New test traces underground forest carbon.

A novel carbon-tracking method offers scientists a way to solve one of the big mysteries of what forests do with their enormous flows of carbon, according to an international research team.

The technique, in which researchers strip a ring of bark off trees, reveals that tree photosynthesis drives much of the flow of carbon dioxide from forest soil, says Peter Hogberg of the Swedish University of Agricultural Sciences in Umea. Most soil-respiration models ignore that factor, he adds.

In the June 14 NATURE, he and his colleagues say that about half the carbon dioxide rising from the soil around pines in northern Sweden comes from the respiration of tree roots and symbiotic fungal partners. The rest presumably comes from the respiration of soil creatures.

The main force controlling the tree-and-fungus share of soil carbon dioxide, the researchers say, is the stream of sugars and other compounds flowing from the leaves to the roots. Most models of respiration in soil have pegged physical conditions, such as soil temperature, as the primary drivers of carbon dioxide release.

Precisely how carbon wafts in and out of huge forests remains a big question for everyone calculating the global carbon cycle and its connection to climate change. It's not too hard to estimate how much carbon plants pull out of the air and use to build their tissues, Hogberg explains. As for what happens underground, "it's the black box," he says.

Underground, tree roots and their fungal partners breathe out carbon dioxide, as do myriad creatures that eat organic debris down there. Just who's doing what, however, gets difficult to measure. For example, digging trenches changes soil dynamics, and bringing roots into the lab for measurements cuts them off from their fungal partners' huge network of filaments. "If you try to tear apart the system, you're looking at artifacts," Hogberg says.

That's why he and his colleagues left the soil in their 900-square-meter test plots untouched but girdled the trees. These cuts severed the tree plumbing that carries food to the roots and their fungal partners. The scientists then collected samples of air above the soil to monitor any changes in carbon emissions.

When the researchers girdled trees early in June, soil respiration started dropping fast and within 7 weeks had sunk to 48 percent of respiration in undisturbed forests. When the researchers girdled trees in August, soil respiration dropped faster, sinking to 44 percent in just 2 weeks.

Forest-carbon specialist John Grace of the University of Edinburgh says he's eager to try the technique in other kinds of forests. "It's so simple, one pinches oneself for not having done it," he says.