Gentle Giants

Who doesn’t love a good mystery? Stand amidst a grove of sequoias, the largest trees on Earth, and you’ll notice the shade, the peace, and the contemplative silence of other visitors. But you may also notice another novelty: the confounding inability to wrap your mind around the scale and age of these 20-story-tall plants.

Sequoias fascinate us simply because as living things, they surpass our understanding in more ways than almost anything else,” says Bill Tweed, the former chief naturalist of Sequoia and Kings Canyon National Parks and author of Challenge of the Big Trees. “They grow so much larger than we do. They live so much longer than we do. They are so much more resilient than we are. They just appeal to human nature in a very fundamental way.”

Over centuries, these trees have weathered blizzards and droughts, thunderstorms and forest fires. Despite their longevity, however, giant sequoias will encounter novel challenges this century. Climate change will present a number of new threats, as fires grow more severe and winter snowpack diminishes. Scientists are working to understand how this mysterious species will respond, but many are concerned that the groves won’t be able to adapt quickly enough.

“It’s highly probable we’ll see lots of tree die-off,” says Nate Stephenson, an ecologist at U.S. Geological Survey’s Sequoia and Kings Canyon Field Station. “Animals can migrate by walking or flying. How do trees migrate? Well, they die in one place and have to reestablish in another.”

Giant sequoias currently grow in a narrow belt of the Sierra Nevada in nearly 80 groves, many of which are protected by Yosemite, Sequoia, and Kings Canyon National Parks. These giants top 300 feet in height, live more than 3,000 years, and, according to recent research, put on a greater mass of wood each year.

Already, the tree species that surround sequoia groves are showing signs of stress under climatic changes. Research led by Stephenson, which evaluated 18,000 trees over about 20 years, found that the firs and pines of the Sierra Nevada have been dying at higher rates—and that the increase in mortality is linked to rising temperatures.

Scientists believe that if temperatures rise as much as projected in California—between three and nine degrees over the next century—sequoias will be threatened, and seedlings would struggle to establish themselves at all.

Young sequoias thrive in environments that are moist but see frequent, mild ground fires. One projected effect of global warming is that the mountains’ deep snowpack—a critical reservoir for these giant trees—will melt sooner in the spring, meaning that the groves will need to subsist on less water for longer. A drought of only two years can kill new seedlings.

Forest fire patterns also could change. Already, a 100-year history of fire suppression has allowed small plants to grow rampant in American forests, acting as kindling for an increasing number of large, destructive blazes. Mature sequoias can bounce back from fires, even when more than 95 percent of their canopies are scorched; seedlings, however, are wiped out. Under prolonged fire conditions, it could be difficult for any seedlings to grow successfully.

Much is still unknown about how mature sequoias will react in the face of environmental changes—and when. Researchers from Humboldt State University and the University of California–Berkeley are looking for answers with a landmark study of coastal redwoods and sequoias. The team surveyed 16 plots in state and national parks, recorded baseline data on the trees, and established climate-monitoring stations in canopies.

Todd Dawson, a professor at Berkeley and a leading scientist on the study, hopes that the data collected will help conservation organizations, national parks, and legislators make management decisions to protect the species in the future. “It takes a lot of resources and energy to maintain such a massive organism,” says Dawson. “If you change the environment abruptly or in the wrong way, these really big organisms seem to be affected first. That’s why we think these trees could be a pretty important barometer for monitoring climate changes in California.”

Other scientists, such as Stephenson, believe that mature individuals might be more resilient than pines and firs because they have survived nasty droughts and spent millennia digging their roots deep into cracks in the granite. Over their 100-million-year history, they certainly have endured a lot. Still, they have not faced the speed of change the 21st century brings.

But the Park Service is taking action. In Sequoia and Kings Canyon National Parks, fire ecologists perform prescribed burns to prevent massive forest fires and to create the bare, mineral-rich soil in which seedlings thrive. This summer, Yosemite is launching a $20-million restoration of Mariposa Grove, which will remove a parking lot, gift shop, and road so workers can restore soil and ensure that water flows through the area more naturally.

Of course, no one knows precisely what conditions this new century will bring, and it’s likely these trees still have a thing or two to teach us. “The human mind can become habituated to even the most amazing things over time,” says Tweed. “But the more time you spend with sequoias, the more fascinating and compelling they become.”