Guilds of trees share their sugar

by Scott K. Johnson

Oona Räisänen

We use many words to describe groups of trees: forest, wood, grove, stand, copse… But what about “guild”? (Don’t worry, neither druids nor dryads are about to be invoked.) Woven into the roots of forest trees are mycorrhizae, fungi that are close partners with the trees. The fungi help free up nutrients from the soil, and the trees hook the fungi up with some sugar for their trouble.

Researchers have found that mycorrhizae can actually bind trees into a community by facilitating the transfer of nutrients among them. And experiments with saplings have even shown that sugar can be traded via the “myconet.” Individual trees have always been viewed as separate organisms that compete for light and water, but now it appears that real forests may collaborate in secret ways below ground.

A group of researchers including Tamir Klein, Rolf Siegwolf, and Christian Körner took advantage of a unique experimental setup In a Swiss forest. There, a construction crane houses a carbon dioxide source that runs to five 40-meter-tall Norway spruce trees, where thin porous tubes tied to the branches slowly leak CO₂ for the trees to breathe in.

Part of the long-term experiment here is to monitor how the trees respond to higher CO₂ concentrations. But because the isotopic signature of the carbon in that CO₂ is different from regular air, you can also follow that carbon through the tree like a tracer. And that means you can also find out if that carbon hits the "myconet."

Samples from all over the treated pines show the presence of the experimentally added carbon, which includes less carbon-13 and more carbon-12. Samples from the canopies of the neighboring trees showed that only the nearest tips were catching whiffs of the CO₂—not enough to muddy the experiment.

Samples of the mycorrhizae around the roots of the treated trees also contained an altered isotopic carbon signal. So there’s step one, as expected—the fungi were definitely getting carbon from the tree.
Step two is where this gets really interesting: the roots of neighboring trees contained the same signal despite the fact that they were different species. (Larch, beech, and Scots pine.) Remember, the tops of these trees had isotopic signatures indistinguishable from the rest of the forest. Yet their roots had almost the same signature as the Norway spruce that were getting the carbon treatment.
Near the ground, samples from the trunks of the untreated trees even showed a slight signal. So sugar produced by photosynthesis in the Norway spruce, after being released to the mycorrhizae, was being taken up for use by neighboring trees. (Insert cliché about borrowing a cup of sugar here.)
Looking at the numbers more closely, it appears the transfer went both ways. The isotopic signature in the Norway spruce trees’ roots was weaker than in the rest of the tree, as if the tracer was diluted with other carbon. Part of that can be explained by the presence of older carbon preceding the experiment ending up in the young roots that were sampled.  But it also seems that around 40 percent of the carbon in the young roots was coming from other trees in the guild. Sugar (and possibly some carbon in other forms) wasn’t just flowing away from the treated trees—it was being exchanged.
Scaling up their numbers, the researchers estimated the carbon exchange between trees in a given area at around 4 percent of the total conversion of atmospheric CO₂ into plant stuff. So while trees are making the most of their own food, there could be a surprising amount of sharing going on.

In an article accompanying the paper in Science, Marcel van der Heijden of Switzerland’s Institute for Sustainability Science points out that this experiment will need to be replicated—particularly in other ecosystems. But it raises some pretty interesting questions. For example, van der Heijden writes, “Further work now needs to investigate whether trees benefit from this resource sharing and whether being interconnected through such mycorrhizal fungal networks enhances plant fitness and forest stability over evolutionary time.”

While there are plenty of details to work out, the researchers write that this exchange of carbon among networked guilds of trees adds “a new dimension and level of complexity to known ecosystem processes.”