By Mike Amaranthus, Ph.D., and Tonya Gordon, MS
Even though you can’t see most of it, a complex soil food web lives beneath trees in natural areas. This web is teeming with earthworms, mites, bacteria and fungi — all kinds of mostly microscopic, interdependent organisms that release mineral nutrients and create the loose soil structure trees need to thrive. These microorganisms also help prevent diseases, as well as process and recycle organic material. Beneficial mycorrhizal fungi form the cornerstone of this below-ground menagerie growing in and around tree roots, mining subsoil for nutrients and water to share with trees.
Trees actually help feed all this underground life. “While trees invest a substantial amount of their photosynthesis in feeding soil microbes, the trees are obviously getting benefits back,” said David Steinfeld, USDA Forest Service soil scientist. Between 40 percent and 70 percent of a tree’s total carbohydrates are allocated below the soil surface. It’s a big investment — so what’s in it for the trees?
Think of plants, with their green chlorophyll, as little solar-powered engines that pipe a steady flow of carbohydrates out through their root systems. Sugar exudates from the roots are consumed by bacteria, fungi, protozoa, nematodes, earthworms and insects. As carbon flows throughout the food web, it provides energy and the building blocks for the organisms that are consuming it, whose waste or bodies provide the energy and building blocks to other organisms in the web. In other words, although each organism is working to find energy and food for its own survival, the interactions of individual organisms are providing food and protection to the trees and other soil organisms.
Fungus among us
There is an awful lot of fungus in the world. There are an estimated 1.5 million species with a total biomass around twice that of all animals. Despite its widespread occurrence and diversity, this group of organisms is not well understood. Fungi are famous for their effects on moldy bread, itching toes and degrading the foundations of our homes. Although some people do acknowledge the importance of fungi to make bread, beer and blue cheese, overall fungi are vastly under appreciated. One group, soil fungi for example, is incredibly important to tree health.
Hidden from view beneath the soil surface, the relationship between fungi and trees has been successful for millions of years. Fungi can’t make their own food. They have to absorb their nourishment from living or dead organic matter. We have come to understand that in natural habitats tree roots supply the food to fuel the activities of mycorrhizal fungi. In nature, tree roots are a complex mixture of both fungus and plant — mycor-rhiza literally means “fungus-root.”
Mycorrhizal fungi attached to and supported by roots come in and “hunt” down nutrients and water in the soil. These fungi, which are made up of hyphae that look like fine threads in the soil, capture and carry these nutrients back to the plant roots.
The fungal hyphae along with fine plant roots form a “sticky” net called “glomalin” in the soil where organic matter, sand particles and clay minerals become bound together. Glomalin is similar to chewing gum, and it glues soil particles together into small packets, called aggregates. It is these aggregates that give soil its porosity and allow for the movement of air and water into the soil, which is necessary for healthy trees.
But this complex, mostly invisible web of mycorrhizal fungi can easily be damaged. Clearing of native vegetation, compaction, removal of surface soil, certain chemical fertilizers, pesticides, grading and tilling can all reduce or eliminate beneficial mycorrhizal fungi in the soil. Landscape and tree care professionals are often faced with dead soil that has limited, if any, potential to form the mycorrhizal relationship to support their trees.
Early investment in mycorrhizal fungi yields dividends
Mycorrhizal inoculum has become more commonly used in the tree industry throughout the United States. Nurseries, landscape professionals and arborists are continually seeking ways to increase tree survival, growth and vigor, while reducing the input costs of managing them. Recent studies in Oregon and Montana incorporated mycorrhizal inoculum at the nursery in the production of shrubs and trees to determine if survival and growth was improved in the outplanting environment.
In the first study (Native Plant Journal, 2008), mycorrhizal inoculum was incorporated into the potting soil during production of five shrub and tree species (Ribes aureum, Prunus pumila, Potentilla fructicosa, Amorpha canescens and Juniperus scopulorum). Shrubs and trees that were well colonized with mycorrhizal fungi at the nursery were outplanted in disturbed field conditions in Oregon survived significantly better than control shrubs and trees. Following outplanting, seedling survival was twice as high for the mycorrhizal-colonized shrubs and trees compared to control seedlings. The average survival for the mycorrhizal-colonized plants was 80 percent compared to 38 percent for the control seedlings.
In Montana, the Montana State University Agricultural Research Service implemented a similar study with Rhus trilobata, Prunus virginiana and Rosa woodsii. All of these plants were grown together in the same greenhouse as 10-cubic-inch stock and were pruned to approximately six-inch height at outplanting. One-half the seedlings received mycorrhizal inoculation at the nursery. After only one season of growth, the difference in outplanting performance was dramatic.
These studies emphasize the need to consider mycorrhizal inoculation for improving mycorrhizal colonization, and to achieve the benefits of mycorrhiza under field conditions. In artificial nursery conditions, high levels of water and nutrients and sterile soils at the nursery produce vast quantities of trees for sale. The high levels of water and nutrients and the lack of mycorrhizae discourage the trees to produce the extensive root/fungus system they will need for successful transplantation, rapid establishment and good health. The result are non-mycorrhizal trees poorly adapted to the eventual outplanted condition “in the real world” that need to begin to fend for themselves. Nursery-grown plants already colonized with mycorrhizal fungi can more rapidly establish and take advantage of soil resources in the outplanting environment. If not established at the nursery, the mycorrhizal relationship should be established as early as possible in the field to achieve the dividends of the fungus-and-tree relationship.
Mike Amaranthus, Ph.D., spent more than 20 years with Oregon State University and the U.S. Department of Agriculture where he wrote more than 60 research papers on mycorrhizae. He is president and chief scientist for Mycorrhizal Applications, Inc., located on the Web at www.mycorrhizae.com. He is the recipient of the USDA Highest Honors for scientific achievement, and has been featured on several major national and international programs — most recently on “Modern Marvels” on The History Channel on an episode detailing the future of fertilizer.
Tonya Gordon, MS, is project coordinator working on special needs and programs for Mycorrhizal Applications, Inc. Gordon holds an MBA, and for the past four years has been working with arborists and landscape professionals to improve outplanting performance and maintain healthy, sustainable landscapes.