20
— COMMUNITY HOUSING —
PROJECTS
EVEN IF MATURE trees have now grown too thick for many of the activities I have described so far, animals are happy to go on using them. These giants can become coveted living spaces, a service the trees do not offer voluntarily. Birds, martins, and bats are particularly partial to the thick trunks of older trees. They like thick trunks because the sturdy walls provide especially good insulation against heat and cold.
In Europe, it’s usually a great spotted woodpecker or a black woodpecker that gets things started. The bird hacks out a hole in the trunk that may be only an inch or two deep. Contrary to popular opinion, the birds don’t restrict themselves to rotten trees, and they often start construction in healthy trees. Would you move into a ramshackle home if you could build a new one next door? Just like us, woodpeckers want the place where they bring up their families to be solid and durable. Even though the birds are well equipped to hammer away at healthy wood, it would be too much for them to complete the job all at once. And that’s why they take a months-long break after the first phase, hoping fungi will pitch in. As far as the fungi are concerned, this is the invitation they have been waiting for, because usually they can’t get past the bark. In this case, they quickly move into the opening and begin to break down the wood. What the tree sees as a coordinated attack, the woodpecker sees as a division of labor. After a while, the wood fibers are so mushy that it’s much easier for the woodpecker to enlarge the hole.
Finally, the day comes when construction is complete and the cavity is ready to move into. But that’s not enough for the crow-sized black woodpecker, and so he works on a number of cavities at the same time. He uses one for the kids, one for sleeping, and the others for a change of scene. Every year, the cavities are renovated, and wood chips at the base of the trees are evidence of this activity. Renovation is necessary because the fungi that have invaded the space are by now unstoppable. They keep eating deeper into the trunk, transforming the wood into damp mush, which isn’t an ideal environment in which to raise a family. Every time the woodpecker cleans out the soggy mess, the nesting cavity gets a little larger. Sooner or later, the cavity becomes too large and, above all, too deep for the baby birds, which must climb up out of the opening to make their first flight. And now it’s time for the subletters to move in.
The subletters are species that can’t work with wood themselves. There’s the nuthatch, which is somewhat like a woodpecker but much smaller. Like woodpeckers, it hops around on dead wood, pecking away to get at beetle larvae. It loves to build its nests in abandoned great spotted woodpecker nesting cavities. But there’s a problem. The entrance is way larger than it needs and could let in predators intent on eating its brood. To prevent this, the bird makes the entrance smaller using mud, which it arranges artfully around the perimeter.
While we’re on the subject of predators: trees also offer their subletters a special service on the side, thanks to the characteristics of their wood. Wood fibers conduct sound particularly well, which is why they are used to make musical instruments such as violins and guitars. You can do a simple experiment to test for yourself how well these acoustics work. Put your ear up against the narrow end of a long trunk lying on the forest floor and ask another person at the thicker end to carefully make a small knocking or scratching sound with a pebble. On a still day, you can hear the sound through the trunk incredibly clearly, even if you lift your head. Birds use this property of wood as an alarm system for their nesting cavities. In their case, what they pick up is not benign knocking but scrabbling sounds made by the claws of martins or squirrels. The sounds can be heard high up in the tree, which gives the birds a chance to escape. If there are young in the nest, they can try to distract the attackers, though such attempts are usually doomed to failure. But at least the parents escape with their lives and can compensate for their loss by raising a second brood.
Acoustics are not so important for bats, for they have completely different concerns. Some species of these tiny mammals need lots of tree cavities at the same time to raise their young. In the case of Bechstein’s bats, which live in Europe and western Asia, small groups of females raise their offspring together. They spend only a few days in the same quarters before it’s time to move on. The reason for this is parasites. If the bats were to spend the whole season in the same cavity, there would be a parasite population explosion and they would torment the winged nocturnal hunters mercilessly. Short moving cycles take care of this by simply leaving the parasites behind.
Owls don’t fit very well into woodpecker cavities, and so they must be patient for a few more years. Over time, the tree continues to rot, and sometimes the trunk splits open a bit more so that the entrance gets bigger. And sometimes there is a series of woodpecker cavities up the trunk that speeds the owls’ entrance. These are like woodpecker apartments stacked one on top of the other. As the process of decay progresses, they slowly merge into each other, and when that happens, they are ripe for the arrival of the tawny owl and his friends.
And what about the tree? Its efforts to defend itself are in vain. And it’s too late to mount an attack against fungi anyway, because by now the floodgates have been open to them for years. But the tree can lengthen its life-span considerably if it at least manages to get a grip on its external wounds. If it manages to do this, it will continue to rot on the inside; however, externally it will be as stable as a hollow steel pipe and can survive for another hundred years. You can spot these attempts at repair if you see bulges around the edges of a woodpecker hole. Despite its best efforts, the tree rarely makes headway on closing the entrance. Usually, the merciless builder simply pecks the new wood away.
The rotting trunk now becomes home to a complex living community. Wood ants move in and chew the moldy wood to make their papery nests. They soak the nest walls with honeydew, the sugary excretions of aphids. Fungi bloom on this substrate, and their fibrous web stabilizes the nest. A multitude of beetles are drawn to the mushy, rotten interior of the cavity. Their larvae can take years to develop, and therefore, they need stable, long-term accommodations. This is why they choose trees, which take decades to die and, therefore, remain intact for a long time. The presence of beetle larvae ensures that the cavity remains attractive to fungi and other insects, which keep a constant supply of excrement and sawdust raining down into the rot.
The excrement of bats, owls, and dormice also drops down into the dark depths. And so the rotten wood is constantly supplied with nutrients, which feed species such as the blood-necked click beetle,47 or the larvae of the European hermit beetle, a big black beetle that can grow up to 1.5 inches long. Hermit beetles are very reluctant to move and prefer to spend their whole lives in a dark hole at the base of a rotting tree trunk. And because these beetles rarely fly or walk, whole generations of the same family can live for decades in the same tree. And this explains why it is so important to keep old trees. If they are cleared away, these little black guys can’t just wander over to the next tree; they simply don’t have the energy to do that.
Even if one day the tree gives up and breaks off in a storm, it has still served the community well. Even though scientists haven’t fully researched the relationships yet, we do know that higher species diversity stabilizes the forest ecosystem. The more species there are around, the less chance there is that a single one will take over to the detriment of the others, because there’s always a candidate on hand to counteract the menace. And even the dead tree trunk can offer a valuable service managing water for living trees merely because it is there, as we’ve already seen in chapter 17, “Woody Climate Control.”
21
— MOTHER SHIPS OF —
BIODIVERSITY
MOST ANIMALS THAT depend on trees don’t harm them. They just use the trunks or the crowns as custom-built homes that offer small ecological niches, thanks to varying amounts of moisture and light. Innumerable specialists find places to live here. Little rese
arch has been done, particularly in the upper levels of the forest, because scientists need to use expensive cranes or scaffolds to check them out. To keep costs down, brutal methods are sometimes employed. And so, in 2009, tree researcher Dr. Martin Gossner sprayed the oldest (six hundred years old) and mightiest (170 feet tall and 6 feet wide at chest height) tree in the Bavarian Forest National Park. The chemical he used, pyrethrum, is an insecticide, which brought any number of spiders and insects tumbling down to the forest floor—dead. The lethal results show how species-rich life is way up high. The scientist counted 2,041 animals belonging to 257 different species.48
Tree crowns even contain specialized wetland habitats. When a trunk splits to form a fork, rainwater collects at the point where the trunk divides. This minuscule pool is home to tiny little flies that provide food for rare species of beetles. It’s more difficult for animals to live in trunk cavities where water collects. The cavities are dark, and the murky, moldy brew that lurks there contains very little oxygen. Larvae that develop in water cannot breathe in places like that—unless, of course, they are endowed with snorkels, like the offspring of the bumblebee hoverfly. Thanks to breathing tubes that extend like telescopes, these larvae can survive here. Bacteria are almost the only things stirring in these waters, so they are probably the larvae’s food source.49
Not every tree is targeted by woodpeckers as a nesting site and doomed to gradual rot, and by no means do all slowly waste away, offering many specialized species hard-to-find habitats as they do so. Many trees die quickly. A storm might snap a mighty trunk, or bark beetles might destroy a tree’s bark in a few short weeks, causing its leaves to wither and die. Then the ecosystem around the tree changes suddenly. Animals and fungi that are dependent on the tree pumping a steady supply of moisture through its veins or sugar to its crown must now leave the corpse or starve. A small world has come to an end. Or has it just begun?
“Und wenn ich geh, dann geht nur ein Teil von mir.” “And when I go, only a part of me is gone.” This phrase from a hit by German pop singer Peter Maffay could have been written by a tree. For the dead trunk is as indispensable for the cycle of life in the forest as the live tree. For centuries, the tree sucked nutrients from the ground and stored them in its wood and bark. And now it is a precious resource for its children. But they don’t have direct access to the delicacies contained in their dead parents. To access them, the youngsters need the help of other organisms. As soon as the snapped trunk hits the ground, the tree and its root system become the site of a culinary relay race for thousands of species of fungi and insects. Each is specialized for a particular stage of the decomposition process and for a particular part of the tree. And this is why these species can never pose a danger to a living tree—it would be much too fresh for them. Soft, woody fibers and moist, moldy cells—these are the things they find delicious. They take their sweet time over both their meals and their life cycles, as demonstrated by the stag beetle. The adult beetle lives for only a few weeks, just long enough to mate. This animal spends most of its life as a larva, which slowly eats its way through the crumbling roots of dead deciduous trees. It can take up to eight years for it to get big and fat enough to pupate.
Bracket fungus is similarly slow. It gets its name because it sticks out from the dead trunk like a shelf made from half a broken plate. The red belt conk is one example. It feeds on the white threads of cellulose in the wood, leaving brown crumbs as evidence of its meal. Its fruiting body, the aforementioned broken plate, is attached to the trunk at a neat horizontal angle. This is the only way it can ensure that its reproductive spores will trickle out of the small tubes on its underside. If the rotten tree it is attached to falls over one day, the fungus seals the tubes and continues to grow at right angles to its former fruiting body so that it can form a new horizontal plate.
Some fungi fight bitterly over feeding territory. You can see this clearly on dead wood that has been sawn into pieces. You’ll find marbled structures of lighter and darker tissue clearly separated by black lines. The different shades indicate different species of fungus working their way through the wood. They wall off their territory from other species with dark, impenetrable polymers, which look to us as though they are drawing battle lines.
In total, a fifth of all animal and plant species—that’s about six thousand of the species we know about—depend on dead wood.50 As I have explained, dead wood is useful because of its role as a nutrient recycler. But can it also be a threat to the forest? After all, perhaps if there’s not enough dead wood lying around, organisms might decide to eat live trees instead. I hear this concern voiced time and again by people who come to visit the forest, and there are a few private forest owners who remove all dead trunks for exactly this reason. But this is neither a necessary nor a useful practice. All removing dead wood does is destroy valuable habitats, because live wood is of no use to organisms that live in dead wood. Live wood is not soft enough for them, it is too moist, and it contains too much sugar. This is quite apart from the fact that beeches, oaks, and spruce defend themselves from colonization. Healthy trees growing in their natural range withstand almost all attacks if they are well nourished. And the armada of decomposers helps feed the living trees as long as the little guys can find a way to make their livelihoods.
Sometimes dead wood is directly beneficial to trees, for example, when a downed trunk serves as a cradle for its own young. Young spruce sprout particularly well in the dead bodies of their parents. This is known as “nurse-log reproduction” in English and, somewhat gruesomely, as Kadaververjüngung, or “cadaver rejuvenation,” in German. The soft, rotten wood stores water particularly well, and some of the nutrients it contains have already been released by fungi and insects. There is just one teeny problem: the trunk isn’t a permanent replacement for soil, as it is constantly being degraded, until one day it disintegrates completely into humus on the forest floor. So what happens to the young trees then? Their roots are exposed and lose their support, but because the process plays out over decades, the roots follow the disintegrating wood into the forest floor. The trunks of spruce that grow up this way end up being elevated on stilts. The height of the stilts corresponds to the diameter of the nurse log on which they once lay.
22
— HIBERNATION —
IT’S LATE SUMMER, and the forest is in a strange mood. The trees have exchanged the lush green in their crowns for a washed-out version verging on yellow. It seems as though they are getting increasingly tired. Exhaustion is setting in, and the trees are waiting for the stressful season to end. They feel just like we do after a busy day at work—ready for a well-earned rest.
Grizzly bears hibernate and so do dormice. But trees? Do they experience anything that could be compared to our nightly time-outs? The grizzly bear is a good candidate for comparison, because it follows a similar strategy to trees. In summer and early fall, it eats to lay down a thick layer of fat it can live off all winter. And this is exactly what our trees do as well. Of course, they don’t feed on blueberries or salmon, but they fuel themselves with energy from the sun, which they use to make sugar and other compounds they can hold in reserve. And they store these under their skin just like a bear. Because they can’t get any fatter (only their bones—that is to say, their wood—can grow), the best they can do is fill their tissues with food. And whereas bears can go on eating everything they can find, at some point, the trees get full.
You can see this very well especially if you look at wild cherries, bird cherries, and wild service trees any time after August. Even though there are many beautiful sunny days they could make use of before October, they begin to turn red. And what that means is that they are shutting up shop for the year. The storage spaces under their bark and in their roots are full. If they made more sugar, there would be nowhere to stash it. While the bears happily go on eating, for these trees the sandman is already knocking on the door. Most other tree species seem to have larger storage areas, and they continue to photos
ynthesize hungrily and without taking a break right until the first hard frosts. Then they, too, must stop and shut down all activity. One reason for this is water. It must be liquid for the tree to work with it. If a tree’s “blood” freezes, not only does nothing work, but things can also go badly wrong. If wood is too wet when it freezes, it can burst like a frozen water pipe. This is the reason most species begin to gradually reduce the moisture content in their wood—and this means cutting back on activity—as early as July.
But trees can’t switch to winter mode yet, for two main reasons. First, unless they are members of the cherry family, they use the last warm days of late summer to store energy, and second, most species still need to fetch energy reserves from the leaves and get them back into their trunk and roots. Above all, they need to break down their green coloring, chlorophyll, into its component parts so that the following spring they can send large quantities of it back out to the new leaves. As this pigment is pumped out of the leaves, the yellow and brown colors that were there all along predominate. These colors are made of carotene and probably serve as alarm signals. Around this time, aphids and other insects are seeking shelter in cracks in the bark, where they will be protected from low temperatures. Healthy trees advertise their readiness to defend themselves in the coming spring by displaying brightly colored fall leaves.51 Aphids & Co. recognize these trees as unfavorable places for their offspring because they will probably be particularly vigorous about producing toxins. Therefore, they search out weaker, less colorful trees.
The Hidden Life of Trees: What They Feel, How They CommunicateDiscoveries from a Secret World Page 11