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Is the Climate Killing Our Trees?

Aerial_Shasta forestsA new collaborative study suggests that warming temperatures are taking a toll on trees in old-growth forests across the western US.

The study concluded that the near doubling in the mortality rate over several decades transcends forest types, elevation, tree size and species. The study will by published in Science this week.

Phil van Mantgem, who co-led the research team at USGS, said the spike in dying trees could lead to habitat destruction for forest wildlife. And while living trees absorb greenhouse gases, dying trees actually release carbon dioxide into the atmosphere, furthering the effects of global warming.

Usually, van Mantgem said, a small number of trees in a forest die each year and are replaced by new growth. However he’s observed that trees are dying so quickly that new growth is having trouble keeping up. He said one of the causes could be the West’s rising average temperature. While it rose only 1 degree (F) during the past few decades, he said it’s been enough to reduce the snowpack and melt the snow earlier, causing longer periods of dry weather and distressing forests.

Warm weather might also nurture insects and diseases that attack trees. Some reports have already tied destructive bark-beetle outbreaks to higher temperatures.

Nate Stephenson, another research team co-leader with the USGS, said the deaths, over time, could reduce the age of the western forests. “Tree death rates are like interest on a bank account – the effects compound over time,” Stephenson said. Stephenson worries that the increasing rate could lead to a bigger and more abrupt change in forests, similar to sudden and extensive die-backs observed in the southwest, Colorado and British Columbia.

Scientists from the U.S. Forest Service, and six universities collaborated on the study. Van Mantgem appeared on KQED’s Forum program today, along with host Dave Iverson, Climate Watch Sr. Editor Craig Miller and Inez Fung, author of a new study on seasons shifting from rising temperatures. Van Mantgem then popped up on NPR’s Science Friday. New York Times correspondent Andrew Revkin, author of the widely followed Dot Earth blog, also appeared and responded to recent polling on attitudes toward climate change.