Trees Operate A Below-Ground Carbon Trading Network

Norway spruce are such community-minded trees, they share carbon even with neighbors of other species. Aleksander Bolbot/ Shutterstock

Trees in temperate forests share carbon through their root systems, with up to 40 percent of the element in finer roots coming from their neighbors, a new study published in Science revealed. The finding may lead to better ways to protect forests or bolster the storage of soil carbon.

Living in a forest can force trees to fight for nutrients and sunlight with often larger neighbors. However, University of Basel's Dr. Tamir Klein has revealed that in a Swiss mixed tall forest, trees of different species also cooperate beneath the Earth.

Tree roots are known to sometimes graft together, leading to the sharing of resources, but this was thought to be the exception, not the norm. However, Klein noted evidence for the exchange of carbon and other nutrients through networks of mycorrhizal fungi and set out to measure it.

To distinguish the carbon atoms' sources, Klein and colleagues fed the leaves of five 40-meter-high (130-foot-high) Norway spruce (Picea abies) trees in northwest Switzerland with carbon dioxide depleted of carbon-13 for five years. Carbon-13 is a stable but rare isotope of carbon with an extra neutron compared to the more common carbon-12. Five other trees of the same species were used as controls.

When Picea abies were fed low carbon-13 gas, their roots became depleted of the isotope, as did the roots of neighboring trees. Klein et al./Science

The fine roots of the trees fed the carbon-13-depleted gas contained 2.6 percent less carbon-13 than the controls. However, concentrations of carbon-13 were almost as low in the roots of neighboring trees, including those of different species. There was also a small but significant decrease of carbon-13 in the newly laid down rings of neighboring trees at chest height.

The implication is that trees neighboring the targeted spruce got proportionally almost as much carbon from the leaves of these neighboring trees as they did from their own leaves. Trees of different species were acting almost like outcroppings of a single united organism, rather than independent plants.

“The carbon transfer that we observed most likely occurred through common ectomycorrhiza networks, which are very abundant at this site,” the authors wrote. Confirmation came from the carbon-13 depletion of fungal fruit near the spruce. Most ectomycorrhiza fungi species are specific to particular species, but some colonize the roots of multiple species, including Norway spruce and all three neighbors studied here. This would enable them to facilitate transfer between trees of different species.

The carbon also appears to have transferred in all directions, at a rate of 280 kilograms/hectare/year (252 pounds/acre/year), rather than certain trees acting as suppliers for others. This tallies with the observation that all the studied trees were healthy and well established. However, the benefits of such an interchange are not obvious.

Nevertheless, the authors suggest that carbon transfer may be valuable under conditions such as drought or fire, when some species are more affected than others. It is possible that more vulnerable trees survive such outbreaks with a little help from their friends. If so, the discovery may help us protect forests endangered by threats such as climate change.

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