From outer space, a new dilemma for old-growth forests
WASHINGTON — A new study using laser pulses shot from satellites has found that the world's tallest forests are those along the Pacific Northwest coast.
Though the findings shouldn't shock anyone who grew up in the region, they offer another indication of how important these ancient trees eventually could become.
The temperate forests of Douglas fir, Western hemlock, redwoods and sequoias that stretch from northern California into British Columbia easily reach an average height of more than 131 feet. That's taller than the boreal forests of northern Canada and Eurasia, tropical rainforests and the broadleaf forests common in much of the United States and Europe. The only forests that come close are in Southeast Asia, along the southern rim of the Himalayas and in Indonesia, Malaysia and Laos.
As scientists try to unravel the mystery of missing carbon, increasing attention is focused on these forests.
From 15 percent to 30 percent of the 7 billion tons of carbon that are released globally every year is unaccounted for, government scientists say. About 3 billion tons remain in the atmosphere, and the oceans absorb 2 billion tons. Vegetation, including the forests, probably absorbs the remaining 1 billion to 2 billion tons, but no one knows for sure how much and where.
Scientists suspect that the forests with the biggest trees store the most carbon, and the Northwest forests are probably among the largest carbon sinks in the world. However, they also say that while slower-growing older trees store more carbon, younger trees also absorb more carbon as they grow rapidly.
That sets up a debate about how forests should be managed, particularly whether older trees should be cut to make way for younger ones or whether they should be protected to store the carbon they contain.
"It's a hot topic," said Elaine Oneil, a research scientist at the University of Washington's School of Forest Resources and the executive director of a consortium that's been studying the issue. "We can't afford a one-size-fits-all solution. We can't lock it all up, and it's not feasible to cut it all for two-by-fours."
Ongoing studies using the satellites and lasers may provide valuable information on how fast the forests are growing and how much carbon they store.
"All of the remote sensing is providing us with the ability to monitor changes in the environment in a way you might not see on the ground," said Michael Lefsky, an assistant professor in the department of forest, rangeland and watershed stewardship at Colorado State University. "We are expecting under global warming that the productivity of the forests will change."
Lefsky used data from a laser technology called LIDAR that's capable of "capturing vertical slices" of surface features on Earth from satellites. It's the same technology that geologists are using to map earthquake faults in western Washington state.
With the help of computers, Lefsky put together a global forest height map based on data from 250 million laser pulses collected during a seven-year period.
LIDAR measures the height of forest canopies by shooting laser pulses and measuring how much longer it takes for them to bounce back from the surface than from the top of the forest canopy. The pulses can penetrate through the canopy to the ground.
"It's like an echo," said Lefsky, whose findings were published in the journal Geophysical Research Letters.
Overall, LIDAR offered direct measurements of only 2.4 percent of the Earth's forested surfaces.
"This is really just a first draft and it will certainly be refined in the future," he said.
Lefsky, who previously had done forest research at Oregon State University, said he wasn't surprised that the temperate conifer forests of the Northwest coast had the tallest canopies. While the Northwest forests include the world's tallest trees — redwoods and sequoias — they represent only a small fraction of the region's timberlands, he said, but there are thousands of acres of other tall trees.
In contrast to the Northwest stands, the boreal forests of mostly spruce, fir, pine and larch have canopies that are typically less than 66 feet tall. Relatively undisturbed tropical rain forests have canopies of 82 feet, and the broadleaf forests of oak, beech and birch in much of the U.S. and Europe have roughly the same canopy height.
Trees absorb carbon dioxide, a greenhouse gas, and convert it using water into sugar and oxygen. Much of the sugar becomes cellulose, the key ingredient in wood. From 45 percent to 50 percent of a tree's wood is carbon-based.
The trees and soil in national forests in Washington state, Oregon and southeast Alaska store 10.8 billion tons of carbon, according to a Wilderness Society analysis of U.S. Forest Service data earlier this year.
The analysis also found that of the 120 national forests, the 10 with the highest carbon density were in Washington, Oregon and southeast Alaska.
Oneil's group, the Consortium for Research on Renewable Industrial Materials, suggests that rather than leaving all the tall trees in place, where they could be susceptible to bugs and fires, they be cut and used for wood products such as building materials.
The carbon in those wood products would be stored permanently and their use would reduce the need to manufacture cement and steel, a process that produces greenhouse gases. In addition, the leftovers from milling the logs, such as chips and sawdust, can be used for everything from bark mulch to biofuel for power plants.
Carbon absorption by trees in the Northwest slows when they reach 30 to 70 years of age, Oneil said. By no means, Oneil said, is her group arguing that all the old-growth forests in the Northwest be cut. The stands have other important uses, such as providing habitat for wildlife and recreational opportunities, she said. However, she suggested that the forests, particularly on private lands, could be managed to absorb even more carbon.
"If you don't pick the carrots, you can't plant the next crop," she said.
Environmentalists said cutting the tall forests wasn't the answer, because that could release up to 60 percent of the carbon that was stored in trees and the soil.
"We need to preserve the old growth for existing (carbon) storage," said Mike Anderson, a senior resource analyst with The Wilderness Society in Seattle. Anderson didn't rule out using private lands to increase carbon storage.