BLUE RIVER — Most of us slow down as we get older. Not trees.
In fact, according to findings announced last week in the scientific journal Nature, the vast majority of trees actually increase their growth rates as they age, pulling more carbon out of the atmosphere and providing a hedge against global warming.
That conclusion goes against the grain of conventional wisdom, which has long held that young trees do a better job of packing on wood and locking up carbon, an argument that has been used in the past to justify shorter harvest rotations on timberland.
The latest research turns that argument on its head, painting a picture of moss-bearded ancients that keep adding volume at a faster and faster clip as the decades and centuries pass.
“We know that larger, older forests store more carbon, but the basic idea is they stop growing,” said Mark Harmon of the Oregon State University College of Forestry, one of three OSU researchers listed as co-authors on the paper published Wednesday in Nature’s online edition.
“What we found here, actually, is that large old trees not only store more carbon but they also take up more carbon.”
The paper was the work of 38 researchers who reviewed records from studies on six continents. The other OSU scientists were Rob Pabst of the College of Forestry and Duncan Thomas of the College of Agricultural Sciences.
What made this study unique was the use of long-term records of tree growth. The research involved repeated measurements of 673,000 individual trees, some going back 80 years and more.
That includes some 40-year-old plots at the H.J. Andrews Experimental Forest, located in the Cascade Mountains about 50 miles east of Eugene near the town of Blue River. The 16,000-acre site is jointly administered by OSU, the Forest Service’s Pacific Northwest Research Station and the Willamette National Forest.
One of those plots, known as Reference Stand 2, is a good example. It’s a 2½-acre patch of old-growth forest on a steep hillside dominated by soaring, 450-year-old Douglas firs mixed with hemlock, Pacific yew and western red cedar, with an understory of salal, Oregon grape, vine maple and rhododendron.
When the plot was laid out in 1971, the entire site was staked out on a 10-meter grid. Any tree thicker than a baseball bat was fitted with a numbered aluminum tag. Each tree’s location was marked on a map, and its diameter at breast height was noted in the records.
Every five years or so, a crew would come back and measure each tree again, carefully recording the data to determine growth and mortality rates.
“The longer the plots go, the better the data is,” said John Moreau, an OSU research technician who’s been working at Andrews since 1974. “It’s amazing what you can learn from these old studies.”
There are 38 such sites in and around the Andrews Experimental Forest, part of a larger regional network of reference stands scattered across western Oregon and Washington. Some of them were established nearly a century ago by the U.S. Forest Service and have been continually monitored ever since.
Similar long-term studies have been undertaken at other sites all over the world, and the authors of the Nature article pulled all that data together and analyzed it to reach their conclusions. Among other things, they found that growth rates continued to accelerate with age in almost all of the trees studied — 97 percent of 403 tropical and temperate zone species.
Harmon said he wasn’t completely surprised by the results.
“I would have been surprised if tree growth rates slowed down as they grew older,” he said.
“I thought they might stay about the same, but we found that most grew faster as they grew older.”
Perhaps the most gratifying part of the outcome from a scientist’s perspective is the consistency of the findings across multiple species, Harmon noted.
“It does sort of run counter to what people often think, but it was also a very clear result,” he said. “It was not ambiguous.”
Harmon pointed out that the study applies only to growth rates of individual trees and cautioned that calculating growth for tree stands and entire forests is subject to lots of variables, including the fact that some trees will die over time.
But the new information can now be used to refine computer models used to study a host of complex problems, including carbon sequestration to moderate global warming.
“This really gives you a different way to think about that,” Harmon said.
“To get it out of the atmosphere, we can put it into trees. The question is, do we put it in large trees or small trees?”
Mike Cloughesy, director of forestry for the Oregon Forest Resources Institute, welcomed the study and said its conclusions reaffirm most contemporary thinking about the importance of protecting old-growth trees.
For the most part, he said, the timber industry has adjusted to the fact that old-growth stands on public lands are off-limits to logging and has retooled lumber mills to process smaller logs. These days, the greatest threat to big trees may be the danger of fire sweeping through overstocked forests.
“A lot of old growth is still being lost, but it’s not being lost to chain saws,” Cloughesy said. “It’s being lost to wildfire.”
What’s needed now, he added, is a policy that encourages the harvest of smaller trees to promote forest health while creating jobs in the woods and increasing the flow of timber to the mills.
“Where we can, we should do some active management on those stands to protect those big trees,” Cloughesy said.
“We need to be doing some thinning, some fuels reduction, maybe even taking out some of those big trees so others can continue to grow.”