Trees share their metabolic spoils

If you go down in the woods today, you might catch a clutch of trees sharing a carbon picnic. That's the conclusion of a study that involved carbon labelling five Norwegian Spruce trees in a forest over a five year period.

Tamir Klein and his colleagues at the University of Basel in Switzerland, writing in Science, used tall cranes and a system of pipes to administer carbon dioxide that had been depleted for a heavier form of carbon called carbon-13.

Normally, carbon dioxide in the atmosphere consists of a mixture of both carbon-12 and carbon-13.

Dosing the trees with this lighter form of carbon dioxide gave them a characteristic carbon signature that the team were then able to follow through the trees' tissues and also down into their roots.

The researchers studied the trees they were treating and also other trees growing nearby that were from a range of different species.

There was no evidence that the neighbouring trees were picking up any of the carbon dioxide label via their leaves to any great extent, and understory plants and fungi growing on the forest floor did not show any of the carbon label applied to the spruce trees.

But the roots and stems of the neighbouring trees were strongly labelled with the carbon signature showing that carbon-containing chemicals made by the labelled spruce trees were being traded with other nearby tree species.

The level of labelling was so strong that it suggests that up to 4% of the carbon extracted from the air by a forest - or nearly a quarter of a tonne of carbon per hectare per year - is being exchanged between tree species in their roots.

The scientists think that the "middle-man" involved in the bulk of the nutrient exchange are species of so-called "mycorrhizal" fungi, which form close relationships between the roots of different plant species and supply essential elements like nitrogen and phosphorus in return for sugars. These fungi - and their fruiting bodies - were also heavily labelled with the carbon signature.

The findings are a surprise. Although scientists knew previously that plants exchange some nutrients with underground fungi, and there was some evidence that nearby trees of the same species can occasionally link up and form root grants that allow nutrient exchange, the scale of the carbon exchange detected in this new study is quite staggering.

According to the team, "the observed interspecific carbon transfer among tall trees in our study can become increasingly important for forests under stress conditions (e.g., drought or spring frost) or after disturbance such as wildfire, when divergence in species' responses come into play. The magnitude, direction, and control of these transfer fluxes and their importance are yet to be resolved, and they add a new dimension and level of complexity to known ecosystem processes."