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Trees share carbon: new discovery may be key to saving trees in a warming world

  • A five-year experiment, the first of its kind, shows that tall healthy trees share resources, specifically carbon, with trees of different species.
  • Mycorrhizal networks — the underground filaments of fungi and fine plant roots that have grown together — allow for the exchange of nutrients, water, and carbon with and between plants. However, neither of these underground transportation highways have ever been shown to transfer carbon among trees of different species — until recently.
  • Researchers believe this resource sharing may play a significant role in the survival of forests as they increasingly come under stress from climate change.

A new discovery has forced ecologists to shift how they think about forest systems from hotbeds of competition to cooperative, interdependent structures. A five-year experiment, the first of its kind, shows that tall healthy trees share resources, specifically carbon, with trees of different species. This carbon-trading is driven by collaborative underground networks of mycorrhizal fungi which exist in almost all of the world’s forests. Researchers believe this resource sharing may play a significant role in the survival of forests as they increasingly come under stress from climate change.

“The entire finding was a surprise, my co-author didn’t believe it at first, we thought it was a root sorting error,” said Dr. Tamir Klein of the University of Basel of his results published in Science.

View of the study site at the Swiss canopy crane research station in a mixed forest 12 km southwest of Basel, Switzerland. Photo courtesy of Christian Körner and Tamir Klein.

Ecologists have long understood that trees compete for nutrients and resources. The tallest and oldest trees are the champions, having wrested the necessary resources from their surroundings and neighbors. But the results of the experiment in a temperate forest in Switzerland show that interactions between trees may be more complicated than was believed. Trees, in fact, share resources with their neighbors via underground highways of fine roots and symbiotic fungi. Scientists already knew that mycorrhizal networks — the underground filaments of fungi and fine plant roots that have grown together – are mutually beneficial to both fungi and trees. They exchange nutrients, water, and carbon with and between plants. Fungi allow for increased uptake of nutrients and water from the soil while green plants export sugars created through photosynthesis to fungi and other microbes in the soil. Scientists consider mycorrhiza networks an integral part of forest ecosystems. Neighboring trees of the same species can also exchange carbon through the physical fusion of their roots. However, neither of these underground transportation highways have ever been shown to transfer carbon among trees of different species — until recently.

Professional tree climbers ascending one of the five control unlabelled Picea abies trees for sampling of canopy twigs beyond the reach of the crane jib. Photo courtesy of Christian Körner and Tamir Klein.

In order to track carbon exchange, researchers used a construction crane and a network of thin tubes to flood the crowns of 120-year-old and 40-foot tall spruce trees with Carbon-13, a form of carbon heavier than that found in normal air. Using Carbon-13 allowed them to distinguish it from regular carbon and track the transfer of Carbon-13 as it was photosynthesized in the leaves and transported into twigs, stems, and fine roots of the trees.

Researchers found that the spruce trees transferred the Carbon-13 not only to its own twigs, stems, and roots to support growth as was previously known, but also to other tree species, including neighboring pine, birch and larch trees via mycorrhizal networks. Previously, scientists had no knowledge of the involvement of large trees in carbon transfer because there was no method to expose large trees to Carbon-13 until recently. “The most advanced knowledge was that seedlings can transfer carbon among each other,” said Klein. The finding that tall trees transfer carbon to other healthy tall trees of different species came as such a surprise that the scientists doubted their work.

Researchers used a construction crane to distribute 13C to the tops of spruce trees via tubes. Photo courtesy of University of Basel, research group C. Körner.

To make sure nothing went amiss, the researchers went back into the forest, carefully excavated the fine roots, and followed them through the dirt to the source tree to verify that the Carbon-13 had indeed traveled from the labeled spruce to neighboring trees of different species.

This is significant says Klein. “It’s a shift in how we look at trees in a forest. Neighboring trees can actually share fixed carbon and not solely compete with each other.” Trees in a ‘guild’ – those connected by resource transporting mycorrhizal networks — seem to balance the surplus carbon among themselves. While this experiment, conducted in a temperate forest, was the first evidence of carbon trading among different tree species, Klein said, “in principle it can happen in any forest where there is mycorrhizal fungi, that’s most forests on earth.”

Tree mortality connected to climate change is a major global issue today. While direct causes of death may include everything from drought, spring frost, wildfires or disease, all of these impacts are exacerbated by rising global temperatures due to burning fossil fuels. Different trees respond differently to stresses. But when trees are connected to these mycorrhizal networks, “they might get just enough surplus carbon [from their neighbors] to make the difference between life and death under stress,” explained Klein.

Sampling of verified fine roots from overlapping root spheres. Roots were excavated down to 5 cm depth and traced to the trunk of origin. The red arrow denotes the sampling point of fine roots belonging to labelled Picea abies (front) and belonging to neighbouring, unlabelled Pinus sylvestris (rear). Photo courtesy of Christian Körner and Tamir Klein.

While it is not yet known the magnitude or specifics of how this carbon sharing works, it adds a new level of complexity to how forest ecosystems function. Scientists also believe it could become an important factor in better managing forests for resilience and survival in an age of climate change.

 

Citation:

Klein, T., Siegwolf, R. T., & Körner, C. (2016). Belowground carbon trade among tall trees in a temperate forest. Science, 352(6283), 342-344.