Chris Maser

All things in Nature's forest are neutral. Nature assigns no values. Each piece of a forest, whether a bacterium or an 1,000-year-old tree, is allowed to offer its prescribed structure, carry out its prescribed function, and interact with other components through their prescribed, interrelated processes. None is more valuable than another; each is only different from the other.

Assigning a value to something in the forest begins to adjust that object in our focus, where bringing one thing into focus simultaneously forces almost everything else out of focus. For example, rodents were poisoned for decades in the name of forestry, because they were perceived as having only a negative value. They ate the seeds that industry wanted to grow into merchantable trees. Today, forest rodents are viewed differently. Some still eat tree seeds, but at the same time they disperse viable spores of mycorrhizal fungi, nitrogen-fixing bacteria, and yeast. The following vignette of the northern flying squirrel in western Oregon illustrates but one example of the myriad dynamic, interdependent relationships among mammals and forests the world over.1


The northern flying squirrel of the temperate coniferous forest nests and reproduces in the treetops and feeds primarily on belowground-fruiting fungi (truffles and false truffles), which it finds at night by odor. In so doing, it uses the forest from the tops of the trees down into the upper layers of the soil. As the squirrel eats and digests the fungi, the fungal spores are emitted in fecal pellets anywhere in the forest the squirrel happens to be when it defecates. As these fungal spores are washed into the soil by rain or melting snow, they inoculate the root tips of trees and act as viable extensions of a tree's root system thereby aiding the tree in its uptake of water, phosphorus, nitrogen, and other metabolites from the soil that the tree itself is incapable of extracting with any efficiency.

The fungi in the soil thus feed the crowns of the trees high in the air, and the trees, in their turn, send sugars from photosynthesis in their crowns down to feed the fungi in the soil. The fungal fruiting bodies feed the northern flying squirrel, and the squirrel for its part "seeds" the floor of the forest with viable fungal spores that inoculate the trees' roots, whereupon the spores germinate, grow, and begin to feed the trees.2


Consider that most higher plants worldwide—from the northern tundra, to the desert, to the tropical rainforest—have an obligatory, symbiotic relationship with certain fungi that are central to their processes of capturing nutrients from the soil. In fact, fossil evidence, as well as current molecular studies, of the earliest terrestrial plants show that plant roots and certain fungi co-evolved as symbiotic partners around 400 million years ago. This mutualistic relationship formed structures known as mycorrhizae, a word that comes from the Greek mykes (a fungus) and rhiza (a root). The term mycorrhiza, which literally means "fungus-root," denotes the symbiotic relationship that is today generally distributed throughout plant communities.

The hyphae, or individual fungal strands, each one to two cells thick, form the main structural elements of the mycorrhizal fungi. ("Hypha," singular, comes from the Greek hyphe, a web.) The hyphae either penetrate the cells of a plant's roots to form an "endomycorrhiza" (endo = mycorrhiza inside the root) or ensheath the root of form an "ectomycorrhizae" (ecto = mycorrhiza outside the root).

Note the light-colored tissue of an ectomycorrhizal fungus forming a "mantle" around the tree's root tips.

In the nutrient-deficient conditions of most forests, at least ninety percent of a tree's "feeding" roots are colonized by ectomycorrhizal fungi, which results in a layer of fungal tissue or "mantle" that forms around the tree's feeding roots, thereby creating an interface between the tree's feeding roots and the soil. From this mantle, individual hypha aggregate and organize into root-like structures called "rhizomorphs" (rhizo = root + the Greek morphe = shape) that grow out from the feeding roots into the soil, where they act as an extension of the tree's root system. The aggregate of hyphae are also referred to as "mycelia" (singular = mycelium), which in New Latin means "made of mushrooms." A mycelium is a given mass of hyphae that forms the non-reproductive part of a fungus.

Some mycorrhizal fungi are host-specific, meaning that a given mycorrhizal fungus colonizes a single species of plant, whereas others are generalists and colonize a number of host plants. Douglas-fir or birch, for example, can be colonized by may species of mycorrhizal fungi. In turn, these fungi extend from tree to tree and so form linkages among trees. The generalized nature of host compatibility ensures that almost all trees and many other plants in an undisturbed forest ecosystem, regardless of species, are interconnected by billions of miles of hyphae, organized into mycelial systems, that stem from a diverse population of mycorrhizal fungi. If, therefore, you could pull up a whole forest and gently wash the soil from the roots of the plants, you would find a mycelial net connecting the entire forest—one of the reasons an old forest is so retentive of its soil-nutrient capital.3


The most obvious flying squirrel-forest relations are those that occur on the surface of the ground, such as foraging. Even the nesting and reproductive behavior of these squirrels remains relatively obscure because of their nocturnal habits. In probing the secrets of the flying squirrel, however, at least four functionally dynamic, interconnected cycles emerge.

The Fungal Connection

Ectomycorrhizal fungi profoundly affect the forest ecosystem. Fungal hyphae penetrate the tiny, fleshy, feeder rootlets of the host plant to form a balanced, beneficial mycorrhizal symbiosis with the roots. Through the obligatory, symbiotic mycorrhizal association, a plant, such as Douglas-fir, provides carbohydrates and other metabolites to its mycorrhizal symbiont, which lacks chlorophyll and generally is not competent saprotroph (a living organism that derives its nutrients by decomposing organic material).

For its part, the fungal symbiont mediates the plant's uptake of nitrogen, phosphorus, other minerals, and water and translocates them into the host. In addition, the mycorrhizal association promotes the development of fine roots; produces antibiotics, hormones, and vitamins useful to the host plant; protects the plant's roots from pathogens and environmental extremes; moderates the effects of heavy metal toxins; and promotes and maintains soil structure and the forest food web. This mycorrhizal association is expensive, however, with an estimated fifty to seventy percent of the host plant's net annual productivity being translocated to the roots of the plant and their associated mycorrhizal fungi.4

When access to nutrients is increasingly restricted in a forest ecosystem that is already nutrient-impoverished, mycorrhizal fungi can influence both the interactions among plants and the species composition of the plant community itself. It appears there is an additive, beneficial effect that comes with each species of mycorrhizal fungus that colonizes a given plant, which, in turn, could mean that both biodiversity and ecosystem productivity will increase with an increasing number of fungal symbionts. This scenario seems likely because experimentation has shown that as the number of fungal symbionts is increased, so too is the collective biomass of roots and shoots, as well as the species diversity of the plant community. Conversely, as a forest is disturbed through exploitive forestry practices and the use of artificial fertilizers, the function of the mycorrhizal system can be impaired

In addition to gleaning nutrients from the soil and translocating them into the host plant, mycorrhizal fungi, along with roots of the host plant and the free-living microbial decomposers in the soil, are significant components of the global balance of carbon. Much of the carbon balance is mediated by photosynthesis and drives the respiration or "breathing" of the soil. Photosynthesis, in turn, is the synthesis of complex organic materials (especially carbohydrates from carbon dioxide, water, and inorganic salts) by using sunlight as the source of energy, with the aid of chlorophyll and its associated pigments. The "photosynthates" (= nutriments) produced by the process of photosynthesis (sent from the green, aboveground portion of a plant to its roots and mycorrhizal symbionts), are critical in maintaining the soil's respiration, through which carbon is extracted from the soil. Although the carbon made available to the soil through photosynthesis helps to balance the loss of carbon from the soil through respiration, the production of photosynthates is mediated more by annual seasonality than by the temperature of the soil.5

As I said before, these fungi, of which the truffles are the reproductive part, depend for survival on the live forest trees to feed them sugars produced in the trees' green crowns. In turn, the fungi form extensions of a tree's root system by collecting minerals, other nutrients, and water that are vital to the tree's survival. The fungi also depend on large, rotting trees lying on and buried in the forest floor for water and the formation of humus in the soil. Humus, which lends soil its dark color, is the Latin word for "the ground, soil" or alternatively the New Latin word humos, meaning "full of earth." Further, nitrogen-fixing bacteria occur on and in the ectomycorrhiza, where they convert atmospheric nitrogen into a form that is usable by both fungus and tree.6

The Truffle Connection

Truffles—the belowground fruiting-bodies—are the initial link between, mycorrhizal fungi, and the flying squirrel. Flying squirrels nest and reproduce in the tree canopy and come to the ground at night where they dig and eat truffles. As a truffle matures, it produces an aroma that attracts the foraging squirrel. Evidence of a squirrel's foraging remains as shallow pits in the forest soil and occasional partially eaten fruiting-bodies.

Truffles contain the nutrients required by the small animals that eat them. In addition to nutritional value, truffles also contain water, fungal spores, nitrogen-fixing bacteria, and yeast, all of which become important in the forest network.

The Squirrel Connection

Keeping the above in mind, let's consider the coniferous forests of the Pacific Northwest in which Douglas-fir and western hemlock predominate in the old-growth canopy. Herein lives the Northern Spotted Owl, which preys on the flying squirrel as a staple of its diet. The flying squirrel, in turn, depends on truffles, which it detects by odor at night and then digs them out of the forest soil.7

When flying squirrels eat the truffles, they consume fungal tissue that contains nutrients, water, viable fungal spores, nitrogen-fixing bacteria, and yeast. Pieces of truffle move to the stomach, where the tissue is digested; then on through the small intestine, where absorption takes place, and then to the cecum. The cecum is like an eddy along a swift stream; it concentrates, mixes, and retains fungal spores, nitrogen-fixing bacteria, and yeast. Undigested material, including cecal contents, is formed into excretory pellets in the lower colon; these pellets, which are expelled through the rectum, contain the viable spores and accompanying micro-organisms necessary to inoculate the root tips of trees.

The Pellet Connection

Flying squirrels dine heavily on truffles. The fate of their fecal pellets varies, however, depending on where they fall. In the forest canopy, the pellets might remain and disintegrate in the tops of the trees. Or a pellet could drop to a fallen, rotting tree and inoculate the wood. On the ground, a squirrel might defecate on a disturbed area of the forest floor, where a pellet could land near a feeder rootlet of a Douglas-fir that may become inoculated with the mycorrhizal fungus when the spores, having been washed into the soil by rain, germinate. If environmental conditions are suitable and root tips are available for colonization, a new fungal colony may be established. Otherwise, hyphae of germinated spores may fuse with an existing fungal thallus (the non-reproductive part of the fungus) and thereby contribute and share new genetic material.8

A fecal pellet is more than a package of waste products; it is a "pill of symbiosis" dispensed throughout the forest. Flying-squirrel pellets usually contain four components important to the forest:  (1) viable spores of mycorrhizal fungi, (2) yeasts, (3) nitrogen-fixing bacteria, and (4) the complement of nutrients necessary for the survival and function of the nitrogen-fixing bacteria—like the yolk that feeds the chicken forming in the white of an egg.

The yeast, as a part of the nutrient base, has the ability to stimulate both growth and nitrogen fixation in the bacterium Azospirillum spp. Nitrogen thus made available can be used both by the fungus and the host tree. Yeast cells may also aid spore germination because spores of some mycorrhizal-forming fungi are stimulated in germination by extractives from other fungi, such as yeasts.9

The northern flying squirrel thus exerts a dynamic, functionally diverse influence within the forest. The complex of effects ranges from the crown of the tree inhabited by the squirrel, from which it can rain fecal pellets to the soil. The pellet's house spores, microorganisms, and nutrients, which work into the soil's root zone, where the fungi form mycorrhizae that absorb nutrients, which are conducted into the roots, up the trunk, and into to the crown of the tree, perhaps into the squirrel's own nest tree. The trees, in turn, help to create the forest, which may be part of a catchment basin that collects and stores water required for human consumption—drinking, irrigation, or electricity, which says nothing of wood products, foods, and medicines used as products to sustain the material comfort of society—all thanks to the help of forest-dwelling rodents and fungi that touch the quality of our lives in so many, many ways.


  1. S. Pyare and W. S. Longland. Mechanisms of truffle detection by northern flying squirrels. Canadian Journal of Zoology, 79 (2001):1007-15.

  2. (1) Chris Maser, James M. Trappe, and Ronald A. Nussbaum. Fungal-small mammal interrelationships with emphasis on Oregon coniferous forests. Ecology, 59 (1978):779-809; (2) Chris Maser, James M. Trappe, and Douglas Ure. Implications of small mammal mycophagy to the management of western coniferous forests. Transactions of the 43rd North American Wildlife and Natural Resources Conference, (1978):78-88; (3) Daniel L. Luoma, James M. Trappe, Andrew W. Claridge, and others. Relationships Among Fungi and Small Mammals in Forested Ecosystems. Chapter 10. In: Cynthia J. Zable and Robert G. Anthony, (editors). Mammalian Community Dynamics in Coniferous Forests of Western North America: Management and Conservation. Cambridge University Press, New York, NY. 2002; (4) J. G. P. Calvo, Zane Maser, and Chris Maser. A Note on Fungi in Small Mammals from the Nothofagus Forest in Argentina. Great Basin Naturalist, 49 (1989):618-620; (5) Andrew W. Claridge, M.T. Tranton, and R.B. Cunningham. Hypogeal Fungi in the Diet of the Long-nosed Potoroo (Potorous tridactylus) in Mixed-species and Regrowth Eucalypt Stands in South-eastern Australia. Wildlife Research, 20 (1993):321-337; and (6) Chris Maser, Andrew W. Claridge, and James M. Trappe. Trees, Truffles, and Beasts:  How Forests Function. Rutgers University Press, New Brunswick, NJ. 2008. 288 pp.

  3. The general discussion of mycorrhizae is based on:  (1) David Read. The ties that bind. Nature, 388 (1997):517-518; (2) David Read. Plants on the web. Nature, 396 (1998):22-23; (3)Marcel G. A. van der Heijden, John N. Klironomos, Margot Ursic, and others. Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity. Nature, 396 (1998):69-72; and (4)Anna S. Marsh, John A. Arnone, Bernard T. Bormann, and John C. Gordon. The role of Equisetum in nutrient cycling in an Alaskan shrub wetland. Journal of Ecology, 88 (2000):999-1011.

  4. This paragraph is based on:  (1) Michael P. Amaranthus, Debbie Page-Dumroese, Al Harvey, and others. Soil Compaction and Organic Matter Affect Conifer Seedling Nonmycorrhizal and Mycorrhizal Root Tip Abundance and Diversity. Research Paper PNW-RP-494. U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station, Portland, OR. 1966. 12 pp; (2) Daniel L. Luoma. Monitoring of Fungal Diversity at the Siskiyou Integrated Research Site, with Special Reference to the Survey and Manage Species Arcangeliella camphorata (Singer & Smith) Pegler & Young. Unpublished Final Report, Order #43-0M00-0-9008. Department of Forest Science, Oregon State University, Corvallis, OR. 2001. 18 manuscript pages; and (3) J.E. Smith, R. Molina, M.M.P. Huso, and others. Species richness, abundance, and composition of hypogeous and epigeous ectomycorrhizal fungal sporocarps in young, rotation-age, and old-growth stands of Douglas-fir (Pseudotsuga menziesii) in the Cascade Range of Oregon, U.S.A. Canadian Journal of Botany 80 (2002):186-204.

  5. Peter Högberg, Anders Nordgren, Nina Buchnamm, and others. Large-scale forest girdling shows that current photosynthesis drives soil respiration. Nature, 411 (2001):789-792.

  6. (1) C.Y. Li, Chris Maser, and Harlan Fay. Initial Survey of Acetylene Reduction and Selected Mircoorganisms in the Feces of 19 Species of Mammals. Great Basin Naturalist, 46 (1986):646-650 and (2) C.Y. Li, Chris Maser, Zane Maser, and Burce Caldwell. Role of Three Rodents in Forest Nitrogen Fixation in Western Oregon:  Another Aspect of Mammal-Mycorrhizal Fungus-Tree Mutualism. Great Basin Naturalist, 46 (1986):411-414.

  7. (1) Eric D. Forsman, E. Charles Meslow, and Howard M. Wight. Distribution and Biology of the Spotted Owl in Oregon. Wildlife Monographs, 87 (1984):1-64; (2) Jack Ward Thomas, Eric D. Forsman, Joseph B. Lint, and others. A Conservation Strategy for the Northern Spotted Owl: Report of the Interagency Scientific Committee to Address the Conservation of the Northern Spotted Owl. U.S. Government Printing Office, Washington, D.C. 1990. 427 pp; (3) Andrew B. Carey, Janice A. Reid, and Scott P. Horton. Spotted Owl Home Range and Habitat Use in Southern Oregon Coast Ranges. Journal of Wildlife Management, 54 (1990):11-17; and (4) Chris Maser. Mammals of the Pacific Northwest: From the Coast to the High Cascade Mountains. Oregon State University Press, Corvallis, OR. 1998. 406 pp.

  8. (1) Zane Maser, Chris Maser, and James M. Trappe. Food Habits of the Northern Flying Squirrel (Glaucomys sabrinus) in Oregon. Canadian Journal of Zoology, 63 (1985):1084-1088 and (2) Chris Maser, Zane Maser, Joseph W. Witt, and Gary Hunt. The Northern Flying Squirrel:  A Mycophagist in Southwestern Oregon. Canadian Journal of Zoology, 64 (1986):2086-2089.

  9. The preceding three paragraphs are based on:  (1) C. Y. Li and Michael A. Castellano. Nitrogen-fixing bacteria isolated from within sporocarps of three ectomycorrhizal fungi. page 164. In: Randolph Molina (Editor). Proceedings 6th North American Conference on Mycorrhiza. Forest Research Laboratory, Oregon State Univeristy, Corvallis, OR. 1985; (2) C. Y. Li, Chris Maser, Zane Maser, and Bruce A. Caldwell. Role of three rodents in forest nitrogen fixation in western Oregon:  another aspect of mammal-mycorrhizal fungus-tree mutualism. Great Basin Naturalist, 46 (1986):411-414; and (3) C. Y. Li and Chris Maser. New and Modified Techniques for Studying Nitrogen-Fixing Bacteria in Small Mammal Droppings. USDA Forest Service Research Note PNW-441. Pacific Northwest Forest and Range Experiment Station, Portland, OR. 4 pp. 1986.

Photo credits

Photographs of the flying squirrels are by Jim Grace, USDA Forest Service

The root-fungus photograph is by C.P.P. Reid, USDA Forest Service

The photograph of the hyphae in the soil is by Kermit Cromack, Oregon State University

Photograph of the truffles by James M. Trappe, USDA Forest Service

The photograph of the flying-squirrel pooparoonies is by Chris Maser

©chris maser 2009. All rights reserved.

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