Warming Climate Blamed for Increased Mortality in High-Elevation Western Forests

The journal Science, one of the biggies in the science world, just published an article in its most recent issue about the cause of the increased death rates of trees in the Sierra Nevada and elsewhere in high-mountain areas of western states. From Tom Knudson’s article in the Sacramento Bee:

Tree mortality rates have doubled in old-growth forests across the Sierra Nevada and western United States because of rising temperatures associated with climate change, a new study has found.
The study, which will be published Friday in the journal Science, suggests that if the trend continues, the region’s majestic conifer forests may become younger, smaller and far more sickly — making them vulnerable to massive die-offs from insect attacks and other causes.
“Effectively, everywhere we looked we saw mortality rates increasing,” said Nathan Stephenson, co-leader of the study and a research ecologist with the U.S. Geological Survey in Three Rivers near Sequoia and Kings Canyon National Parks.

Many factors can cause tree death, including insects and overcrowding. The researchers controlled for those factors and were led to the conclusion that the main cause is the overall rise in temperature. Basically, winter snow is melting sooner in spring and summer, depriving the trees of needed moisture. Also, the temperature increase means the trees require more moisture to stay healthy.

I live in Lake Shastina on the north side of Mount Shasta. It’s high-desert country, with a mix of western juniper and ponderosa pine. Just a bit to the northeast in the Shasta Valley the ponderosas end because there is not enough moisture to support them. I’ve wondered if the trend toward decreased precipitation we’ve had in recent years will eventually kill many of the ponderosas around Lake Shastina.

For more information, see the Sacramento Bee page on climate change and the Sierra Nevada. Also visit Bee writer Tom Knudson’s excellent Sierra Summit blog.