At the root of the problem

Trees may have trouble growing in changing environments

For the past two decades, forest scientists have been predicting that trees will try to migrate to new regions as climate change alters their environments. But forest ecologist Suzanne Simard says it’s not that simple; organisms living below ground will play a large role in whether or not trees can settle in new regions.

British Columbia’s Interior Douglas fir forests are predicted to move north, following the climate they thrive in. To do this, trees will not only have to move at record-breaking rates to keep up with an ever-changing climate, they will also have to contend with foreign soils and plants competing for the same space, says Simard.

“Predictions about where trees will grow in the future have been based primarily on climate models,” she says, “but there are other factors, like the soil environment, that may limit whether a tree species will be able to move into a new area.”

Simard’s research focuses on how organisms living in soil – like fungi – help trees establish and grow. Some fungi live inside the roots of trees and form mycorrhizas (literally “fungus-roots”). These fungi help trees acquire nutrients and water from the soil in exchange for carbon.

In 1997, Simard was part of a team of researchers that discovered that trees were connected to one another through an underground web of mycorrhizal fungi. This network allows trees to communicate by transferring carbon, nutrients and water to one another.

Simard also helped identify something called a hub tree, or “Mother Tree.” Mother trees are the largest trees in forests that act as central hubs for vast below ground mycorrhizal networks. They support young trees or seedlings by infecting them with fungi and ferrying them the nutrients they need to grow.

Prof. Suzanne Simard talks about Mother trees. Video by: Dan McKinney; Producer: Julia Dordel

“We really haven’t determined whether the mycorrhizal fungi will migrate along with their tree hosts,” says Simard. “And without an appropriate web of fungi to help the establishment of seedlings, forests may not migrate to new locations where climate becomes hospitable for them.”

Simard’s now working with the BC Forest Service on a new research project that spans from southern California to northern British Columbia. They have planted 50 trees at 50 sites across that range. In two to three years, the team will see whether mycorrhizal fungi have been able to infect the seedling roots.

This will be among the first research to show how soil organisms will affect the ability of forests to migrate with changing climate.

“We are predicting massive changes in our ecosystems in the next century and we don’t know if trees will be able to keep up,” says Simard. “This research will help us understand the changes and provide some tools on how to facilitate the transitions.”

As climate change causes greater catastrophic events, like severe forest fires and insect outbreaks, existing forests will die back and open up for new trees and plants to move in, explains Simard. Her worry is that if trees can’t move into these new areas, weeds will instead.

Forest trees capture carbon dioxide, a greenhouse gas that causes climate change. If forests are replaced with weeds, more carbon dioxide will be released into the atmosphere and the effects of climate change will be more pronounced. Species that depend on forests for habitat will also disappear.

“It’s critical that we figure this out,” says Simard. “We need to be able to facilitate change so we can keep forests in our landscape. Conservation of forests will not only mitigate climate change, it will also slow the loss of biodiversity.”

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UBC Reports | Vol. 57 | No. 7 | Jul. 7, 2011

Trees are connected underground through a web of fungi that live inside the roots of trees. In forests, Mother trees act as central hubs for this network and support young trees by infecting them with fungi and ferrying them the nutrients they need to grow. Drawing from Mother Tree video by Dan McKinney

Trees are connected underground through a web of fungi that live inside the roots of trees. In forests, Mother trees act as central hubs for this network and support young trees by infecting them with fungi and ferrying them the nutrients they need to grow. Drawing from Mother Tree video by Dan McKinney

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Mother trees are the largest trees in forests that act as central hubs for vast below ground mycorrhizal networks.

How important is fungus?

For some trees, the question of whether fungi can infect their roots in a new location is more critical than for others.

Some trees, like red alder, have as few as one species of fungus living in its roots. Others, like Douglas firs, are host to up to 2,000 fungal species, where some are specialists and others are generalists. The specialist fungi link together trees of the same species while generalist fungi can link together trees of many different species and even other plants like shrubs. Typically, the more specific the fungi, the more specialized its role for the tree.

For trees depending on specific fungi, it is vital that the fungus can grow in new environments. About 80 per cent of the roots of old Douglas fir trees are colonized by a specific fungus that is especially important in water uptake, but it only fruits and disperses its spores below ground. If this fungus can’t migrate, the trees won’t either, and Douglas fir forests could disappear. Otherwise, the tree will need to find new fungi to fill this fairly specialized role through its life cycle.

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