Global warming is permitting bark beetles to produce more generations in a season, and this climate-induced high reproductive rate is causing massive forest destruction in Alaska, northern Canada, and parts of the western United States; the extended warm season is allowing bark beetles to attack en masse and overwhelm the trees’ defenses, taking more and more healthy trees that would otherwise be immune to their attacks.
WOOD IS ATTRACTIVE not only to beetles. One group of Hymenoptera, the order to which bees, ants, and wasps belong, are solitary rather than social insects, and their larvae feed on wood. These, the horntails (Siricidae), are large, robust wasps who get their name from the straight, stiff ovipositor sheath at the female’s tail end. When ready to lay eggs in a log, she withdraws her needlelike ovipositor from this sheath and points it straight down, at right angles to the sheath and her body. She then drives the hollow ovipositor almost its whole length into the solid wood. If she senses that the wood is suitable, she propels an egg down into the wood, along with fungi and a mucous secretion that promotes fungal growth and helps the larva digest the wood. Like the wood-boring beetle larva, the wasp larva creates a burrow behind itself as it chews through the softening wood.
An insect larva is relatively safe from most predators and parasites deep inside still-solid wood. But one type of ichneumon wasp, Megarhyssa ichneumon, has specialized to parasitize horntail larvae. The female of this species has an ovipositor that is up to 10 centimeters long—longer than her entire body (in contrast to the 1-centimeter-long horntail ovipositor). In flight it looks like a long black thread dragging along behind her. This “thread,” however, consists not only of the ovipositor but also of two other small threads wrapped around it, forming a protective sheath. Unlike the horntail’s ovipositor, this one is very flexible, yet the wasp can drive it several centimeters into solid wood and propel an egg all the way through it and into a horntail wasp larva.
Unlike the horntails, the Megarhyssa female cannot use brute force to drive the whiplike, flexible ovipositor. She has to take it out of its protective sheath and loop it over her back in a big bow in order to get the tip of it to even touch the wood, in what looks like an acrobatic maneuver. Her egg-laying task is long and dangerous (while so engaged she is effectively attached to the wood and cannot quickly withdraw, occasionally getting stuck there), so it’s unlikely that she would invest in it without somehow knowing where her target is located deep within the wood. How she finds that out is not known.
WHEN THE WOOD-BORING beetles and horntails emerge from a recently dead tree to complete their life cycle, they leave behind a new habitat that is suitable for numerous other insects. The galleries in the wood made by beetle and wasp larvae are used by a variety of insects. First, those beetles that feed on fungi rush in, followed by specialized predators such as Colydium lineola, which feed on the first colonizers. One group of bark beetle predators, the clerids, colorfully decorated in patterns of red, orange, white, and black, have thick heads that anchor the powerful mandible muscles needed to crunch their beetle prey. (In contrast, note the tiny head of a pollen-eating beetle.)
Eventually the bark starts to loosen from the tree, creating even more habitat for other insects and spiders, who find food and shelter there. These colonizers then attract their own predators, such as the red flat bark beetle, Cucujus clavipes.
As long as wood stays dry, it resists decay, yet the larvae of some specialist beetles do eat dry wood, including various species of tiny brown beetles known as powderpost (Lyctinae; Bostrichidae), and deathwatch or furniture beetles (Anobiidae). The larvae process the wood for the small amounts of nutritious starch in it; their burrows, which may be only 1 to 3 millimeters wide, spill powdery sawdust as they chew. Moisture can now enter the wood through these pores, allowing decay to proceed more rapidly.
Increasingly softened by burrows and then fungal and bacterial decay, the wood eventually becomes suitable for the larvae of the large spike-edge long-horned beetles. Meanwhile, the fungal undertakers produce fruiting bodies on which certain beetles feed.
In the tropics, moist decaying wood and other plant matter become the habitat of scarabs, including the world’s largest beetles, the South American Hercules beetle, Dynastes, and the African Goliath beetle, Goliathus giganteus. The Goliath lives up to its name; it may be up to 10 centimeters long, and the larvae weigh up to 120 grams, about ten times the weight of a warbler. Here in the northeastern forest, the only wood-eating scarab beetle I am familiar with is the all-black Osmoderma scabra, whose fat white larvae I find routinely in almost any hardwood tree that has damp decaying wood. This larva is partially transparent, so you can see the dark wood mush in its digestive tract. Farther south one would also find the larvae of the mostly tropical fruit and flower chafers in a subfamily of scarabs, the Cetoniinae, of which the Goliath beetle is a member. Worldwide, there are an estimated four thousand cetoniine species. Most are tropical and many of them are so far undescribed.
The cetoniines’ size range is enormous, as is the spectacular variety of their bold, bright, usually metallic markings. Although all the larvae are white and live on decaying vegetation, the largest ones primarily eat rotting wood, while the adults feed on decaying fruit. The adults of intermediate-sized beetles of this group eat flower petals, and the smallest ones eat pollen.
Fruit and flower scarabs. All but one of these cetoniines are from East Africa; the South American elephant beetle, Megasoma alphas (lower left), is shown in four views, male and female. The two giant scarabs, the rhinoceros and the African Goliath beetle, Goliath giganteus (top right), are fruit eaters as adults, while the others are pollinators mostly of trees. All of the larvae feed on decaying wood and other dead vegetation. Flower scarabs are renowned for their brilliant colors. These are resplendent in metallic greens, yellows, and rich browns.
Because of the adults’ feeding patterns, the cetoniines are major tropical pollinators; the flowers of many plants are specifically adapted to be pollinated by them. In a recent study of two orchid species in South Africa that offer no food rewards (no nectar or pollen is available to feed visitors to the flowers), it was discovered that the plants did not set fruit if cetoniine beetles did not visit. The beetles apparently visited (and pollinated) the orchid flowers because they mimicked those of a plant that did offer food. Thus both plant species must exist in the habitat at the same time in order for the beetles and the plants to live there. I recall with pleasure seeing and hearing flower scarabs zooming around flowering acacia trees on the southern African savanna. Near Dar es Salaam they were flying around, and presumably pollinating, blooming mango trees, and I found their large white grubs in rotting tree trunks on the ground. Thus, in the continuing processes of death turning to life, some of the beetles that help in tree undertaking also play a crucial role as surrogate reproductive organs in a direct interactive living system. These interrelationships exist in all biological communities but are seldom so direct and simple.
ONCE SEEDED INTO the dead tree, fungi account for most of the decomposition of wood. Indeed, according to the mycologist Paul Stamets, fungi can “save the world.” Their role in our lives includes the provision of food and antibiotics and the neutralization (as well as production) of toxins. But I suspect that all of these services dwarf their role in decomposing wood, which helps build soil.
A fungus assumes many forms. Sometimes you see it, but most of the time you don’t. After extracting sufficient nutrients from a tree, the fungus converts them for its own reproduction, growing what may be a highly visible and even spectacular fruiting body. These fungal reproductive organs, which produce and spread spores, are usually known as mushrooms, conks, or brackets. The main body of the fungus that produces these structures is a threadlike net growth called, in aggregate, the mycelium, which can grow for many years in a tree trunk before producing fruiting bodies in response to particular conditions of temperature and moisture. Tubes or lamellae on the ventral surfaces of the fruiting bodies then rel
ease millions of spores, which travel mostly by the wind. After landing on a suitable place, a spore germinates and produces a new mycelial web. Two mycelia of opposite mating types may meet and join to produce sexual spores.
Most mushrooms last only a few days before decaying or being eaten, often by gnat (fly) larvae. However, some mushrooms—conks—last for many years, each year adding a new spore-bearing layer at the bottom. The age of other fungi, such as some that grow on soil, can be measured by their periodic production of fruiting bodies. On the lawn of a neighbor of ours, a flush of mushrooms comes up some years, always in a circle, which year by year becomes larger. After a week or so the mushrooms are rotted and gone, but the mycelial body that produces them remains underground, to send up its fruiting bodies in subsequent summers.
Fungi living on trees are similarly hidden most of the time. Take the maple-decaying fungus Armillaria mellea, which forms a white fungal mat under the bark of the tree. This fungus is bioluminescent—it glows in the dark—but of course you cannot normally see this from the outside. I have observed only the later stage of the fungus, when the bark is dead and loosened. You then see a dense network of black “shoestring” fungal organs called rhizomorphs (root forms), which are visible for months or years. A third form of A. mellea is its fruiting bodies, little brown mushrooms that produce the reproductive spores. These honey mushrooms come up at the base of the infected tree, shedding their spores and deteriorating in only a week.
Edible mushrooms are a gourmand’s delight, and when my family was living in the Hahnheide forest, we indulged. We were scavengers living on scavengers, mainly the ones we called Rehfusschen and Steinpilzen, along with many others whose names I no longer remember. One mushroom that is currently a big hit in the United States is Lentinula edodes, which has been cultivated in Asia for thousands of years. It is generally known by its Japanese name, shiitake (from shi, meaning “oak,” the tree on which it grows). Shiitake mushrooms are prized for their taste and for their reputed enhancement of immune function and high protein content. They are cultivated on freshly cut—but not too fresh—logs and are currently being grown on oak and maple logs even in my neighborhoods in Vermont and Maine. Shiitake “spawn” is commercially available, and I plan on using it to recycle the sugar maple logs that I must weed out of my maple grove. Instead of waiting for beetle larvae to inject the fungal spawn into their logs, growers make cuts with a chain saw, rub the spawn in, and then seal each inoculum in with melted wax.
Here in New England, several other mushrooms that live on freshly dead or dying hardwood trees, especially oaks, are sought as food by many people. We scour the woods every late summer and fall for the sulfur shelf (Laetiporus sulphureus), also called chicken-of-the-woods, because it tastes like . . . chicken. The fruiting bodies produced by one fungus in a flush of “fruiting” may weigh more than fifty pounds. Another mushroom, the hen- of-the-woods (Grifola frondosa), whose fruiting bodies are almost as big, is delicious, as is a third disposer of wood, the oyster mushroom (Pleurotus ostreatus), which grows on dead deciduous trees, especially beech. The fruiting bodies of these fungi may tickle our palate and serve as a vital food resource for many animals, but their unseen forms do far more service as tree undertakers.
Fruiting bodies of some of the many fungi that recycle dead wood. Colors range from brilliant red, yellow, and green to black and brown.
TREE UNDERTAKING PROCEEDS at a glacially slow pace in comparison to that of animals, but on their way to death, before the process is complete, many trees supply life. In this transitional phase, the body of the tree serves important ecofunctions even before it falls to the ground.
Even after a dead tree starts to decay, it may remain standing for decades. These standing dead trees are a prime indicator of the health of a forest because, well—because they are an indicator of its life. More than a third of the bird species in a forest depend on standing dead trees, both for their food if they eat beetle grubs and for nesting places, because the partial decay makes nest-hole construction possible. Without this interim stage, most woodpeckers would not be able to exist. Very few can hammer a nest hole solely out of solid live wood (although they may hammer through an outer solid layer to get to the softer, partially fungus-softened wood beneath it, where they will make their main nest cavity).
One of the more explicit examples of this phenomenon involves the tinder polypore (also called hoof fungus), Fomes fomentarius, and the false tinder mushroom, Phellinus igniarius, which grow in old aspen (poplar) trees. The tinder mushroom has been used since ancient times to start fires from sparks (Ötzi, the 5,300-year-old Iceman discovered in 1991 in an Italian glacier, carried it). Now it serves mainly sapsuckers.
The late Lawrence Kilham, a physician and ornithologist of note, made a study of the yellow-bellied sapsuckers near his home in Lyme, New Hampshire. He determined that the sapsuckers have an apparent search image for the mature fruiting bodies of the tinder mushroom. The fungus grows in the aspens’ heartwood, leaving the sapwood intact as a tough shell, while its fruiting bodies are on the outside, attached to the bark. These fruiting bodies lead the birds to the trees in which they excavate their nest holes. Unlike many other woodpeckers, the sapsucker doesn’t excavate wood to get insect larvae. Instead, it makes sap licks by puncturing the bark of maple, birch, basswood, oak, and other trees. Perhaps these woodpeckers prefer softened-up poplar trees for their nest holes because they are unwilling or unable to excavate solid wood.
Before knowing of Kilham’s 1971 study, I had confirmed his results; I had wondered if these woodpeckers had a preference for aspen trees with the fungus, because I always saw the fungus whenever I found a sapsucker nest hole. I surveyed the trees in my Vermont neighborhood, where there are many poplars. I examined 176 poplars along our road. Of these, 12 had Fomes conks, and 5 of these 12 had sapsucker holes; there were no sapsucker holes in any of the aspens that didn’t have the fungus. Sapsuckers seem to deliberately choose as their nest tree one that has a soft core. Perhaps they identify it by the fruiting bodies of the fungus. The other local woodpeckers also use poplars, but they do not have a preference for them or for trees with the fungus. Downy and hairy woodpeckers appear to choose a high, still-solid dead maple stub for excavating a nest hole in which to raise their young. However, in the fall they often excavate a much more rotten, lower stub as an overnighting cavity. Two holes made by hairy woodpeckers that I recently found were in a long-dead balsam fir and in a birch softened by the parchment fungus, Stereum rugosum. Two downy winter shelters were made about 2 meters above the ground in sugar maples taken by the turkey-tail fungus, Trametes versicolor.
Despite these preferences, if they are available, woodpeckers are flexible, though not necessarily with the best results. My cabin site in Maine has few poplars, and I once found a sapsucker pair who had made their nest hole in a dead maple stub. I made this discovery inadvertently after the stub broke off in a storm, spilling the still-naked babies onto the ground, where I found them dead.
Progression of a fungus in a live sugar maple tree that was being shaded and would likely soon die. The fungus probably entered near the base (lower left) where live tissue had grown around a physical injury, leaving three exterior wounds. The lighter heartwood is dead, rotting (though still solid) tissue; the black areas are where the tree is fighting the infection. The cross-sections show that the fungus has extended up the tree to a little over fifteen feet.
The babies of all the woodpecker species I know are very noisy, making a nearly constant raspy din. Possibly the noise helps motivate their parents to continually feed them. It must also attract predators, although for the most part the young woodpeckers remain safe inside their fortress, the solid tree. Kilham found, however, that raccoons could sometimes extract young sapsuckers from their nest, provided they could break into the nest hole. Raccoons had little success reaching the noisy sapsucker young in aspens, which have the hard shell of sapwood on the outside, but they could break into nests when
the woodpeckers chose dead maples, birches, or beeches—trees that lack the tough sapwood shell.
An aspen infected with false tinder fungus can be a valuable resource for yellow-bellied sapsuckers. Once they find one, they may return to it for several years in succession to nest; sapsuckers are the only woodpecker species that returns to the same tree (but, like other woodpeckers, it makes a new nest hole each time). A sort of “tenement house” results if the birds return for six or seven years. Some of the empty holes are recycled as homes by northern flying squirrels and also by nuthatches, titmice, and chickadees.
Hairy, downy, and pileated woodpeckers preferentially choose hardwood trees for their nest holes, using parts of the tree that are dead but still solid (at least on the outside), and usually very high up in the tree. Unlike the sapsuckers, these woodpeckers do not migrate south in the winter. The hairy and downy woodpeckers are the only birds I know in New England who build themselves a shelter in which to stay overnight in the winter. In October they excavate cavities similar to nest holes, except that they are almost always in decayed, easy-to-excavate tree stumps. In turn, in and at the edges of forests, three owl species, as well as wood ducks, mergansers, nuthatches, crested flycatchers, tree swallows, bluebirds, and sparrow hawks (American kestrels), nest in one or another kind of woodpecker hole. Black-capped and boreal chickadees sometimes excavate nest holes, but their tiny bills are much weaker than the sapsuckers’; they cannot penetrate solid wood. They seek out wood that is quite decayed. Brown creepers do not excavate nest holes, but they also require dead trees because they build their nests under hanging bark, mainly on dead conifers. In the tropics, almost all parrots, hornbills, barbets, and many flycatchers depend on holes in trees for their nests.
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