Hidden Allies Aid Survival of the West's Largest Trees

Most people are oblivious to the vast, interconnected network that underlies a forest floor, but it’s a system that plants literally depend on for survival — especially big trees. They are able to fend off attackers better when they are connected to this underground system, according to the new research from Sara Germain and Jim Lutz from Utah State University’s Department of Wildland Resources.

For large, mature trees, the connection made the difference between life and log: if enemies such as bark beetle or invasive pathogens are close, big trees need a diverse group of underground allies even closer in order to survive.

‘Mycorrhizal mutualism’ is a partnership between underground fungi and 95 percent of the world’s plants. Plants offer sweet products of photosynthesis to the fungi connected to their roots. Fungi enhance a plant’s nutrient uptake from the soil; shift nutrients between plants; regulate genetics to produce better-armed plants; and allow inter-plant communication. As vital as they are to plant well-being, the relationships are so seamless and ubiquitous that determining exactly how much the plants benefit from connection to the underground system is tricky to disentangle — observers often credit survival advantages to the plant itself, or to partnerships between plants.

This newly published research measures whether mycorrhizal connection could increase chances for survival for plants under direct attack from enemies.

“Trees all over the west have been dying at higher rates than they used to from things like bark beetles, invasive pathogens, fire, and drought. And all of these are being exacerbated by warming temperatures. This is especially true for the largest trees, which is alarming. Large trees capture vast amounts of carbon, help filter the air and water, and provide habitat for countless wildlife species. Their conservation has become a goal of land managers, forest ecologists, and recreationists alike,” said Germain.

To understand the interaction, the researchers looked wide and long – at more than 101,000 woody plants from three mixed-conifer forests across western North America for over a decade. Each tree and shrub stem was mapped relative to its neighbors so that competitive interactions between plants could be assessed. Every year, each of these plants were revisited to determine whether it had died. If so, extensive pathology exams were conducted to determine the causes of its demise. Researchers found that connection to a strong mycorrhizal network particularly benefited large trees — the same effects weren’t as apparent for small trees or shrubs.

“We were excited to find that more diverse forests provided more mycorrhizal benefits that helped large trees survive attacks from bark beetles and pathogens,” Germain said. “Forest managers can use this information to update silvicultural treatments to keep the mycorrhizae in mind. I am hopeful that continued discovery of big trees’ fungal friends will help managers protect old-growth forests and maintain all the ecosystem services provided by big trees.”

As managers work toward healthy forests, a more nuanced understanding of mycorrhizal mutualisms can help move forests toward greater biodiversity, both above the soil and below.