The fungal networks that connect trees in a forest are a major factor determining the nature of forests and their response to climate change. These webs are also seen as a way for trees to help their offspring and other tree friends, according to the increasingly popular “mother tree hypothesis”. An international group of researchers has re-examined the evidence for and against this hypothesis in a new study.
The trees in the forest are interconnected by thread-like structures of symbiotic fungi, called hyphae, which together form an underground network called the mycorrhiza network. While it is well known that mycorrhizal fungi provide nutrients to trees in exchange for the carbon provided by trees, the so-called mother tree hypothesis implies an entirely new purpose for these networks. Through the network, the largest and oldest trees, also known as mother trees, share carbon and nutrients with seedlings growing in specially shaded areas where there is not enough sunlight for adequate photosynthesis. The structure of the network should also enable mother trees to detect the ill health of their neighbors through distress signals, and alert them to send the nutrients they need to heal back to these trees. In this way, the mother trees are thought to act as central hubs, communicating with both the young seedlings and other large trees around them to increase their chances of survival.
This is a very attractive concept that attracts the attention of not only scientists, but also the media, as this hypothesis is often presented as fact. According to the authors of the study just published in New BotanyHowever, the hypothesis and theory are difficult to reconcile, prompting researchers to re-examine the data and conclusions from publications both for and against the mother tree hypothesis.
The study, which was led by Nils Henrikson of the Swedish University of Agricultural Sciences, found that experimental evidence for the mother tree hypothesis is actually very limited and theoretical explanations for the mechanisms are largely unavailable. While large trees and their interdependence with their neighbors are still essential to a forest ecosystem, the fungal network does not function as a simple resource-sharing pipeline between trees. This means that the apparent sharing of resources between trees is more likely the result of trade between fungi and trees rather than directed transfer from tree to tree. Often, this exacerbates competition between trees rather than supporting seedlings.
“We have found that mycorrhiza-mycorrhizal networks are indeed essential to the stability of many forest ecosystems, but rarely through sharing and caring among trees. Instead, they act as a trading floor for individual trees and fungi, each trying to strike the best deal for survival.” Life,” explains Oscar Franklin, study author and researcher in the Agriculture, Forestry and Ecosystem Services Research Group of the IIASA Biodiversity and Natural Resources Program. “The forest is not a superorganism or a family of trees helping each other. It is a complex ecosystem with trees, fungi and other organisms, all interconnected but not guided by a common goal.”
“Although the mother tree hypothesis narrative is rarely supported by scientific evidence and is controversial in the scientific community, it has inspired research and public interest in the complexity of forests. It is critical that future management and study of forests take the true complexity of these important ecosystems into account, Franklin concludes.