With Deaths of Forests, a Loss of Key Climate Protectors

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In every forest, carbon is constantly being absorbed as trees and other organisms grow, then released as they die or go dormant. These carbon fluxes, as they are called, vary through the day. They vary with seasons, with climate and weather extremes, with the health of the forests and with many other factors. Across the world, scientists are struggling to track and understand this ebb and flow.

A 100-foot tower stands in the middle of the Harvard Forest, studded with instruments. Put up in 1989, it was the first permanent tower of its kind in the world, built to help track the carbon fluxes. Now hundreds of them dot the planet.

Meticulous measurements over the decades have established that the Harvest Forest is gaining weight, roughly two tons per acre per year, on average. It is characteristic of a type of forest that is playing a big role in limiting the damage from human carbon emissions: a recovering forest.

Not so long ago, the land was not a forest at all. Close to where the men were working stood an old stone fence, a telltale sign of the land’s history.

“When the European colonists came to America, they saw trees, and they wanted fields and pastures,” explained J. William Munger, a Harvard research fellow who was supervising the measurements. So the colonists chopped down the original forest and built farmhouses, barns, paddocks and sturdy stone fences.

By the mid-19th century, the Erie Canal and the railroads had opened the interior of the country, and farmers plowing the thin, stony soils of New England could not compete with produce from the rich fields of the Midwest. So the old fields were abandoned, and trees have returned.

Today, the re-growing forests of the Eastern United States are among the most important carbon sponges in the world. In the Harvard Forest, the rate of carbon storage accelerated about a decade ago. As in much of the world, the temperature is warming there — by an average of 2.3 degrees Fahrenheit in the last 40 years — and that has led to longer growing seasons, benefiting this particular forest more than hurting it, at least so far.

“We’re actually seeing that the leaves are falling off the trees later in the fall,” Mr. Werden said.

Scientists say that something similar may be happening in other forests, particularly in cold northern regions that are warming rapidly. In some places, the higher temperatures could aid tree growth or cause forests to expand into zones previously occupied by grasslands or tundra, storing more carbon.

Forests are re-growing on abandoned agricultural land across vast reaches of Europe and Russia. China, trying to slow the advance of a desert, has planted nearly 100 million acres of trees, and those forests, too, are absorbing carbon.

But, as a strategy for managing carbon emissions, these recovering forests have one big limitation: the planet simply does not have room for many more of them. To expand them significantly would require taking more farmland out of production, an unlikely prospect in a world where food demand and prices are rising.

“We’re basically running out of land,” Dr. Kurz said.

Even in forests that are relatively healthy now, like those of New England, climate risks are coming into focus. For instance, invasive insects that used to be killed off by cold winters are expected to spread north more readily as the temperature warms, attacking trees.

The Harvard Forest has already been invaded by an insect called the woolly adelgid that kills hemlock trees, and managers there fear a large die-off in coming years.

Wildfires and Bugs

Stripping the bark of a tree with a hatchet, Diana L. Six, a University of Montana insect scientist, pointed out the telltale signs of infestation by pine beetles: channels drilled by the creatures as they chewed their way through the juicy part of the tree.

The tree she was pointing out was already dead. Its needles, which should have been deep green, displayed the sickly red that has become so commonplace in the mountainous West. Because the beetles had cut off the tree’s nutrients, the chlorophyll that made the needles green was breaking down, leaving only reddish compounds.

Pine beetles are a natural part of the life cycle in Western forests, but this outbreak, under way for more than a decade in some areas, is by far the most extensive ever recorded. Scientists say winter temperatures used to fall to 40 degrees below zero in the mountains every few years, killing off many beetles. “It just doesn’t happen anymore,” said a leading climate scientist from the University of Montana, Steven W. Running, who was surveying the scene with Dr. Six one recent day.

As the climate has warmed, various beetle species have marauded across the landscape, from Arizona to Alaska. The situation is worst in British Columbia, which has lost millions of trees across an area the size of Wisconsin.