VISALIA – Federal researchers warned Thursday that warming temperatures could soon cause California's beloved giant sequoia trees to die off more quickly, so forest managers must start considering the impacts of climate change and a longer, harsher wildfire season.

Hot, dry weather over the last two decades already has helped to kill an unusual number of old-growth pine and fir trees growing in Yosemite and Sequoia national parks, according to recent research from the U.S. Geological Survey.

In the next decade, climate change also could start interfering with the giant sequoias' ability to sprout new seedlings, said Nathan Stephenson, one of several scientists speaking at an interagency symposium in Visalia, a small city at the base of the towering Sierra Nevada mountains.

“The first effects of climate change that we're likely to see is that the giant sequoias will have trouble reproducing because their root systems don't work as well when temperatures warm,” said Nathan Stephenson, a research ecologist with the USGS Western Ecological Research Center, whose desk sits just a few miles from General Sherman, the planet's largest living tree. “After that, I wouldn't be surprised if in 30 years we see their death rates go up.”

Sequoiadendron giganteum, an inland cousin to the tall California coast redwood, can grow to be more than 2,900 years old and bulk up to more than 36 feet in diameter.

Stephenson was among a team of tree demographers who monitored the health of pines and firs growing in the two southern Sierra Nevada parks from 1982 to 2004. As both temperatures and summer droughts increased over that period, he found those trees' normal death rate more than doubled, and the stands became more vulnerable to attacks from insects or fungus, he said.

While those species have a faster life cycle than the ancient sequoias, scientists say the mortality rates can help predict what may happen to the massive organisms as temperatures are predicted to increase an average of 3 to 10 degrees Fahrenheit statewide by the end of the century.

“We've got a lot of our most cherished species at stake,” said Constance Millar, a senior research scientist with the U.S. Department of Agriculture Forest Service. “Rather than just managing forests for the plants we see growing there today, we're now having to look forward to think about what might thrive there in 100 years.”