Underground fungal 'internet' supports tree growth
Plant biology, Forest ecology
Reading the word “fungi,” you probably envision mushrooms. Maybe the buttery porcini in a risotto you paid way too much for on a blind date from hell. Perhaps the poisonous Amanita muscaria that has somehow been immortalized as furniture for garden gnomes. In reality, mushrooms are only the cute, familiar tips of the fungal iceberg. Most of a fungus’ body lies underground, connecting with other fungi to form a network of branched threads called a mycelium. Since the late 1800s, scientists have known that plants are able to associate with the mycelium to form the mycorrhiza – from the Greek mykēs, “mushroom” and rhiza, “root” – but only over the past half century have they begun to uncover its complexity. Incredibly, different plant species connect to one another using the mycorrhiza, exchanging resources underground much like remote computers access the internet to exchange information.
In 1997, Suzanne Simard and her colleagues from the University of British Columbia in Vancouver published some of the first evidence that young plants use the mycelium to give and receive carbon. To show the exchange of carbon in action, Simard used two different species of seedling: Douglas fir and paper birch trees. Into each seedling they pumped in carbon dioxide containing either of two carbon isotopes: one with 13 neutrons and the other with 14. Having a different carbon isotope in the firs and birches allowed the team to determine if one plant was receiving carbon from the other when they had a look at total carbon content later. Imagine two connected rooms full of identical looking people. If these people were suddenly allowed to travel between rooms, you would have no way of keeping track of where anyone started or how many had left. If you introduced a way to tell the two starting rooms apart – say, if everyone in one room wore red t-shirts and those in the other wore blue – you could get an idea of net transfer by counting red shirts in the blue room later. Reciprocal isotope labelling works in a similar way, but with different neutron numbers instead of shirt colours. Simard was able to show carbon exchange between the seedlings in both directions and, amazingly, found that the amount of carbon exchanged depended on whether the recipient seedling was grown in light or in shade. Seedlings grown in deep shade, which were less capable of photosynthesising on their own, received more carbon from their partner. In forest ecology, this is referred to as a source-sink relationship. By sharing resources through the mycelium, different species of plants can support one another when nutrition is limited.
A lot has happened since Simard’s study first appeared in the prestigious Nature journal in 1997. Since then, scientists have figured out that mycorrhizal networks are used to transport nutrients, water, and even chemical signals that warn other plants of incoming threats from harmful fungi or aphids.
The studies that have followed have mainly focused on the relationship between seedlings and the mycelia but still had not demonstrated an interaction between the fungal network and adult trees – until now. Simard once again features in this 2020 publication from the Journal of Ecology, this time led by Joseph Birch (with a name like that, how could you not be a forest ecologist?) and Kevin Beiler at the University of Alberta. Beiler mapped out the mycorrhizal network linking 350 Douglas fir trees and used genotyping to identify which fungal species were making these connections, while Birch took samples from the core of each tree to assess ring growth. Birch found that the more connected an adult Douglas fir was – to other trees as well as to unique species of fungi – the better its growth rate. Not only do these findings highlight the community activity of forests, but they may have crucial implications for the expected stresses accompanying climate change. Rising atmospheric carbon dioxide levels and drier conditions could place greater importance on trees sharing their water supply underground using the fungal ‘internet.’ Emerging studies on mycorrhizae continue to drive home the age-old message that organisms may be more interconnected and interdependent than meets the eye.
Isotope: Variants of a chemical element that have different numbers of neutrons. All isotopes of an element have the same number of protons (carrying a positive charge) but different numbers of neutrons (carrying no charge). The most common isotope of carbon has 12 neutrons.
Genotyping: Differentiating between different organisms using their unique DNA sequences.
“The artwork is absolutely stunning and captures the intricacy and beauty of belowground symbioses. I think James did a fantastic job capturing the beauty of the idea of belowground individualism and multi-species cooperation.”
— Joseph Birch, University of Alberta
Journal references
Simard, S., Perry, D., Jones, M., Myrold, D., Durall, D., and Molina, R. Net transfer of carbon between ectomycorrhizal tree species in the field. Nature. 1997. https://doi.org/10.1038/41557
Birch, J., Simard, S., Beiler, K., and Karst, J. Beyond seedlings: Ectomycorrhizal fungal networks and growth of mature Pseudotsuga menziesii. Journal of Ecology. 2020. https://doi.org/10.1111/1365-2745.13507.