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Warming Temps Killing More Sierra Trees

Increasing death rates have been occurring mostly in small trees in the Sierra Nevada, but some larger trees have also died over the past two decades, such as the lodgepole pines in this photo. Credit N. Stephenson, USGS

Trees in the Sierra Nevada are dying faster than usual as temperatures rise and bring summer droughts with them, the U. S. Geological Survey says.

USGS scientists have observed a rise in the tree death-rate across a wide variety of forest types in the Sierra Nevada mountain range over the past two decades, as summer temperatures also rose.

The 20-year study, detailed in the July 20 online edition of the journal Ecology Letters, showed that short-term variations in climate, such as temperature increases, correlated with short-term variation in tree mortality.

"Our findings suggest Sierran forests, and potentially other forests of dry climates, may be sensitive to temperature-driven increases in drought, making them vulnerable to extensive die-back during otherwise normal periods of reduced precipitation," said USGS scientist Phil van Mantgem.

The USGS tracked the fates of more than 20,000 individual trees in old-growth forest in Sequoia and Yosemite national parks and found a significant increase in tree mortality across most zones of elevation and for the two dominant groups of conifers, firs and pines.

Small trees have taken the heaviest toll, most likely because their roots do not reach as deep as large trees, whose long root systems better equip them to survive moderate droughts and to store resources.

In the study, the authors state that if the climate continues to warm in such water-limited areas, tree deaths will likely accelerate.

Potentially exacerbating the problem are other stresses to trees such as air pollution.

"Much like people, when stresses weaken a tree, it becomes more susceptible to further complications," van Mantgem said.

Andrea Thompson
Andrea graduated from Georgia Tech with a B.S. in Earth and Atmospheric Sciences in 2004 and a Master's in the same subject in 2006. She attended the Science, Health and Environmental Reporting Program at New York University and graduated with a Master of Arts in 2006.