Last year, Wired magazine published a brief article, "Old Growth Forests Can Actually Contribute to Global Warming." It called for cutting the oldest trees, landfilling "the scrap" and replacing old-growth forests with short-rotation tree farms.
While the author's presentation of ecology and global carbon accounting seemed one part ignorance, one part provocation, his was only a more extreme version of a long-held view of old-growth forests: not only are they decadent, overmature, biological deserts — but they also release more heat-trapping carbon than they absorb.
Old forests, this perspective contends, are a net source of greenhouse gases. So protecting old growth shouldn't be given credit in new carbon markets or international treaties, like the 1992 Kyoto Protocol, that work to slow global warming. Only young forests and reforestation projects, some policymakers have argued, are of value in sopping up carbon dioxide.
"That's plain wrong," says Bill Keeton, an expert on old-growth forests and an associate professor of forest ecology and forestry at the University of Vermont.
A bank of carbon
Keeton's latest research findings, published in the journal Ecological Applications, show that instead of reaching peak carbon storage at middle age, old-growth forests in the northeastern U.S. continue to build biomass for hundreds of years, pulling carbon from the atmosphere faster than they give it up through decomposition.
"Our data, and other recent studies, have shown that not only do old forests store very large amounts of carbon, they can continue to sequester new carbon for centuries," Keeton says. Old-growth forests do have slower rates of uptake than their younger brethren, but they are far from being leaky geriatric ecosystems.
Keeton recently presented data at a forestry conference in Austria showing that old-growth temperate forests around the world have much higher levels of total carbon storage than young or middle-age stands. In the Pacific Northwest, for example, an acre of middle-age forest holds only 47 percent of the carbon that an acre of true old-growth forest there holds. For the Adirondack Mountains of New York it's 67 percent, the Carpathian Mountains in eastern Europe, 64 percent, and northeastern China, only 31 percent.
"Would you rather have a loaded bank account that has been steadily building for hundreds of years," Keeton asks, "or one that grows a little faster, but has only a small amount of money in it?"
His work is part of a growing body of evidence poised to redraw theoretical projections that have dominated forest science for thirty years. These models predicted that many forests would reach a peak accumulation of biomass and carbon at about 170 years — and then tail off. Keeton's real-world data from the Adirondacks and other temperate forests around the world yield a model that shows biomass continuing to build — stored in living trees, dead limbs and carbon-rich soil — for 300 to 400 years and possibly longer.
Natural disturbances, like wind storms or fires, can set this process back for individual stands of trees. "Carbon storage in old-growth is dynamic, it doesn't just build forever and remain static," Keeton says. But the net effect averaged out for many stands is the development of a very large carbon reservoir over time.
As policymakers turn their attention to carbon markets and new agreements to reduce greenhouse gas emissions, a debate rages: "To optimize forest carbon storage, should we emphasize intensive forestry and young vast growing forests," Keeton asks, "or lower-impact forestry, production of durable wood products and structurally complex high-biomass forests?" His findings strongly support the second path.
But that's abstract. It's one thing to value old trees as a carbon sink in the global atmospheric cycle; it's another to stand in the middle of the bark-rich creaking glory of an old-growth forest.
Into the woods
That's why I'm thrashing through two-foot-deep snow, trying to keep up with Keeton and his graduate student Jared Nunery, not far from Ampersand Mountain in the Adirondack High Peaks Wilderness.
A few minutes later, Keeton stands beneath a black wave of leaves and roots. About twenty feet high, this swatch of forest-floor-turned-vertical looks ready to crest over his head. The top edge is covered by a lip of snow and saplings glinting in the sunshine — like arboreal sea-spray. In front of him, the trunk of a downed hemlock tree extends far out into the snow.
Keeton smiles. "This is a great tip-up," he says, "come take a look."
In the shadowy trough, I can see frozen maple leaves, dead hemlock seedlings hanging almost upside-down, veins of strangely bright moss, and several mysterious black holes.
"Some kinds of birds like to nest in here," Keeton says, prodding at one of the holes, "maybe a winter wren." Hiking around to the backside of the downed tree, another wave of roots rises, interlarded with big rocks. Where the tree has ripped out of the ground, a hole remains. "Black bear sometimes make their dens down there in the winter," he says.
Nearby, other huge trees still stand, forming a glowering green shade over a streambed. One large dead tree sticks up like a gnawed pencil, barkless and pocked with woodpecker holes. Upslope, lumpy mounds and patches of saplings appear where older trees have tumbled. The overall effect is a ragged mosaic of sunlight, moldering logs, jumbled branches and openings to the sky.
We're in the midst of an old-growth hemlock-hardwood forest. It's a remnant of the primeval forest that once covered the eastern US, a rare place that has never felt an ax or furrowing plow. While old growth dominated most of New England before European settlement, less than half of one percent of it remains. "There are less than a thousand acres in Vermont," Keeton says. And globally, less than ten percent of remaining temperate forests are old growth, according to one estimate.
But these remaining fragments are of great value, Keeton says. They form high-quality habitat for creatures that depend on mature forests, build pools and structures in streams that some trout and other fish require, harbor rare plants, and make a home for lichens and mosses and insects found nowhere else.
Cutting for conservation
And these old-growth forests may have a lot to teach us about how to harvest trees too. For nearly a decade, Keeton has been studying old-growth forests to develop ideas of how to better manage woodlots and commercial timberland (see associated video story above.) His experimental techniques, what he calls "structural complexity enhancement," mimic many of the characteristics of true old-growth forests.
Six years ago, in experimental plots on the west side of Mt Mansfield and at UVM's Jericho Research Forest, Keeton had a team of loggers push some trees over with a giant machine called a feller buncher, leaving artificial tip-ups. His plan called for many of the biggest trees to be left behind instead of harvested. Some of the healthiest middle-aged trees got a boost of sunlight by a "crown release" in which surrounding trees were cut down, to let the target tree grow more quickly into the (now rare) largest category of trees — up to three-and-a-half-feet across. With chainsaws, the team girdled some trees, allowing them to die slowly, while still standing, to form bird-friendly snags. And, yes, some trees were cut and trucked to a sawmill.
This approach is "just one tool in a forester's toolbox," Keeton's quick to point out. But, so far, his technique looks a very sharp tool for conservation-minded landowners.
In several of his plots, Keeton's team was able harvest commercial timber with a modest profit and enhance wildlife habitat for several indicator species, like red-backed salamanders and some sensitive herbaceous plants. Keeton's computer simulations, recently published in the journal Forest Ecology and Management, predict that over the next fifty years the plots will redevelop more than 90 percent of the carbon and biomass that would have developed if the forest remained uncut. "That's much higher than a conventional forestry prescription," he says.
And other landowners, not seeking a profit, say the Nature Conservancy, could apply Keeton's techniques to enhance carbon storage and accelerate the development of old forest characteristics in younger forests.
A fine mess
Keeton rests his mitten on the huge horizontal trunk of the hemlock, thrown down by a recent windstorm. To some old-fashioned foresters this might look like a wasted saw log. But to Keeton it looks like a sign of well-being. "In a lot of ways, a messy forest is a healthy forest," he says.
"This place is really spectacular. It's intrinsically beautiful and complex," he says. "But it also has a lot to teach us about how to do good forestry. And we can also look at these old trees — living, dead, returning to the soil — as a huge bank of stored carbon. That's really important too."