Communism in trees goes underground.

Although plants don't plot to overthrow capitalist regimes, their actions demonstrate a clear communist bent. At least some species of trees seem to give according to their abilities and receive according to their needs, report Suzanne W. Simard of the Ministry of Forests in Kamloops, British Columbia, and her colleagues in the Aug. 7 Nature.

The team showed that some trees give their neighbors carbon that they have captured from the atmosphere. An underground network of fungi collaborates in transporting the goods.

Scientists had previously found that carbon flows between plants, but they had not established whether individual plants show any overall profit or loss. To address that question, the researchers provided adjacent trees with one of two brands of carbon dioxide, each labeled with an isotope of carbon. By examining how much of the different isotopes ended up in each tree, the team could measure net transfer of the element. Birch trees, for example, gave fir trees more carbon than they got in return, the researchers observed.

The scientists discovered that shade enhances a tree's ability to receive. Because plants require energy from the sun to grab carbon dioxide from the air, they become carbon-starved when light is scarce. Birches subsidized firs that were shaded by heavy cloth canopies even more generously than firs in sunnier conditions, the team reported. Birches and firs grow together naturally, so the findings may have implications for life in the forest.

"A plant grows in the shade for long periods early in its life," says David Read of the University of Sheffield in the United Kingdom. "This study provides an explanation for how it gets what it needs."

The carbon appears to travel via a subterranean web formed by a common group of fungi. The network envelops the roots of both types of trees. Much more carbon travels between the fir and the birch, which share fungi, than between either of these trees and cedars, which associate with a different fungal group.

The fungi normally receive carbon--in the form of sugar--from the trees. In return, they dispense some of the nitrogen and phosphorous they scavenge from the soil. The new results suggest that fungi can donate carbon as well.

"We don't yet know how the second tree tricks the fungus into giving up carbon," says Read. "There's nothing in it for the fungus as far as we can see--at least in the short term." This scheme, however, may help the second tree survive, so the fungus may be "planning for its next meal," Read conjectures.

The fungi "even out" the carbon supply in the community, says team member David A. Perry of Oregon State University in Corvallis. "When we look above ground, we see a bunch of individuals. When we look below ground and see all the connections, that individualism becomes much less clear." This view challenges current ecosystem models, which assume that plants constantly compete with one another for resources.

"Perhaps cooperation increases the fitness of the community," says Perry. Different plants specialize, performing better with different amounts of light and moisture, he explains. If these organisms share the fruits of their labor, community members receive what they need in a wide variety of situations.