Walking through a forest is like sailing past an iceberg: you’re missing a big part of the picture. Trees hide nearly half their biomass in a vast tangle of roots, which in turn are usually woven into an even bigger web made of fungus. This benign fungal infection is a classic example of symbiosis, a relationship in which both organisms benefit. Trees can make new tissue from sunlight, water, and air, but their roots can’t extract enough vital nutrients such as nitrogen and phosphorus from the soil. Fungi produce digestive enzymes that free these compounds, but they can’t draw carbon from the air. So the organisms collaborate, the fungus taking carbon from the roots and pumping in soil nutrients in return.

Scientists have known about this relationship for decades, but recent experiments are revealing unsuspected complexity. One was conducted by Suzanne Simard, a forest ecologist at the British Columbia Ministry of Forests in Kamloops. Simard had been taught to view trees as rugged, competitive individuals, each trying to struggle above its neighbors to get as much light as possible. But she couldn’t help being struck by the subterranean partnerships trees form with fungi--and that the same fungal threads often connected to other trees, even trees of other species. I was always perplexed by the fact that we could explain only 10 to 20 percent of the variation in how these species grew--their height, their density--by competition, says Simard.

Simard’s experiment shows just how interlinked the trees are. She planted seedlings of Douglas fir and paper birch, letting them become infected by local fungi. After a year Simard returned and put tents over some of the trees. A Douglas fir trapped in the shade would photosynthesize less, while a paper birch in the sunlight would continue to draw its usual amount of carbon from the air.

After six weeks Simard began to track what was happening to the carbon the trees were capturing. She put sealed plastic bags over the trees and injected carbon dioxide loaded with different carbon isotopes into the bags. (Isotopes are atoms of a given element that have varying numbers of neutrons.) After nine more days Simard uprooted the trees, ground them into a paste, extracted the isotopes, and measured how much of each the trees had.

She discovered that the isotopes absorbed by one tree often ended up in another, and that shaded trees took far more carbon from their sun- drenched neighbors than they gave. This happened even if it meant that carbon absorbed by a paper birch traveled not to another birch but to a Douglas fir. Simard could only conclude that the fungus was managing the trees, extracting carbon from healthy ones and pumping it to shaded ones, regardless of species. The fungus gave shaded trees 6 percent or more of their carbon, an amount that can ultimately make the difference between being able to produce seeds and being barren.

Simard’s results force a fresh look at some conventional notions in biology. How, for example, can the standard view of evolution--an every- organism-for-itself scramble for resources--be squared with trees that surrender precious carbon to trees from another species? For one thing, Simard suggests, this arrangement aids the fungus: There’s definitely something in it for the fungus if the trees are doing well. And perhaps the trees themselves can evolve only in a partnership. The survival of a group of plants may depend on an individual and its neighbors as well. From a strictly evolutionary perspective it may not make sense, but from an ecological one it does.

That perspective should give pause to foresters. In many forests Douglas fir is the preferred species and paper birch--a fast-growing tree that can shade the slower-growing firs for decades--is considered a weed. But Simard says the birches may be nurturing the firs. These species that we think of as weeds are serving as critical links, and once we sever these links, we affect the stability of those ecosystems. Our practices are still based on the notion that forests act like gardens, and we should weed out what we don’t want. But forests are far more complex than that, and we need to maintain this diversity.