Life Everlasting

by Bernd Heinrich

  Of the many people who should be held accountable for the population collapse of the vulture crowd, most are nameless but not blameless. Yet some of us played a more central role than others, and neither ignorance nor personal justification can be a defense when it comes to heinous crimes leading to genocide, ecocide, or the extinction of species. Effects really matter, and any chemical that is synthesized—meaning that it has never before existed as a component of an ecosystem—should be assumed harmful on an ecosystem level until proven otherwise. This is not wild extrapolation; it is commonsense biology. And by common sense it is also obvious that “the market” as such will not solve the problem but will create and maintain it, if left to run free like a high-tech car without a steering wheel or brakes.

  III

  PLANT UNDERTAKERS

  Plants aren’t undertakers, but they are the ultimate biochemists. Except for several minor exceptions (such as Venus flytraps), they do not consume gobs of flesh or even complex organic molecules. They use water, sunshine, and a few minerals to build themselves out of carbon from carbon dioxide pulled out of the air. However, what they make from these simple beginnings can, by animal standards, become extraordinarily massive and nutritious.

  Plants are the intermediary agents in our own recycling into and back out of the soil, and we cannot understand our recycling unless we consider theirs. They are highly adapted organisms whose lives develop according to opportunities and constraints similar to those that apply to animals. Like us, they reproduce, grow, and have developed genetic codes inscribed by natural selection on deoxyribonucleic acid (DNA). I concentrate here on the recycling of trees, because they are the most visible plants and overall are perhaps also the most central to the recycling process.

  The undertaking of trees (and other plants) is so commonplace in nature that it is easy to take for granted; I admit I used to pay little attention to it. Many animals injure and kill plants, and in any ecosystem where plants live they also eventually die. This process may not be dramatic, unlike an animal carrying off another and tearing it apart in minutes. The death of a tree neither spills blood nor smells bad. Instead, trees may be nibbled on for years by insects, and after they die, through the agency of beetles, fungi, and bacteria, they slowly and unobtrusively disintegrate into the soil, a recycling that makes the rest of life possible and is life as well. The process occurs on a vast scale; were it not for the tree undertakers, the forest would in a few years become an impenetrable tangle of dead wood, soon stopping all plant growth. I hadn’t seen much normal tree undertaking in my forest, because most of the tree carcasses that I and others created were cut up and hauled away to be made into lumber, paper, or firewood. In a natural ecosystem, however, the dead trees would have been left in place.

  Trees of Life

  LIKE THE BODIES OF ANIMALS, TREES ARE PREFERENTIALLY eaten while they are still fresh. And since trees have formidable defenses against being eaten when they are alive, they are usually taken only when they are weakest or about to die. The most nutritious part of a tree, the inner bark, which is attacked first, is protected by the tough outer bark shield. But once a tree has fallen, the inner bark is usually available to be eaten for several months, while some of the wood—the framework built to raise the living part of the tree up to the light—may persist for decades.

  With the exception (to be discussed later) of possibly one fungus, the world’s largest organisms are trees, and some of them are also the oldest, a testament to their ability to resist death by the ravages of parasites and predators. By our own—animal—standards, some trees seem to live forever and may thus appear to be almost inanimate. Every species has its own maximum life span. Some, like the bristlecone pines, redwoods, and sequoias of western North America, may live several thousand years. Some of those individuals living now were already giants at the time of Christ, and they may indeed seem immortal. Most oaks can live several hundred years. The oldest white pines, red spruce, cedars, and sugar maples in my forest can live to about two hundred years. The balsam firs and gray birch may not live to fifty, and striped maples seldom make it beyond twenty. These maximum life spans, however, have almost no bearing on the actual life span of individual trees; most die young.

  The trees that we see in a forest are a small proportion of the whole; they are only the survivors and the recently dead. A healthy forest(as opposed to a plantation) has dead trees standing and lying about. But the majority of its trees died and were recycled while they were so small that we didn’t notice them. Most of my forest has dozens of trees per square foot, but few will produce more than two leaves before they die because of shading or crowding, which amount to the same thing. For the most part, trees in a forest live or die not, as often supposed, by some genetic advantage but simply by the good or bad luck of the location where they became rooted relative to others, which determines whether they win or lose in the competition for light and other necessary resources.

  The fate of soon-to-be-dead trees that reach maturity usually begins with insects, and, as with their animal-system counterparts, the “predators” are often not clearly differentiated from the “undertakers” or scavengers. However, a tree is disposed of by a crew or guild of organisms at least as diverse as those that dispose of a mouse, a moose, or an elephant. As in the animal analogue, some of the actors in the tree-undertaking process have evolved to take parts of the body while it is still healthy. Others have a chance only when the tree is weakened, and the majority have to wait until it is almost or fully dead, or even long dead. Scavengers facilitate the latter stages of disposal and bring about transitions. As with animal carcasses, a progression of scavengers attacks the carcass, one species after another, until the feeding queue ends and the tree has returned to soil.

  I WANTED TO SEE how quickly the attack on a downed tree and the subsequent recycling would take place. While building my log cabin in Maine, I axed down about sixty balsam fir, spruce, and pine trees, and I sometimes saw sawyer beetles flying in to lay their eggs on the logs even as I was chopping the limbs off. The sawyer beetles, commonly known as longhorns because of their long antennae, belong to the Cerambycidae family. The antennae of a male sawyer are about twice its body length. They are the insects’ chemical detectors, and their great length attests to their importance in detecting the scent of the beetles’ specific oviposition sites and of potential mates. The longhorns are so good at finding fresh dead trees that I had to peel the bark off every log I cut. Otherwise they would have attacked the logs by the hundreds and rendered them useless to me.

  Besides the longhorns, the jewel beetles (Buprestidae) and bark beetles (Scolytidae) lay eggs on or in bark; the larvae burrow into the cambium of the inner bark and then into the sapwood. As they bore in, they inoculate the tree with fungi, which starts the process of digesting wood, much the way bacteria hasten the decay of animal carcasses. I myself can smell conifers, and surely they can as well, but I never saw a pine sawyer beetle on a healthy vertical pine tree. So how, I wondered, were they able to home in on one I had just chopped down?

  I forgot about that question for years because I was studying the foraging behavior of ravens, not beetles. But it came back to me when, in the course of thinking about plant undertakers, I was also thinning out my sugar maple grove to give the trees more space. This time I deliberately left some white pine carcasses on the ground in the woods, and as I’ll show, some of these carcasses were not, to my surprise, visited by beetles for months. Why, or why not, and how were the beetles attracted?

  It seemed impossible to me that the beetles would sometimes arrive on the scene so quickly, since a healthy tree that is chopped or sawed down is really not properly dead as far as its tissues are concerned. It is merely condemned to be dead. The beetles that attack it must be banking on its future vulnerability, after the larvae hatch. On the other hand, I knew that jewel beetles are attracted to forest fires from perhaps hundreds of miles away, probably to get a jump on the feeding frenzy on the just-killed
trees.

  Most of these beetles cannot “hunt” in the sense of finding and then killing trees, since healthy trees effectively defend themselves. In the conifers, most famously, that defense is the exuding of sticky, turpentinic resins, similar to the defenses employed by some insects, such as stinkbugs, and even skunks. These defenses are a byproduct of the ancient arms race between tree and beetle, a race that has also spurred an intense war among beetle species. Specialization is a must for beetles. Beetle recyclers have to go after the helpless or the dead; except for some notable exceptions, they are scavengers.

  I decided to be deliberate in recording what I observed about my felled pines. On May 11, 2011, I left the trunks in a clearing by the cabin. The temperature was a balmy 60 degrees F. Expecting to see beetles flying in at any second, I waited and waited and waited—for over a month. And still I saw no pine sawyers (the most common longhorn on pine) on any of the logs. I doubted they were extinct. Far from it, as I soon learned.

  I woke up in my cabin in the middle of the hot night of July 23, startled by a large insect walking on my naked back. I jumped up and caught the first pine sawyer I had seen that year. The next night another one disturbed my slumbers. The following morning I saw one on the inside of the window, and still another came crawling up my pant leg when I sat down. I had kept the cabin door closed and the screens on, which had effectively held off the black flies and mosquitoes—and presumably the 32-millimeter-long, 8-millimeter-wide beetles as well; the beetle source had to be inside the cabin.

  It was then that I thought of the pine log, one foot high and wide, that I had been using as a chair. The year before, I had cut the log and peeled off its bark from a live white pine that had been felled by a spring storm. Inspecting it now, I found some rather large, perfectly round holes—about 8 millimeters in diameter—in its side. I counted nine holes and knew that none had been there in the previous weeks; they were most likely the exit holes of the beetles I had just caught. I sawed my chair into “cookies” and discovered tunnels going clear through the center of the foot-thick pine log. Mature longhorn larvae, pupae, and adult beetles were scattered throughout the wood, deep inside the log. The adults, however, had tunneled almost to the surface; to emerge into daylight they would have had to chew through less than a centimeter more. Apparently at this latitude, late July was the time when the beetles completed their development, which had started the summer before. This explained why I had not seen beetles arrive at my freshly cut pine trees during the last two months of spring and early summer.

  As I had predicted, from that time onward and into mid-September, pine borers started to appear at the logs I had put out during the spring, and by early August I could hear the grubs “sawing.” This sound, which might be likened to scraping, varies in frequency with the temperature—the pitch is much higher on warm days—and I heard it day and night. The product of this “sawing” appeared to be 1- to 5-millimeter-long wood fibers or splinters, known as “frass,” which accumulates on the ground in conical piles below the holes the larvae chew through the bark and deep into the wood. Sometimes the frass almost spewed from the holes, as if the logs were leaking their innards. The frass could not have passed through the beetles’ guts: I examined the gut contents of both adults and larvae and found no trace of this material. The adults had empty guts, and their tunnels, just before they exited in late July, contained only a fine, powdery, sawdustlike material. The larval guts contained a smooth creamy paste. Apparently the frass is a byproduct of the larvae’s chewing through the wood and perhaps eating part of it, the way we eat nuts and discard the shells.

  The view of a cross-section of log showing a pine sawyer larva entering the wood during its first summer (top), and in its expanded burrow in the second summer (bottom).

  A month later, in early September, I used my chain saw to gain a view into the pine logs to trace the progress of the grubs. In early August the first larvae that hatched from the eggs deposited by the adult beetles in midsummer had begun making their feeding burrows at the interface of the inner bark and the sapwood. Now, however, there were no larvae under the bark; they had all burrowed deep into the logs. The larvae, which I had often seen in logs during the winter, would pupate there the next spring and then metamorphose to adult beetles and emerge in July or August. I hadn’t answered my question regarding how beetles arrived so quickly at downed trees, but I had found out why they were (usually) not around until late summer.

  Adult pine sawyer beetle after emerging from the tree, next to the exit hole it has chewed to emerge. For scale is the larva’s entrance hole from the previous summer, where it entered the wood after feeding under the bark.

  Temperature is perhaps the main variable affecting beetle emergence. Beetles can emerge even in mid-winter, I found out to my surprise one February 1st. Outside temperatures had been at or below 0°F, and on this day I had heated my cabin from its normal 30–50°F to a balmy 75°F. Within hours many beetles started arriving at one of my upstairs windows. Within a day I had collected 353 of them! They were all bark beetles of a species that to my naked eye looked like black specks. None were longer than 2 millimeters or wider than 0.4 millimeters. Their source was the legs of a table I had made in the fall, of a dying white birch tree. I had left the bark on. The volume of the 353 beetles came to one level teaspoon.

  ALMOST EVERY SPECIES of wood-boring beetle leaves behind distinctive “tracks” as it burrows through the wood, and each species uses particular species of trees. I went to one of the “healthiest” forests I could think of, the William O. Douglas Wilderness, adjacent to Mount Rainier, not far from the Pacific coast, and hiked along the Yakima Indian Trail under never-logged giant cedars and Douglas firs. I saw living firs of all ages and dead ones in all stages of moldering back into the soil. I peeled bark from one of the recently fallen giants and found hardly a square inch that was not beautifully patterned by beetle tracks—burrows inscribed on the inner bark with a corresponding image on the underside of the wood. They were like the tracks left by the pine borer larvae in Maine, though most of these in the Douglas firs were made not by long-horned beetles but primarily by bark beetles (family Scolytidae), which are generally tiny and inconspicuous but potentially devastatingly destructive. I find many of these beetles in my woods in Maine in trees that are about to die.

  The tracks of bark beetles and their larvae make beautiful tattoolike patterns on the wood surface beneath the bark. A recently felled American white ash tree in my woods showed prominent, almost straight lines etched across the grain of the wood—the lines would be horizontal in a standing tree. Each line was an inch or two long. Smaller grooves, largely aligned with the grain, abutted the larger groove at right angles on both sides. Forty to sixty of these vertical tunnels, each one excavated by one larva, radiated from both sides of the central horizontal line. A piece of bark I peeled off a dead balsam fir only a few steps from the ash was being processed by a different species of bark beetle. This one, instead of leaving one main horizontal line, leaves a repeating pattern resembling a brittle star. As in the feeding patterns on ash, numerous small grooves radiated from both sides of each arm. The question naturally arises: How do such strange and “artistic” feeding patterns occur, and why does one species’ pattern differ from another?

  Almost all of the feeding patterns (except for the short exit tunnels) of longhorn beetles in wood are made by the larvae, but those made by scolytid bark beetles include a significant contribution by the adults to their larvae. Feeding begins when a lone adult male burrows through the bark on a just-dead tree or a sick one that cannot mount a sufficient defense. Each beetle, after reaching the outer sapwood, makes a small cavity below his point of entry through the bark. Then one or several females, depending on the species, come in to join him in this “nuptial chamber.” After mating, each female excavates a gallery or tunnel radiating out from the mating chamber below the entry hole. In the above example of the white ash, the horizontal line is actually two a
djoining galleries; the balsam fir usually has four radiating galleries, although I saw up to seven, each made by a different female in the male’s “harem.” Each female deposits eggs right and left at intervals down her gallery, and the larvae that hatch from them make their own, smaller galleries at right angles to their mother’s. The number of galleries reveals the number of that female’s offspring, and the length of each side gallery is an indication of the amount of sapwood the larva ate before it pupated at the end. After about a month (depending on the temperature) the new beetles leave the tree through the hole that the male made on entry. (The fungi and bacteria introduced by the beetles now speed up the tree’s undertaking.) The family feeding pattern is thus not artistry but a record of beetle social behavior, sexual pairing modes, and role as tree undertakers.

  Feeding tracks of bark beetles on logs of balsam fir and pine (top right) and of another species on American ash (bottom). The center burrows are made by the adults, and each radiating burrow by a larva. For comparison, see the beginning feeding track (left) of a long-horned beetle larva on pine.

  My peek at the beginning of the recycling of trees at my doorstep highlights how these highly specialized beetle undertakers are in some ways reminiscent of those associated with animal carcasses. It also suggests the important role of temperature in the trees’ defenses. Only one generation of pine sawyers is produced each year, and that one cycle takes almost a full year. But very small beetles, such as bark beetles, reach maturity much more quickly. With warm enough temperatures and a long summer, up to six generations of bark beetles can be produced in a single year.