influence the amount of carbon the forest can store and, ultimately, how the planet responds to climate change.

The trend the researchers have identified applies to trees of different sizes and types, growing at different elevations and in forests with different fire histories, suggesting that the changes are not simply due to increasing competition for resources or aging of large trees.

In their study, the authors note that from the 1970s to 2006, the period that includes the bulk of their data, the mean annual temperature of the western United States increased at a rate of 0.3 degrees C to 0.4 degrees C per decade, and increased even more at the higher elevations typically occupied by forests. They suggest that regional warming may be to blame for the increased tree mortality, since it may have led to stress from drought or pests, or both.

Kathleen Wren

22 January 2009