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To Each (Tree) According to Its Need

Although trees stand alone, their roots keep in touch through an underground network of fungal hairs. Now, an ecological study reveals that this fungal network can redistribute nutrients in the forest, shifting them from sun-soaked trees to shaded ones--even if they belong to different species. The finding, which will be published in tomorrow's issue of Nature, could shake up ideas about plant competition and, the researchers say, reemphasizes the value of forest diversity.

The symbiotic fungi that often grow on tree roots have threadlike extensions that collect soil nutrients such as nitrogen and phosphorus and funnel them to the tree; the tree, in turn, benefits the fungi by supplying them with carbon for energy. Because different tree species can share the same kind of fungus, these threads form a network that links many different kinds of trees. Experiments on excised roots and potted plants have shown that these fungal pipelines can also transfer carbon--the energy currency of plants--between plants, but the effect hadn't been seen or measured outside of the lab.

Working in the Kamloops Forest Region of British Columbia, Suzanne Simard of the British Columbia Ministry of Forests and colleagues planted birch, fir, and cedar seedlings in groups of three with various degrees of shading for the firs. To label the carbon made by each seedling, the team briefly enclosed each plant in a plastic bag filled with carbon dioxide containing a specific carbon isotope. The birch and fir seedlings, which had been planted in pairs within half a meter of each other, received different isotopes. The researchers expected that as the plants took up the carbon dioxide and transformed it into carbohydrates, the isotope label would go with it.

After a year, the researchers pulled up some of the seedlings and analyzed the carbon in their tissues to see where it had come from. The isotope labels showed that the birch and fir had exchanged roughly equal amounts of carbon--about 4% of their total carbon intake. But following the second year, fir seedlings had an advantage, netting an average of 6% of their carbon from the birch seedlings. What's more, fir seedlings in deep shade did best, pulling in 10% of their carbon from birch seedlings. The cedar seedlings didn't get any carbon from the birch or fir--as expected, because cedars don't share fungi with the other two species.

The finding that plants in shady spots seem to get a nutritional boost from their better lighted fellows could help explain how young seedlings manage to survive in shady forests, and it could shake up thinking about forest ecology. "It indicates that we should place less emphasis on competition between plants, and more on the distribution of resources within the community," says David Read of the University of Sheffield in the United Kingdom. That cooperation, the researchers say--with "weedy" birch helping out shade-bound fir--highlights the value of a diverse forest to foresters.