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MYCORRHIZAE: UNSUNG HEROES OF FOREST HEALTH
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They’re nearly invisible, their name is all but unpronounceable, and few of us are even aware of their existence. Despite this, mycorrhizae (my-ka-RY-zee) are emerging from obscurity as the unsung heroes of forest health. Jane Braxton Little is a freelance journalist based in Plumas County, California. This article is reprinted from California Trees, Fall 2003, Volume 14 (2).
Feature Article
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| A new staking method and the addition of mycorrhizal fungi may increase this San Francisco street tree’s chances of survival. |
(Ontario Arborist, November 2004)
Urban forestry groups are beginning to recognize what scientists have understood for years: Mycorrhizae are essential biological components for plants. Without them, trees and entire forests would languish.
Mycorrhizae are root structures created when specific fungi colonize the young lateral roots of plants. Ectomycorrhizae – one of the two general types of mycorrhizae formed by woody plants – develop sheaths around the cortical cells of short lateral roots. Endomycorrhizae invade individual cells within the cortex. Together they form symbiotic associations that benefit both the plant and the fungus. The fungus helps the plant absorb water and nutrients. The plant provides the fungus with food. The vast majority of the world’s land plants form some type of mycorrhizae, says Joe Morton, professor of microbiology at West Virginia University.
Forests are particularly dependent on mycorrhizae for health and endurance. Trees have developed specialized relationships with fungi as safety measures, increasing their ability to withstand drought, high soil temperatures and acidity. Mycorrhizae also function as biological deterrents to root pathogens.
“They are part of the intricate interrelationships we need to pay attention to. We’d be foolish not to,” says Rick Hawley, Executive Director of Greenspace–The Cambria Land Trust.
Urban Fungi
This newfound awareness of mycorrhizae as key players in forest ecosystems has spawned interest in adding mycorrhizal-forming fungi to the soil around newly planted urban trees. That can help some trees dramatically, scientists say. But they are cautious about overrating the promise of mycorrhizae and they disagree about whether inoculating trees with commercial fungi is effective – or even beneficial.
Although a German scientist described the relationship between plants and fungi over a century ago, it has only recently commanded prominent attention. David H. Adams, a forest pathologist now retired from the California Department of Forestry and Fire Protection (CDF), remembers his first encounter with the effects of mycorrhizae on trees some years ago. He noticed that some of the redwoods and giant sequoias planted in a CDF nursery were thriving while others were dying.
Curious about the difference, Adams collected the top three inches of soil from the beds where the seedling trees were flourishing. He then fumigated all of the beds, worked the soil he had removed back into a sampling of beds, and reseeded the trees. Those growing in the beds without the replacement soil soon died. The others grew so large Adams and his co-workers had trouble packing them. The difference was mycorrhizal fungi in the soil. “A tree – any plant that depends on having its roots colonized by mycorrhizae – will not live without it,” Adams says.
Treating “Dead Soil”
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| Friends of the Urban Forest (San Francisco) now adds a mycorrhizal fungi mix to all of its tree plantings. |
Most forest soils contain mycorrhizae adapted to particular plants for particular functions. When soils are disturbed, however, these symbiotic relationships are disrupted. As urban forest groups learn more about mycorrhizae and the direct relationship between their efficiency and soil quality, their interest in the potential of mycorrhizal additives is growing. The altered landscapes where most groups work are generally missing key soil components, including mycorrhizae. Why not restore them artificially?
Doug Wildman, Program Director for Friends of the Urban Forest in San Francisco, works almost exclusively in what he calls “dead soil.” He supervises the planting of around 1,500 trees annually in places that have been dug up, stripped of topsoil, compacted, and generally abused. Little healthy soil is left. These are the places where mycorrhizae are really needed, he says.
For the last two years, Friends of the Urban Forest has planted all of its trees with MycorTree, a commercial product available as mulch, liquid root dip and soil injection. It includes an array of mycorrhizal fungi and other components designed to stimulate root growth. At around $2 a tree, Wildman considers it cost-effective insurance.
The trees are all doing well, he says, but it’s hard to know whether to credit the mycorrhizae. Wildman changed his staking methods at about the same time he began adding Mycor. “It’s a shotgun approach. We’ll probably never know what works,” he says.
The Great Fungi Experiment
TreePeople, one of California’s oldest and most active urban forest organizations, tried to find out by conducting a field test of mycorrhizal inoculations. Called “The Great Fungi Experiment,” TreePeople selected a disturbed area near the intersection of two major Los Angeles freeways and planted the roadside shoulders with 76 California pepper trees.
The trees were divided into three groups. The first group was the control and received no treatment. Group two received root inoculations via the “trowel method” – a mixture of mycorrhizal fungi and bacteria was incorporated into the soil in the trees’ root crown via holes dug with hand trowels. The third group received root inoculations via liquid root injections applied by a commercial arborist. For the next three years, TreePeople monitored trunk circumferences to determine general growth rates.
Both sets of trees receiving root inoculations showed greater growth, but those inoculated via the injection method were double the average growth of the control trees, says Jennifer Scott-Lifland, a certified arborist and tree care manager at TreePeople. Of even greater importance, she says, was establishing mycorrhizal colonies that will benefit the trees for years to come, giving them the ability to outperform the control trees.
“Our observations have been so encour-aging that... we hope others engaged in urban tree planting and care will decide to try this new technology in their own field plantings,” Scott-Lifland says.
| Ask before you add ...
1) Are there mycorrhizal fungi present in the soil?
2) Are you adding fungal species that can form effective associations with the plant species?
3) What site conditions need to be modified to improve mycorrhizal relationships?
4) What effect will the inoculant have on native soil microflora?
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Scientists Skeptical
Scientists, however, consider these results anecdotal. Research experiments introducing mycorrhizal fungi to plants have generally failed to demonstrate significant improvement in tree growth, says Nelda P. Matheny, co-author of a new edition of Arboriculture: Integrated Management of Landscape Trees, Shrubs, and Vines. The best results have been in highly disturbed soils, where native mycorrhizae populations are unnaturally low, she says. There, inoculations have proven highly useful.
Friends of the Urban Forest and TreePeople are using mycorrhizal treatments under exactly the conditions that maximize their beneficial effects, says Bruce Hagen, a CDF urban forester. When the soil is poor, adding the fungi at planting time will generally help trees get established. It may also help them survive with less water, he says. Still, Hagen cautions against the tendency to regard mycorrhizal fungi as a panacea for poor tree development. “It would be foolish to say it’s worthless but the routine use of mycorrhizae is not warranted,” he says.
Most plants form mycorrhizal associations without any human interference. Even in disturbed soils, mycorrhizal fungi will be introduced naturally over time as wind and animals carry fungal spores into the area. Introducing mycorrhizae in a commercial cocktail may be “cheap insurance,” Hagen says, but the species introduced will likely be different than what the plants will eventually develop on their own.
And he fears that excitement over mycorrhizae will distract urban forest activists and nursery owners from other critical issues. “It’s not a silver bullet. It won’t change poor tree structure and it won’t change poor soil,” Hagen says.
Although the research is limited, several scientific studies bolster his cautionary words. Bonnie Appleton, a researcher at Virginia Polytechnic Institute and State University, found no apparent measurable growth benefits to inoculating pin oak seedlings with commercial mycorrhizal fungi. Other researchers have found that commercial inoculations may depress native mycorrhizae, slowing the natural process that benefits tree health.
These results have not deterred Greenspace–The Cambria Land Trust, working in the north coast area of San Luis Obispo County. Greenspace volunteers add mycorrhizal fungi to the soil every time they plant Monterey pine seedlings, says Hawley, the executive director. Their method is even more low-tech than Friends of the Urban Forest. They simply dig local duff into their planting holes and seedbeds.
“We’re fortunate to live in a forest that has plenty of the inoculants in the ground,” Hawley says. Like their San Francisco counterparts, Greenspace volunteers are working with soils that have been abused. “Odds are these trees wouldn’t survive without mycorrhizae,” he says.
This winter Greenspace will add riparian plants to its Monterey pine program. Hawley plans to restore eroded landscapes with willow, cottonwood, alder and sycamore trees. Instead of the duff used for the pine seedlings, Greenspace will gather soil from nearby sites where similar trees are doing well. This should provide the appropriate mycorrhizae.
“It’s an experiment for us. We’re not altogether clear about how it will all work out,” he says.
What is clear to tree planters and scientists alike is that we are far from understanding the complexities of healthy soils and the plants that depend on them. We know that the interrelationships between fungi and tree roots are critical, and that human activities have been generally destructive to them. Doug Wildman, FUF’s program director, urges other urban forest activists to read the studies currently being released. The science is evolving rapidly. Once we have a better understanding, we will be able to develop applications that benefit tree plantings.
— Source: Arboriculture: Integrated Management of Landscape Trees, Shrubs, and Vines. 4th edn. Richard W. Harris, James R. Clark, Nelda P. Matheny. Prentice Hall, 2004.
Last Modified: Monday, December 6, 2004
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