Forests, Just Like the Ones Mother Nature Used to Make

If you build it, they will come.

These words, forest ecologist Bill Keeton tells me, are more or less the guiding principle behind his efforts to build not a field, but a forest of dreams: an old-growth forest that supports wildlife and stores carbon.

In the northeastern United States, the “old-growth” designation begins around 150 years old. These senior stands purify water and shelter a wide variety of plants and animals as the wear and tear to their aged limbs and roots provide microhabitats not found elsewhere.

Today, forests cover about 80 percent of the Northeast. That sounds pretty good until you hear that of all the old-growth forests that dominated the region prior to European settlement, less than 1 percent remains. “That is the type of ecosystem conditions most organisms were accustomed to,” says David Foster, director of the Harvard Forest. “And over the last 400 years, we’ve pretty effectively eliminated those kinds of habitats.”

Now, land managers across the Northeast are trying to give old-growth forests a boost. Instead of simply waiting for them to grow up, Keeton, a professor of forest ecology at the University of Vermont, has been taking a more hands-on approach, one that tries to speed up a forest’s maturation process by using silvicultural techniques called structural complexity enhancement (SCE).

Hiking with Keeton through UVM’s 485-acre Jericho Research Forest on a warm day in April, I saw a typical northern hardwood forest—one with its roots in Colonial times, when most of the Northeast’s landscape was cleared for agriculture. Farming culture eventually shifted westward during the mid-19th century, and since then the region’s forests have followed the predictable stages of succession after a major disturbance.

“Everything you see here, 150 years ago, was farmland,” Keeton told me, pointing to the hemlocks, beeches, and sugar maples surrounding us. We were standing in one of the control sections, which, for a point of comparison, the researchers have left untouched since the SCE study began 17 years ago. Keeton tests out his methods on several five-acre plots elsewhere in the Jericho forest, as well as on some plots that grow on the side of nearby Mount Mansfield.

The trees I saw ranged from about 70 to 90 years old, still middle-aged by their species’ standards and a couple of human generations shy of achieving the late-successional sweet spot. “Structurally simple, homogeneous, uniform . . . these are all terms you might use to describe the trees here,” Keeton said.

By contrast, more mature forests are complex, with various tree heights and widths, plenty of woody debris, understory vegetation creeping up into the vertical space, and interruptions in the canopy that allow sunlight to shine through. And while there’s not much foresters can do about age, structure—as “structural complexity enhancement” implies—is where Keeton’s techniques comes in.

Existing sustainable forestry techniques include cutting down single trees of multiple ages throughout a timberland, or logging small clusters from a larger managed tract. These help maintain forest structure and function to a certain extent, but Keeton takes it to the next level.

After a short walk, he and I entered a section treated with SCE. The differences from the control were immediately apparent. A large dirt pile created by a fallen tree, called a tip-up mound, greeted us. Overhead, we glimpsed a patch of sky through a clearing in the canopy. Nearby, several girthy trees stretched upward, while downed branches and trees littered the forest floor.

When nature has time to take its course, natural disturbances like windstorms eventually create these scenes. Here, the tip-up mound, hole in the canopy, and other “natural” features are all man-made. Foresters pull trees over to form tip-up mounds. Girdling, or the removal of a strip of bark from the circumference of a tree trunk, results in snags, or standing dead trees. Certain trees are also strategically harvested to open up the canopy near larger, faster-growing trees.

Keeton has spent most of his career studying old-growth forests. To engineer his own, he made a list of desired characteristics and devised a matching silvicultural treatment for each one—the idea being that the way a habitat is put together is what makes it tick. “If we can understand the architecture of an ecosystem, we can understand a lot about its function,” he says.

For instance, species like yellow birch grow best atop tip-up mounds, their leggy roots performing a slow-motion split into the soil as their perches rot. Ground-nesting birds like winter wrens like to nest in the overturned roots, and frogs and eastern red-backed salamanders enjoy the pools of rainwater that often form in the hole at the tree’s base. Cavity-nesting birds favor standing snags, and decaying “nurse logs” provide ideal habitat for seedlings and fodder for fungi, which are important decomposers and drivers of plant diversity.

The biological implications were palpable in the SCE plot in the sheer amount of green and number of limbs and logs to duck under and hop over. To put it in cruder terms, finding a sheltered spot to, ahem, use the facilities was a lot easier there than in the control section.

As all this biomass accumulates, so does the forest’s carbon stock. In a study published this spring in the journal Ecosphere, Keeton and former UVM graduate student Sarah Ford found that SCE-managed units store significantly more carbon than those treated with traditional techniques—a winsome combination in the dawning age of carbon markets and a potential tool in the global fight against climate change.

Using modeling to simulate growth based on pre-treatment data, Keeton and Ford found that after a decade, the SCE plot stored just 16 percent less carbon than an untouched forest—while a conventionally managed swath contained about 45 percent less.

The world’s forests offset approximately 30 percent of global carbon dioxide emissions. As the temperature climbs, interest in increasing carbon sinks is also on the rise. Emerging carbon markets, whether voluntary or mandated, pay forest owners whose land sequesters carbon. The more it absorbs and stores, the higher the sum.

Foster, of the Harvard Forest, says SCE is a promising option for certain parcels of land. “The advantage of this approach is that it gives more immediate results and allows landowners who are interested in diversifying their landscape new options,” he says. But he also stresses that it’s important to balance this type of active management with large-scale preservation.

Foster and Keeton both collaborated on Wildlands & Woodlands, a regional initiative that calls for conserving 70 percent of New England, or 30 million acres, as forests. They envision 27 million acres of woodlands composed mostly of privately owned, managed land (which could include SCE), along with three million acres of wildland growing naturally.

Advocates of old-growth restoration have been criticized for idolizing a historic habitat that no longer fits into the modern landscape. But Keeton says his blend of sustainable forestry and functional habitat is an attempt to build a resilient ecosystem for a future of environmental uncertainty.

“It’s very important to make it clear that this is not a kind of emotional or historical attempt to go back,” Foster says. “It’s an attempt to go forward.”