Beeches are particularly slow. Their seeds are carried off by jays less often than acorns are, and other species spread themselves using the wind and occupy open areas much more quickly. When the easygoing beeches returned about four thousand years ago, the forest was already occupied by oaks and hazels. That was no big deal for the beeches, and you are already familiar with their strategy. They take a lot more shade than other trees and, therefore, have no difficulty sprouting at their feet. The small amount of light that oaks and hazels allow to reach the ground is sufficient for the tiny conquistadors to keep on growing upward and one day to break through the crowns of the competition. What had to happen, happened. The beeches grew up and over the species that had been there earlier and robbed them of the light they needed to survive. Their merciless triumphal march stretches as far north as southern Sweden today, but it is not over yet. Or, rather, it wouldn’t have been over had people not interfered.
When beeches arrived, the European forefathers were beginning to make massive changes to forest ecosystems. They were clearing trees around their settlements to make room for fields for their crops and clear-cutting more areas for livestock. And because even this was not enough, people were simply driving their cattle and pigs into the forest. For beeches, this was catastrophic. Their offspring had to endure centuries at ground level before they were allowed to grow. In those days, their topmost buds were defenseless and at the mercy of browsing animals. Originally, there had been very few mammals around, because dense forests offer little food. Before people arrived on the scene, the odds of beeches hanging out for two hundred years undisturbed and uneaten were high. But then came a constant stream of herders with their hungry livestock gobbling up their tasty buds. In areas where light now fell because trees had been cut down, other species of trees previously overshadowed by the beeches took over. This severely hindered the post–ice age migration of beeches, and to this day, there are areas in Europe they have not yet colonized.
In the past few centuries, hunting has come to European forests as well, which, paradoxically, considerably increased the numbers of deer and wild boar. Thanks to massive feeding programs by hunters, who are mostly interested in increasing the number of antler-bearing stags, the population grew until today it is up to five times its natural level. German-speaking regions have one of the highest concentrations of herbivores in the world, so small beeches are finding it harder than ever to survive. And forestry is restricting their spread, as well. In southern Sweden, where beeches could comfortably grow, it’s one spruce or pine plantation after another. Except for a few individual trees, there are hardly any beeches to be found there. But they are ready and waiting. The moment people stop interfering, they will resume their northward migration.
The slowest of the migrants is the European silver fir, the only species of fir native to Germany. Its name comes from its light-gray bark, which makes it easy to distinguish from spruce, which have red-brown bark. The silver fir, like most tree species, waited out the ice age in southern Europe, probably in Italy, the Balkans, and Spain.62 It migrated from there, following the other trees, at a rate of 300 yards a year. Spruce and pines pulled ahead because their seeds are considerably lighter and better fliers. Even the beeches with their heavy nuts were faster, thanks to the jays.
Apparently, silver firs had developed the wrong strategy because their seeds are not good at flying, even though they are equipped with a small sail to catch the wind, and they are too small to be distributed by birds. Although there are birds that eat the seeds of fir trees, that’s of minimal use to the conifers. The nutcracker—which prefers the seeds of the Swiss pine but will eat the seeds of firs—gathers the seeds and stockpiles them. But in contrast with the jay, which hides acorns and beechnuts in soil all over the place, the nutcracker stashes his provisions in protected, dry locations. Even if a bird forgets a seed or two, because there’s no water the abandoned seeds never sprout.
Life is hard for silver firs. Whereas most of Central Europe’s native trees are well on their way to Scandinavia by now, silver firs have made it only as far as the Harz mountains in northern Germany. But what difference does it make to a tree if it’s a few hundred years late? After all, firs tolerate deep shade and can grow under beeches. They gradually insinuate themselves even into established old forests and can eventually grow into mighty trees. Their Achilles’ heel is that they are delectable to deer. Right now, these herbivores are preventing silver firs from migrating farther north because in some places they are gobbling up every last seedling.
And why is the beech so competitive in Central Europe? Or to put it another way, if it can prevail so well against all other species in Europe, why isn’t it found all over the world? The answer is simple. Its strengths are advantageous only in the region’s climatic conditions, which are influenced by the relative proximity of the Atlantic Ocean. Apart from up in the mountains (where beeches don’t grow on the upper slopes), temperatures don’t fluctuate very much. Cool summers are followed by warm winters, and precipitation is between 20 and 60 inches a year, just the way beeches like it.
Water is one of the key factors for growth in the forest, and this is where the beeches score big time. To produce 1 pound of wood, they need 22 gallons of water. Does this sound like a lot? Most other species of tree need up to 36 gallons, almost twice as much, and that is the deciding factor that enables beeches to shoot up quickly and suppress other species. Spruce are predisposed to guzzle water because in their cool, moist comfort zone in far northern regions, drought is unheard of. In Central Europe, only zones just below the tree line offer the conditions spruce enjoy. It rains a lot here, and thanks to the low temperatures, there’s hardly any evaporation. Trees growing at these elevations can afford to waste water. In most lower-lying areas, however, the frugal beeches come out ahead. Even in dry years, beeches put on a decent amount of growth and quickly tower over the heads of the spendthrifts. The offspring of the competition suffocate in the thick layer of leaves on the ground, but the beech seedlings have no problem pushing their way through. Beeches’ intensive use of light—which leaves nothing for the other species—and their ability to create for themselves the humid microclimate they enjoy, to build up a good supply of humus on the ground, and to gather water with their branches make them unbeatable in Central Europe today. But only in this part of the world.
As soon as the climate warms up and becomes more Mediterranean, these trees are going to have a hard time. They can’t tolerate constantly hot, dry summers and bitterly cold winters, and they will have to step aside for other species, such as oaks. Hot summers and cold winters prevail in Eastern Europe. Although Scandinavian summers are still acceptable, the colder times of the year that far north are also not for the beech. And in the sunny south, they like to settle only the higher elevations where it’s not quite so hot. Because of the climate it needs, therefore, the beech is currently trapped in Central Europe. Climate change is making the north warmer, and so, in the future, it will be able to expand its range in this direction. At the same time, it will eventually get so hot to the south that the tree’s whole range will shift in a northerly direction.
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— TOUGH CUSTOMERS —
SO WHY DO trees live so long? After all, they could grow just like wild flowers: grow like gangbusters for the summer, bloom, set seed, and then return to humus. That would have one definite advantage. Every new generation brings with it the opportunity for genetic modifications. These mutations are most likely to occur during mating and fertilization, and in a world that is constantly changing, adaptation is necessary for survival. For example, mice produce a new generation every few weeks; flies are a lot quicker. Every time hereditary traits are passed down, genes can be damaged, and with a stroke of luck, this damage will introduce a particularly beneficial new characteristic. In short, this is what we call evolution. It helps organisms adapt to changing environmental conditions and, therefore, guarantees the survival of each species. The shorter t
he interval before the next generation, the more quickly animals and plants can adapt.
Trees seem completely uninterested in this scientifically established imperative. They simply live to be ancient—on average many hundreds, but sometimes even thousands, of years old. Of course, they propagate at least every five years, but this doesn’t usually produce a completely new generation of trees. What use is it if a tree produces hundreds of thousands of offspring if they cannot find any vacant posts to fill? As long as their mothers are capturing all the light, nothing much happens at their feet, as I have already explained. Even if the young trees exhibit brilliant new traits, they must often wait centuries before they can bloom themselves and pass these genes along. Quite simply, everything moves along very slowly, and you might expect this to put the trees in an almost impossible situation.
If we look back to recent climate history, it is characterized by abrupt changes. A large construction site near Zurich shows just how abrupt. Workers here came across relatively fresh tree stumps, which, at first, they set aside without paying them any attention. A researcher found them, took samples, and investigated their age. The result: the stumps came from pines that were growing there almost fourteen thousand years ago. Even more amazing, though, were the fluctuations in temperature at that time. In less than thirty years, the temperature dropped as much as 42 degrees Fahrenheit, only to finally rise again by about the same amount. That corresponds to the current worst-case climate change scenario we could potentially face by the end of the twenty-first century. Even the last century in Europe, with the bitterly cold 1940s, the record drought in the 1970s, and the way-too-warm 1990s, was very hard on Nature. Trees employ two strategies to stoically endure these changes: behavior and genetic variability.
Trees exhibit great tolerance for variations in climate. And so the native European beech grows from Sicily to southern Sweden. Apart from the capital S at the beginning of the place names, these regions have little in common. Birches, pines, and oaks are also very flexible. But this is not enough to satisfy everything they need to do. When temperatures and rainfall fluctuate, many animals and fungi move from south to north and vice versa. That means that trees must also be able to adapt to unfamiliar pests.
The climate can also change so severely that it falls outside the range the trees can tolerate. And because they have no legs to carry them away and nowhere to turn for help, they have to adapt so that they can deal with the situation themselves. The first opportunity to do this comes at the very earliest stage of life. Shortly after fertilization, when the seeds are ripening in the flower, they react to environmental conditions. If it is particularly warm and dry, appropriate genes are activated. Scientists have proved that under these conditions, spruce seedlings are better able to tolerate warm weather—though they lose the same measure in frost resistance.63
Mature trees can adapt as well. If spruce survive a dry period with little water, in the future they are markedly more economical with moisture and they don’t suck it all up out of the ground right at the beginning of summer. The leaves and needles are the organs where most water is lost through transpiration. If the tree notices that water is in short supply and thirst is becoming a long-term problem, it puts on a thicker coat. The tree toughens up the protective waxy layer on the upper surface of its leaves. The walls of the cells within the leaves keep them watertight, and the tree increases the thickness of the cell walls by adding extra layers. As the tree battens down the hatches, however, it also has a harder time breathing.
Once a tree has exhausted its behavioral repertoire, genetics come into play. As I’ve just mentioned, it takes an extremely long time to produce a new generation of trees. This means speedy adaptation is not an option, but other responses are available. In a forest that has been left to its own devices, the genetic makeup of each individual tree belonging to the same species is very different. This is in contrast to people, who are genetically very similar. In evolutionary terms, you could say we are all related. In contrast, the individual beeches growing in a stand near where I live are as far apart genetically as different species of animals. This means each tree has different characteristics. Some deal better with drought than cold. Others have powerful defenses against insects. And yet others are perhaps particularly impervious to wet feet. If climatic conditions change, the first individuals to die will be those that have the hardest time dealing with the new status quo. A few old trees will die, but most of the rest of the forest will remain standing. If conditions become more extreme, one tree species could even be decimated without this being the end of the forest. Usually, a sufficiently large number of trees remain to produce enough fruit and shade for the next generation. I made a calculation for the old beech stands in the forest I manage using available scientific data. Even if we were to have a Spanish-style climate here in Hümmel sometime in the future, an overwhelming number of the trees would cope. The only proviso is that the social structure of the forest is not disturbed by lumber operations so that the forest can continue to regulate its own microclimate for itself.
SPRUCE
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— TURBULENT TIMES —
IN THE FOREST, things don’t always work out according to plan. Even though this ecosystem is immensely stable, often humming along for many centuries with no drastic changes, a natural catastrophe could still throw everything into turmoil. I’ve already written about winter storms. If a hurricane flattens whole forests, it usually affects commercial spruce or pine plantations. They are often growing on land damaged by machines and so compacted that the roots can’t grow down into it to provide good support for the trees. Moreover, in Central Europe, these trees grow much taller than they do in their original home farther north, and they hold on to their needles even in the winter. This means there is a large surface area to catch the wind and a long trunk to intensify the pressure. So the fact that the weak roots don’t hold is not so much a catastrophic event as simply a logical one.
But there are storm events in which even natural forests sustain at least localized damage. There are tornadoes whose swirling winds change direction in seconds and overwhelm the trees. These turbulent winds often happen in combination with thunderstorms, which, in Central Europe, occur almost only in summer, so yet another factor comes into play: at that time of year, deciduous trees have leaves on their branches. In the “normal” storm months from October to March, Beech & Co. are naked right down to their branches and, therefore, offer little wind resistance. In June or July, however, trees are not expecting these kinds of problems. If a tornado sweeps through a forest, it slams into the crowns and twists them right off with its raw power. The splintered trunks are left standing as a monument to this atmospheric assault, a lasting testament to the forces of Nature.
Tornadoes are rare events, and therefore, in evolutionary terms, it clearly doesn’t make sense to develop a defensive strategy just for them. However, there is another type of damage that happens much more often in connection with thunderstorms: the complete collapse of the crowns of deciduous trees because of heavy rain. When enormous amounts of water land on the leaves in just a few minutes, the trees have to handle loads that weigh many tons. Typically, extra weight from above comes in the form of winter snow, and this falls right through the trees because the leaves are already on the ground by then. In summer, snow is not an issue, and beeches and oaks in full leaf have no problem bearing up under a typical rainfall. Even a downpour is usually fine if a tree has grown normally. Things do, however, get a little dicey for trees if they ignored the etiquette manual while they were growing up and now have structural issues with their trunk or branches.
A typical issue that can lead to branch failure is the so-called hazard beam. The name says it all. A normal branch curves like a bow. It comes out from the trunk, grows upward for a while, and then grows horizontally before gently curving down. This gentle curve does a good job of cushioning the impact of weight from above without breaking. That is extremely important, because
the branches of older trees can be more than 30 feet long. This long lever exerts enormous pressure at the point where the branch meets the trunk. Despite the dangers, some trees clearly don’t want to follow tried-and-true branch patterns. In these trees, the branches start by pointing away from the trunk, only to then bend and grow upward and continue to hold this course. If a branch that grows in this J shape is bent down toward the ground, the force of heavy rain is not absorbed, and the branch breaks because downward pressure compresses the fibers on the underside (that would be the fibers on the outside of the J curve) and overextends those on the inside. Sometimes it is the trunk itself that is malformed in this way, and these trees break apart in the torrential rains that often accompany thunderstorms. It’s just another tough selection process that eliminates unfit trees from the race.
Other times, the breakdown has nothing to do with structural problems in the tree. The pressure from above is simply too great. Such breakdowns mostly happen in the months of March and April when snow is transformed from feather-light fluff to dead weight. You can estimate the point at which snow becomes dangerous by looking at the clusters of flakes as they fall. When the clusters are about the same diameter as a two-euro coin (that would be a quarter for those of you in the United States or Canada, an Australian dollar, or a British ten pence piece), the situation is getting critical. What you have at this stage is wet snow, which holds a lot of water and is very sticky. Instead of falling through tree branches, wet snow adheres to them, accumulating in thick, heavy layers. Wet snow falling on a tall sturdy tree can break a lot of limbs. It is even worse for adolescent trees. They are standing there with their lanky trunks and small crowns, waiting for it to be their turn to grow. They are either broken by snow loads or bent down so far that they can’t right themselves again. Very small trees, however, are not in danger, because their little trunks are simply too short. Pay attention on your next walk out into the forest. Right there among the middle-aged trees, you will find several that have been bent beyond hope of recovery in just such a weather event.
The Hidden Life of Trees: What They Feel, How They CommunicateDiscoveries from a Secret World Page 16
