Research ecologist Nathan Stephenson crawled around magnificent Giant Forest, checking young giant sequoias for damage from California’s three-year drought.
Instead of stressed-out plants, he found young trees that looked pretty happy, he said. But at some point, he glanced upward and saw something startling.
“The foliage had died back on a much larger sequoia above me,” said Stephenson, a sequoia authority who works for the U.S. Geological Survey. “It’s not happening to all of them, but there is a subset of bigger trees showing stress. It makes sense, but it surprised me a little.”
The brown needles on a 3,000-year-old tree are a smart response to drought, Stephenson said. The trees dump their old foliage when they get drought-stressed and focus on new growth.
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But the bigger takeaway: Nature may hold a few surprises as the climate warms this century for giant sequoias and other plants and animals. California’s intense drought is giving scientists a valuable sneak peek.
Though the older sequoias have survived past climate warmups, no one knows how these natural treasures will hold up this time. Climate warming is moving faster than it has in the past, scientists say. Some researchers worry that the Sierra will lose some trees that were alive before the time of Christ. They acknowledge it’s possible the giant sequoia will not survive as a species.
For other parts of nature, the expectation is that many types of plants and animals will retreat to higher ground or move farther north to find habitat.
Some species may tolerate the change and remain in the biological nooks and crannies of the 400-mile-long Sierra Nevada. Some probably will not survive.
Jessica Blois, an assistant professor at the University of California at Merced, teaches paleoecology and has studied the response of California mammals to past climate change. She said the biologically diverse Sierra Nevada should help animals adjust.
“There’s a great complexity in the Sierra, unlike many places,” she said.
But for mature, long-lived Western trees, such as the giant sequoia, bristlecone pine, redwoods or foxtail pine, there won’t be any quick shifting to react. Individual trees that live thousands of years in one place will have to simply hunker down and ride it out.
They’ve done it before. Sequoias, the largest trees on the planet, have lived through a centuries-long dry spell in the last 1,200 years, according to the Tree Ring Laboratory at the University of Arizona.
The largest tree in the world — General Sherman Tree in Sequoia National Park’s Giant Forest — lived through that dry time. It is 36.5 feet thick at its base and 275 feet tall. There are taller trees, but none has more girth — more than 52,000 cubic feet of wood.
Here’s another way to consider the Sherman’s size: In the 1970s, a fallen limb from the Sherman Tree was measured at more than 150 feet long and 7 feet in diameter. At the time, it was considered larger than any tree east of the Mississippi River.
The Sherman and the other giants live in Earth’s last 65 or 70 natural groves. All are in the Sierra Nevada, and the highest concentration of them is in Sequoia-Kings Canyon National Parks and the Giant Sequoia National Monument.
Perched above the San Joaquin Valley, they already face challenges from plumes of Valley air pollution that deposit chemicals in the soil and vegetation. The giants also are limited to a narrow elevation belt between 4,500 feet and 7,500 feet. They require a lot of water and well-drained soil because they have a wide, shallow root system.
Drought is no friend of the giant sequoia, according to researchers. The tree has managed to adapt, not only to warmer times but to the increased number of forest fires that follow. Giant sequoias have a spongy, cinnamon-colored bark that can be 3 feet thick, protecting them from insect infestation and, more importantly, fire.
“The giant sequoia has a phenomenal resistance to fire,” said professor Stephen Hart, a UC Merced researcher who studies biochemical processes.
Frequent and low-intensity fire actually is a benefit to the big trees, setting the stage for giant sequoia seeds to grow, researchers said. Fires tend to eliminate brush and small trees that might create a bigger, more damaging fire.
But giant sequoia seedlings and saplings easily burn up in big fires — and there are likely to be some large wildfires as the climate warms.
For the mature giants, the soil may hold clues to its ability to survive dry times, Hart said. Over time, the giants accumulate a lot of organic matter — decomposing leaves, bark, cones and other forest litter.
“Think of potting soil,” he said. “If you have more organic matter, it helps retain water. So there is more potential for storing moisture from snow or rain.”
Hart, who has studied the soils around giant sequoias in Yosemite National Park groves, said he does not know if the species will survive the warming climate this time. Will they be able to disperse to other areas over time? No one knows that answer.
USGS ecologist Stephenson said he pays attention to seedlings, adding that if the young trees reach 1 or 2 years of age, they tolerate drought pretty well.
He also talks about a particularly dry decade researchers discovered in the late 1200s. The mature giant sequoias survived that intense drought. But no one knows what the forest looked like after that decade.
That’s why scientists cannot let the three-year drought pass without looking closely at the Sierra — both on the ground and through satellite imagery, he said.
“This is a rare opportunity to see what the forest looks like in a warmer, drier time,” he said.