The Hidden Life of Trees: The International Bestseller – What They Feel, How They Communicate. Peter Wohlleben

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       — THE TREE LOTTERY —

      TREES MAINTAIN AN inner balance. They budget their strength carefully, and they must be economical with energy so that they can meet all their needs. They expend some energy growing. They must lengthen their branches and widen the diameter of their trunks to support their increasing weight. They also hold some energy in reserve so that they can react immediately and activate defensive compounds in their leaves and bark if insects or fungi attack. Finally, there is the question of propagation.

      Species that blossom every year plan for this Herculean task by carefully calibrating their energy levels. However, species that blossom only every three to five years, such as beeches or oaks, are thrown off kilter by such events. Most of their energy has already been earmarked for other tasks, but they need to produce such enormous numbers of beechnuts and acorns that everything else must now take second place. The battle for the branches begins. There’s not a speck of space for the blossoms, so a corresponding number of leaves must vacate their posts. In the years when the leaves shrivel and fall off, the trees look unusually bare, so it’s no surprise that reports on the condition of forests where the affected trees are growing describe the tree canopy as being in a pitiful state. Because all the trees are going through this process at the same time, to a casual observer the forest looks sick. The forest is not sick, but it is vulnerable. The trees use the last of their energy reserves to produce the mass of blossoms, and to compound the problem, they are left with fewer leaves, so they produce less sugar than they normally do. Furthermore, most of the sugar they do produce is converted into oil and starch in the seeds, so there is hardly any left over for the trees’ daily needs and winter stores—to say nothing of the energy reserves intended to defend against sickness.

      Many insects have been waiting for just this moment. For example, the beech leaf-mining weevil lays millions upon millions of eggs in the fresh, defenseless foliage. Here, the tiny larvae eat away flat tunnels between the top and bottom surfaces of the leaves, leaving brown papery trails as they feed. The adult beetles chew holes in the leaves until they look as though a hunter has blasted them with a shotgun. Some years, the infestations are so severe that, from afar, the beeches look more brown than green. Normally, the trees would fight back by making the insects’ meal extremely bitter—literally. But after producing all those blossoms, they are out of steam, and so this season they must endure the attack without responding.

      Healthy trees get over this, especially because afterward there will be a number of years for them to recover. However, if a beech tree is already sickly before the attack, then such an infestation can sound its death knell. Even if the tree knew this, it would not produce fewer blossoms. We know from times of high forest mortality that it is usually the particularly battered individuals that burst into bloom. If they die, their genetic legacy might disappear, and so they probably want to reproduce right away to make sure it continues. Something similar happens after unusually hot summers. After extreme droughts bring many trees to the brink of death, they all bloom together the following year, which goes to show that large quantities of beechnuts and acorns don’t indicate that the next winter will be particularly harsh. As blossoms are set the summer before, the abundance of fruit reflects what happened the previous year and has nothing to do with what will happen in the future. The effect of weak defenses shows up again in the fall, this time in the seeds. The beech leaf miners bore into fruit buds as well as leaves. Consequently, although beechnuts form, they remain empty, and therefore, they are barren and worthless.

      When a seed falls from a tree, each species has its own strategy as to when the seed sprouts. So how does that work? If a seed lands on soft, damp soil, it has no choice but to sprout as soon as it is warmed by the sun in the spring, for every day the embryonic tree lies around on the ground unprotected it is in great danger—come spring, wild boar and deer are always hungry. And this is just what the large seeds of species such as beeches and oaks do. The next generation emerges from beechnuts and acorns as quickly as it can so that it is less attractive to herbivores. And because this is their one and only plan, the seeds don’t have long-term defense strategies against fungi and bacteria. The seeds slough off their protective casings, which lie around on the forest floor through the summer and rot away by the following spring.

      Many other species, however, give their seeds the opportunity to wait one or more years until they start to grow. Of course, this means a higher risk of being eaten, but it also offers substantial advantages. For example, seedlings can die of thirst in a dry spring, and when that happens, all the energy put into the next generation is wasted. Or when a deer has its territory and main feeding ground in exactly the spot where the seed lands, it takes no more than a few days for the seedling’s tasty new leaves to end up in the deer’s stomach. In contrast, if some of the seeds do not germinate for a year or more, then the risk is spread out so that at least a few little trees are likely to make it.

      Bird cherries adopt this strategy: their seeds can lie dormant for up to five years, waiting for the right time to sprout. This is a good strategy for this СКАЧАТЬ