A secret alliance, concealed deep beneath the forest floor, has recently been unearthed by Weizmann Institute scientists. Dr. Tamir Klein and PhD student Ido Rog of the Plant and Environmental Sciences Department have discovered intricate networks of fungi connecting the roots of different tree species with one another. These networks enable the trees to exchange of minerals, nutrients, water and carbon, while funneling carbon to the fungi in return. Although symbiotic relationships between trees and fungi have been known for decades, this is the first time that different tree species have been observed cooperating with one another using an underground network of fungi in their natural setting. The findings add evidence to the trend that is forcing ecologists to shift their thinking about how carbon transfer gets controlled within the forest.
It was Klein's research during his stint as a postdoctoral fellow in Switzerland that laid the groundwork – so to speak – for this current discovery. Klein, however, initially set his sights 40 meters higher, using a construction crane to install measuring instruments above the forest canopy. In these experiments, he discovered that carbon was being transferred between neighboring mature trees of different species, including spruce, pine, birch and larch. But the measurements did not add up: The trees had stored more carbon than he could account for by balancing carbon emission and intake measurements at the canopy.
Rog entered the picture as Klein was undertaking a quest to track down the missing component of the carbon sink. For over four years Rog has been painstakingly digging through rock-laden soil, excavating fine roots and carefully following them through the terrain to identify their parent trees. After installing devices deep underground and employing next-generation sequencing techniques, Rog analyzed over 1000 root tips from 12 individuals of four tree species (spruce, pine, larch and beech). He was not only able to quantify the amount of carbon being transferred between the trees, but he also identified the fungal species responsible. These “ectomycorrhizal” (symbiotic fungi) are known for their special relationships with trees, and thus their importance in the web of forest ecology has long been recognized, but no one had yet imagined that trees use them as conduits for carbon sharing. "Out of the fungal species sampled, we identified at least eight different species that are generalists and common to the four tree species we analyzed; 90% of them are associated with at least two host species," says Rog. "We also found that trees that were more closely related, such as spruce and pine, had a more similar mix of fungal species connected to their root tips and shared more carbon between them than the more distantly-related species, say larch and beech. But overall, carbon was being exchanged between all species."
If the fungi are running the show, they could be directing resources between trees depending on their specific needs
From the trees’ point of view, carbon sharing would logically occur only between trees of the same species, giving them an evolutionary advantage over other species. This suggests that forest evolution may be taking place on a different level than simple tree kinship.
"The fact that trees are 'sharing their wealth' across species, suggests that there is some sort of 'hidden' management occurring. We think the management is dominated by the fungi," says Klein. "Fungi need to secure their own carbon sources; it is in their best interest to ensure that all the trees within the network are healthy and strong." If the fungi are running the show, they could be directing resources between trees depending on their specific needs. The fungi's actions would be especially advantageous if a forest is subjected to such stressors as climate change, drought, fire or disease; and the result would be a more resilient forest.
To test their hypothesis, the scientists have been continuing this line of investigation in Israeli forests. They are particularly focused on the carob tree, which has leaves that stay a luscious, shiny green even in extremely hot, dry conditions. Rog: "According to historical and biblical legends, the carob tree has 'mysterious powers' – secret ways of surviving in semi-arid climates. Perhaps fungi are the secret?" The scientists found that when carob trees are planted in a mixed forest of oak, conifer and pine, they too become part of the fungal network. The researchers are now beginning to uncover the specific species of fungi that connect the carobs to other tree species.
In another experiment, the scientists plan to induce conditions of drought in a small area to investigate the role of the fungi in the forest system when it is confronted with an ecological disturbance.
Klein: "On a theoretical level, our findings are altering our understanding of forest ecology, and scientists are beginning to realize that trees are more collaborative than previously believed. On a practical level, we are working with the Keren Kayemet LeYisrael – Jewish National Fund (KKL-JNF), to encourage the mixing of tree species in Israel's forests in order to build greater ecological resilience and stability."
Dr. Tamir Klein's research is supported by the Mary and Tom Beck-Canadian Center for Alternative Energy Research; the Y. Leon Benoziyo Institute for Molecular Medicine; the Larson Charitable Foundation New Scientist Fund; the Yotam Project; Dana and Yossie Hollander; the estate of Emile Mimran; the estate of Helen Nichunsky; and the European Research Council. Dr. Klein is the incumbent of the Edith and Nathan Goldenberg Career Development Chair.