In the forests of British Columbia, where recent wildfires have sent smoke across borders and dimmed blue summer skies, a series of studies from the past 30 years contends that large, old trees send resources and messages to the seedlings around them. The “mothering” could, hypothetically, help burned landscapes recover faster, boost the amount of carbon dioxide stores in soil, and improve the resiliency of natural systems overall.
The idea seems to borrow from bedtime tales about ancient trees and the enchanted forests they foster; to validate beliefs about all types of creatures nourish their young; to vouch for the inherent goodness of nature, where collaboration triumphs over competition.
[Related: Behold the world’s tallest trees]
But two papers have recently called into question the evidence supporting the “mother tree” explanation. Do these veterans of the forest act as guardians for newer generations, protecting them from drought, disease, and deforestation? Or is their relationship much more complicated?
What is a ‘mother tree’?
The term “mother tree” was coined in the 2000s by a Canadian scientist named Suzanne Simard, who grew up in a family of loggers in the Monashee Mountains in British Columbia. The old-growth forests on the range sustained a booming timber industry for more than a century.
In her book Finding the Mother Tree, Simard describes a childhood spent wandering through the forest, gaining keen insight on the intimate connections between long-lived trees like Douglas firs and ponderosa pines and the ecosystems they inhabit. Later, she studied forestry, became a research scientist for the Canadian Ministry of Forests and now teaches and leads a lab at the University of British Columbia.
Her childhood experiences and decades of scientific research led her to draw a connection that was somewhat revolutionary in forestry management and the conventional biological thinking that species must compete to survive. Simard proposed that large trees that are hundreds or even thousands of years old can send carbon, nutrients, water, hormones, and even alarm signals to young plants via a network of underground fungi known as mycorrhiza. She describes these trees as “mothers” in her writing and interviews, and argues that they are essential in making forests around the planet better suited to survive climate change.
“We need to save the legacies, the mother trees and networks, and the wood, the genes, so they can pass their wisdom onto the next generation of trees so they can withstand the future stresses coming down the road,” Simard said in her Ted Talk from 2016. “We need to be conservationists.”
What’s the scientific evidence?
Some forestry researchers warn that the mother tree viewpoint is ahead of the science. A paper published in the journal Nature Ecology & Evolution in February reviewed 26 studies that look at the ability of underground fungal networks to transfer resources and if mother trees send resources to young plants.
The studies spanned continents, experimental design, and forest and soil type. The authors found that in about 80 percent of the studies, access to mycorrhizal networks associated with nearby trees had no benefit to the seedlings planted around them. In 18 percent of the studies the seedlings did benefit. And in a much smaller subset, those trees and their mycorrhizal networks actually harmed the others
“There are lots of important ecological roles for big trees in the forest,” says Justine Karst, the lead author of the paper and a professor at the University of Alberta. “But this sort of popularized idea of their role and how they work with these fungi as these passive conduits in the soil doing things under the direction of trees, there’s just not really evidence for that.”
Part of the problem is that there’s so much variability within the 26 studies, making it difficult to draw conclusions about mother trees—or large, old trees, as Karst prefers to call them—as a whole.
“It differs in which forest the experiment was conducted in: how far the seedlings were growing from the mature tree, the type of seeds or the type of soils, if there’s overstory mortality,” Karst explains. “There’s so many of these background features that there’s just no way to generalize. This is something that we would suggest moving forward is that we need to understand, what is the cause of this variability?
Another issue is that most of the research that supports the mother tree theory comes from studies in labs, Meghan Midgley, a soil ecologist at Morton Arboretum who was not an author on the recent review, explains. “We haven’t been able to observe it in the forest, which is where we’d really want to see this sort of relationship happening.”
The idea might even be so appealing, scientists have let it bias them. “There are alternative explanations that have not been acknowledged in studies,” Karst says. For example, one common experimental design using mesh bag encourages different types of fungal growth, potentially biasing the results.
PopSci reached out to Simard and The Mother Tree Project about these scientific uncertainties, but did not receive a response by the time of publication.
What role do fungi play?
One facet of the mother tree debate experts agree on is that fungi have a unique relationship with trees. Midgley studies this symbiotic subset of organisms, which grow on the tree’s root system and allow it to gain access to water and nutrients deep in the soil. In return, the guests get carbon, which the fungi can’t can’t produce itself. “From the tropics to boreal forests, trees are associating with fungi,” Midgley says. “This is a relationship that has been established for much of evolutionary time.
These collaborative fungi also have an overall beneficial effect on plants. “There are many hundreds of studies showing that when there’s no fungi, plants don’t grow as well as when there are fungi,” Midgeley adds. “There’s also some evidence that they can help protect plants from below-ground pathogens or from being eaten by below-ground invertebrates, so they can play a variety of roles for a plant.”
Knowing this, forest manager might take fungi, as well as large, old trees, into account when restoring an ecosystem after a wildfire. However, there’s not enough evidence right now to support specific strategies, like introducing fungi into a forest that’s been harmed by wildfire, Midgley says.
She and Karst both suggest further research that would help scientists better understand the variability between mature trees and their relationships with fungi and the rest of the forest. “Why do seedlings sometimes show no response, a positive response, or a negative response [to older trees]?” Karst says. “We don’t know those answers, but I think that they’ll be important to find out.”