The lofty leaves seem more resistant to tension from water pressure — from the tree working to pull water from its roots — and they have different levels of carbon and oxygen than lower leaves, the researchers reported at the meeting. Measuring these elements can reveal more information about how sequoias use water, Ambrose said.
At the root level, sequoias influence the surrounding soil, researchers have discovered. On the downhill side of the trees, where more leaves and branches pile up, the pH is higher compared to soil beneath nearby sugar pines, Stephen Hart, an ecologist at the University of California, Merced, reported at the meeting. Hart and his students tested soils in Yosemite National Park's sequoia groves and found more nitrogen, calcium, magnesium and phosphorous in the soils near sequoias. "The fertility of the entire soil is enhanced," Hart said.
A drought could also be hard for seedlings and young trees, which don't have well-developed root systems that can tap water supplies.
But giant sequoia seedlings seem to have an effective drought response — they completely shut down tiny pores in their leaves, called stomata. Closing their stomata blocks water loss, but it also means the plants can't photosynthesize. Instead, "they live on storage water," explained lead researcher Stefania Mambelli, a plant ecophysiologist at UCB. Mambelli and her colleagues have raised giant sequoia and coast redwood seedlings in a nursery and shut off water to the plants for six weeks to mimic drought conditions.
Scientists are also concerned that climate change could bring a new danger to the giant sequoias via diseases. "It doesn't take moisture stress alone to kill a tree," said Koren Nydick, an ecologist at the Sequoia and Kings Canyon National Parks. Sequoias are normally resistant to most diseases that plague California's forests. But a giant sequoia planted by John Muir in the San Francisco Bay 85 years ago, outside its natural range, is now dying of a fungal disease, Nydick said. And coast redwoods are not only carriers for sudden oak death, they are also four times more susceptible to fire damage because of the fungus, which has killed tens of thousands of California oaks, according to a study published in October 2013 in the journal Ecology.
Sequoias alive today have survived droughts. But a 1992 U.S. Forest Service study, of ancient pollen from a mountain meadow in Sequoia National Park, suggests there were fewer giant sequoias 4,500 years ago when the California climate was drier.
"That leads us to ask if they were near extinction," Nydick said. "Despite their resistance and their huge storage capacity, there is a threshold. Are we going to reach that?"
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