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Study: Climate Change, Drought Threaten Giant Sequoias

November 9, 2017

A new study published online in the Journal of Geophysical Research: Biogeosciences finds that the giant sequoia, a fixture of California’s Sierra Nevada forests for the past 2.6 million years, might be in jeopardy from the effects of drought and climate change.

The iconic trees, which only grow in some 70 groves scattered over an area of about 55 square miles on the western slopes of California’s Sierra Nevada mountains, were spared the widespread tree mortality that recently occurred in California forests, claiming 102 million trees over a period coinciding with the state’s 2011-2015 drought.                     

However, researchers from the Sierra Nevada Research Institute (SNRI) at UC Merced, the U.S. National Park Service, the Chinese Academy of Sciences and Sun Yat-sen University found that sequoia groves are showing signs of stress, suggesting even these normally resilient trees are becoming increasingly vulnerable to multi-year droughts, which are projected to continue increasing in severity because of climate change.

“Giant sequoias have this mystique, that once they reach maturity, they are practically immune to the forces that kill other trees,” said Koren Nydick, study coauthor and science coordinator and ecologist at Sequoia and Kings national parks. “But during and just after the recent severe drought, we've seen a small number of sequoias die, at least in part due to the dry conditions. It's an important wake-up call.”

Using data from Landsat, an Earth-observing satellite fleet jointly managed by the U.S. Geological Survey and NASA, researchers evaluated greenness and wetness in sequoia groves and adjacent forests. The analysis did not tease out the effects on just the sequoia trees, but instead presents a picture of changing conditions for the suite of tree species in the groves.

The results indicate that groves experienced a 6 percent increase in greenness from 1985 to 2015. Enhanced greenness could indicate higher forest biomass, which likely led to a 10 percent increase in water usage during the same period.