18
WOODPECKERS, WOOD DECAY FUNGI,
AND FOREST HEALTH

On meadows, where were wont to camp
White mushrooms, rosy gilled,
At dawn we gathered, dewy-damp,
Until the basket filled!

ANON, REMINISCENCES OF CHILDHOOD
FROM SONGS OF LUCILLA, 1901

Henry has been an avid duck hunter since he was a young teen and, as a man, he married the right woman with all the skills needed to make duck and sauerkraut, his favorite dish. Today, decades later, he walks quietly through prime wood duck nesting habitat with not a bird in sight. The mature hemlocks overhang the still river water on this late spring day. As he seeks his quarry, his favorite shotgun lies cradled in his arms, specially hand-loaded cartridges in both barrels. He has gone light on the powder, but packed in with the wadding is a special ingredient that makes this hunt unique. Carefully inserted into the hollow shotgun slug is a softwood dowel colonized with the cultivated mycelium of the red-belted polypore, Fomitopsis pinicola. There across the river is his prey, partially hidden in the gloom of the dense overstory but exposed by the opening over the river. From seventy-five feet he takes careful aim and lets fly. When the smoke clears he can clearly see the gash on the trunk of the large hemlock where his payload has shredded bark, imbedding slugs into the pale softwood cambium some thirty feet above the river surface. His hope and plan is that by forcibly inoculating the tree with the vegetative “seed” of the fungus, sometime in the next five to fifteen years, his small investment in time and shotgun ammo will turn into a decaying mature hemlock sprouting red-belted conks and containing several new woodpecker cavities and their associated tenants. On this particular day he is not shooting to kill a duck, but aiming to create habitat for future generations of woodpeckers, owls, flying squirrels, cavity-nesting ducks, and their kin. But this story starts in another place in time and space.

The Changing Forest Landscape

We have changed the face of this planet through the fruits of our labor, our burgeoning numbers, and our need for homes, food, and stuff: lots of stuff. From almost the first moment Europeans landed on the shores of the Americas, settlers began harvesting the seemingly endless forest that marched inland from every shore. Trees provided fuel for our fires and timber for ships, homes, and towns. They also posed an almost impenetrable obstacle to farming, grazing livestock, and westward expansion in the early days of European colonization. For the next 300 years, as settlers explored, conquered, colonized, and otherwise domesticated much of this great land, the forests became less like a wilderness and more like a renewable resource to be cut, grown, and cut again. Most forested regions of America have been through a number of tree harvesting cycles. The virgin forests are but a dream and the remaining timber is younger—a second, third, or fourth growth following successive clearing operations. Forest management has become a science and a business, designed to maximize marketable timber harvesting and to reduce the time required to mature a generation of trees to marketable size. Over the past century, this timber management strategy led to an increasingly narrow mix of tree species and a movement toward stands of trees all the same age and size. We have actively, even aggressively, managed many forests for the production of softwood conifer species, the most valuable for lumber or pulp production—at the expense of deciduous trees like oaks, beech, and other nut trees. In recent years, foresters and ecologists have found that a decreasing diversity of tree age and species brings with it a decreased diversity and populations of many animals that rely on a mix of tree species and the presence of large snags and mature old trees.

A snag is a dead standing tree, generally defined as at least 8 inches in diameter (though often much larger) and of sufficient height to stand well above the forest floor. Large snags and living trees of large girth now are recognized as a vital part of a healthy forest community because they provide food, shelter, and nesting sites for the birds and mammals that have evolved to live in tree cavities. These include birds that are able to create their own cavities (known as primary cavity nesters or PCNs) such as large woodpeckers and flickers. They also encompass other birds, mammals, and insects that move into abandoned cavities made by woodpeckers and naturally occurring cavities (called secondary cavity nesters, SCNs). One other group of important cavity creators or cavity preparers is the rarely acknowledged wood-degrading, heart-rot fungi.

The Role of Heart-Rot Fungi

Heart-rotters are a group of fungi that specialize in the breakdown of the dead wood fibers that make up the bulk of any tree trunk or sizable branch. The fungi typically are introduced into the living tree through insect activity or an injury that disrupts the bark, exposing the softwood cambium or the heartwood itself. Such an injury can happen when a branch breaks off in high wind or a falling branch or tree strikes the trunk. It also can happen through the action of any number of animal activities, including the foraging of insects, woodpeckers, porcupines, and beavers. Most fungi spread through the dispersal of airborne spores, which, when they land and germinate on the exposed wood, are the beginning of the fungal invasion of the tree. A fungus invades its host by literally eating its way through the wood as its mycelium grows along the wood fibers. The wood colonized by the fungus becomes punky, losing both density and structural integrity. In a standing trunk, the growth occurs more rapidly in a vertical direction than it does horizontally because the mycelial growth faces little obstacle when growing in the same direction as the wood fibers.

In a large living tree, a heart-rot fungus often is able to grow within the trunk for years without any noticeable effect, leaving a sturdy living layer of softwood cambium. The invasion becomes apparent when we see a fruiting body form on the trunk or nearby ground or when the tree is cut or falls in a storm, exposing the rotten or hollow center. A tree often continues growing for years with the fungal mycelium slowly softening and hollowing the center without visible damage or visibly slowing the growth of the tree. Hollow trunks occur only in trees living with a heart-rot fungus where, over time, the softening heartwood collapses. In most cases, this process takes years.

Other wood decay fungi begin their work following the death of a tree and start the process of softening the wood from the bark inward as they extract nutrients by breaking down the wood fibers. A dead tree, without the antifungal defenses present in living tissue, rots much more quickly than a living tree. On a large snag, there typically are several or even many different species of fungi working in concert or in different regions feeding on the tree at any moment in time. Different wood-rotting species grow better in different microhabitats created by variations in sun exposure or shade, near the ground just under the bark, or deeper in the true heartwood. Some of the better-known heart-rot fungi include the red-belted polypore, the artist’s conk, turkey tails, the various varnished conks, the tinder conk, and more fleshy fungi such as hen-of-the-woods and the sulfur shelf. A healthy forest contains scores of different species of wood rot fungi.

Deadwoodology, the study of the ecology of deadwood, is a thriving research field in which wood-decaying fungi play a major role as vigorous ecosystem engineers. The action of wood-decaying fungi increases the availability of resources such as nutrients and humus for plants and other fungi, and feeding and nesting sites for other living organisms including insects, birds, and mammals.¹

Of Woodpeckers and Fungi

Most people know that woodpeckers and their relatives make their nests in cavities in the trunks of trees; fewer are aware that it is an unusually strong woodpecker that is capable of actually excavating a cavity in hard virgin living wood. Most primary cavity nesters seek dead trees or living trees whose wood has already been softened by colonizing fungi. The birds locate and make their nest holes in living trees displaying the fruiting bodies of a heart-rot fungus or ones infected with the fungus but not yet sporting a fruiting body. This is the case with quaking aspen infected with the aspen heart-rot fungus, Phellinus tremulae, which is commonly found on larger mature aspen. In a study of two sites in Wyoming, 71 percent of aspen with cavities created by sapsuckers had visible conks of P. tremulae, though less than 10 percent of all aspen in the area showed conks.² Other studies showed similar though somewhat lower results with other bird species. Cavity-excavating birds choose trees with fungal invasion as a means of finding softer and more easily excavated sites.

One bird famous for the vigor and impact of its ecosystem engineering is the pileated woodpecker, the largest woodpecker in North America. Detritus from the feeding and nest-building activity of these birds litters the ground around the base of the trees, and the noise created by their loud excavations proclaims the presence of the otherwise shy birds. This woodpecker is referred to as a “keystone species,” one whose actions modify the forest habitat to such an extent that they single-handedly increase the diversity of species living in the environment. Most large species of woodpeckers and flickers also significantly affect the forest habitat, but the pileated makes the action of its lesser kin seem puny in comparison. The impacts of the pileated woodpecker that have earned it the keystone species status include:

• The acceleration of the process of wood decay and the associated nutrient recycling through:

• Opening up the bark, sapwood, and even the heartwood in living trees through the activities of feeding and cavity construction, resulting in the wood’s infection by decay fungi and insects

• Transfer of wood decay spores and mycelium from tree to tree in its beak and mouth

• Opening up bark and wood surfaces for the exploration and feeding of other species of woodpeckers, birds, and insects

• Creating resting, roosting, and nesting dens for other birds and animals

• Helping to mediate insect outbreaks through feeding on larvae and adults³

In some forests, deciduous trees are the preferred choice for feeding and nesting of keystone woodpeckers. Studies of aspen, Populus tremuloides, in the western United States have shown that a number of bird species rely on the tree for roosting and nesting sites and at least one, the red-naped sapsucker, is a primary or obligate aspen nester. Across the range of aspen, the aspen heart-rot fungus, Phellinus tremulae, infects the heartwood of mature aspen producing distinctive fruiting conks at the site of old branch stumps, and studies have shown that several species of sapsuckers prefer aspens as nest sites and appear to seek out trees where P. tremulae is fruiting. The fungus typically attacks older living aspen and rots the heartwood while the sapwood remains alive and intact. In one study plot in Wyoming, the average age of aspen with woodpecker cavities was 115 years. Researchers have hypothesized that the birds locate trees with heart rot either by noting the presence of fruiting bodies or by noting the difference in resonance of hollow versus solid trunks when pecked.⁴

Though the cavities in aspen are made by various species of woodpeckers, sapsuckers, and flickers, they then are used by a number of secondary cavity nesters, including chickadees, bluebirds, and smaller woodpeckers as well as birds that require trees of a larger diameter including barn, barred, and screech owls, and even wood ducks and buffleheads. The mammals that use large cavities include squirrels, opossums, raccoons, martens, and fishers.⁵

Big brown bats and silver-eared bats appear to prefer cavities in aspen to several other available tree species as a choice daytime roost.^{6, 7} The living trees offer firm sapwood for roosting and are five degrees cooler than conifers in the heat of summer. Other species of bats have been long associated with tree cavity roosts, as well. All forest-roosting bats are affected by the loss of large-diameter snags and old-growth stands. Some researchers recommend the preservation and restoration of cavity-promoting habitats as a management strategy for ensuring adequate populations of insectivorous bats in forest habitats.⁸

Managing Forests for Cavity Nesters

Decades of forest management practices that recommended the removal of older snags and harvest of wind-downed timber sites, along with other human interventions that remove large, old trees, have severely reduced the optimum habitat for cavity-nesting birds. This habitat loss has reduced populations of primary and secondary cavity nesters and it is thought that these management practices are responsible for dangerously reducing populations of species such as the endangered red-cockaded woodpecker in Texas, the once-assumed extinct ivory-billed woodpecker, and several primary and secondary cavity nesters known from old-growth forests in the Northwest, chief among them the spotted owl. With the disappearance of large-diameter snags and living trees, serious efforts are being made to determine strategies to increase the suitable habitat for cavity nesters.

A number of strategies have been suggested, attempted, and practiced. One challenge, however, is that there is an immediate short-term need for developed snags in many areas where a severe reduction in snags has resulted from decades of past management practices. In the absence of direct manipulation, it will be decades before the maturing forest naturally creates the needed snags sufficient to support optimal cavity users. Suggested short-term strategies include topping live mature trees just below the first main branches using saws or explosives, which is expensive in time, resources, and money; limbing or otherwise wounding trees to create openings for heart-rot fungal invasion (but that’s chancy and requires significant time to produce results); girdling mature trees with either chain saws or fire to kill the tree; and artificial inoculation of live or killed trees with selected hear-rot fungi. This last method is the strategy described in the fictional account of my grandfather, the duck hunter, that opened this story.

In a 2004 paper in the Western Journal of Applied Forestry, researchers reported on the results of artificial inoculation of conifers with two heart-rot fungi in forests located in the Coast Range of Oregon. They employed a hitherto untried delivery mechanism that was controversial and very cost effective. The vegetative mycelium of pine conk (Phellinus pini) and the rose conk (Fomitopsis cajanderi) were grown out onto small wood dowels or sawdust. This “spawn” was then delivered into the trunk of the tree using firearms. The dowels were fitted into specially made hollow slugs for a 0.45-70 rifle, and the sawdust spawn was packed behind 12-gauge shotgun slugs. The spawn was fired into carefully selected sites on live trees or recently artificially topped trees. In a five-year follow-up, all of the topped trees were dead and almost all of the trees showed the presence of decay and fruiting bodies of the target fungi and other species. Almost half of the trees showed evidence of use by primary cavity-nesting species of birds and other wildlife. The live inoculated trees showed little evidence of fungal growth and no sign of wildlife use, but samples of wood collected around the injury site showed that in most cases, fungal invasion was under way. The researchers concluded that topping (killing) a tree was a more rapid method for creating a cavity nester habitat, but use of living trees would likely be effective over a longer period of time.⁹ One difference in use of living versus topped trees is the cost. Topping involves either the risky use of chain saws high above the forest floor or, in some cases, the use of explosives to sever the tree high above the ground. Either method costs hundreds of dollars per tree versus the much cheaper and easily delivered firearm inoculation. A very realistic and more commonly used alternative to blasting a tree with a large-caliber slug involves the use of larger dowels colonized with a target fungus and tapped into predrilled holes in the trunk of a living tree. This technique has been used to increase nesting sites for the endangered red-cockaded woodpecker in the southeastern United States¹⁰ and as a general habitat restoration technique using the red-belted polypore in the Pacific Northwest.¹¹

Certainly the use of explosives to create snags may be needless overkill, but the idea of infecting a living tree with a fungus that will lead to its eventual weakening and death has not been met with open criticism. The intentional use of fungi to perform the role they are naturally suited for is an effective way to undo the damage of narrowly focused forest management practices. Though the opening vignette about my grandfather Henry is fictionalized, the activity described would be a farsighted and effective way for hunters to look out for the overall health of the forest and ensure the long term availability of good habitat for their favored prey.

Man’s past intervention and management of forest environments has resulted in marked reduction in the habitat and presence of cavity nesters, a group of species of vital necessity to a healthy forest. If we can integrate recent lessons about the desirability of cavity nesters and the role of mature snags and wood-rotting fungi into wise management strategies, our positive manipulation of the forest might significantly increase the population of cavity nesters in the decades to come.