13th September 2016 –
One expert says they do have feelings
When I began my career as a forester in western Germany’s Eifel mountains, I knew as much about the hidden life of trees as a butcher knows about the emotional life of animals. The forestry industry produces lumber; it fells trees and then plants new seedlings. If you read the professional literature, you quickly get the impression that the wellbeing of the forest is only of interest insofar as it is necessary for optimising the lumber industry.
But about 20 years ago, I began to organise survival training and log-cabin tours for tourists. In conversations with the many visitors, my view of the forest changed.
Visitors were enchanted by crooked, gnarled trees I would previously have dismissed because of their low commercial value. I began to notice bizarre root shapes, peculiar growth patterns and mossy cushions on bark. Suddenly, I was aware of countless wonders I could hardly explain, even to myself. At the same time, RWTH Aachen University began conducting scientific research programmes in the forest I manage.
During this research, many questions were answered, but many more emerged. I will never stop learning from trees, but even what I have learnt so far under their leafy canopy exceeds anything I could ever have dreamt of.
Trees live in communities
Years ago, I stumbled across a patch of strange-looking mossy stones in one of the preserves of old beech trees in the forest I manage. Carefully, I lifted the moss on one and found tree bark. These were not stones, after all, but old wood; it was obviously attached to the ground in some way. I scraped away some of the bark until I got down to a greenish layer. This indicated the presence of chlorophyll, which makes new leaves green: this piece of wood was still alive! I suddenly noticed that the remaining “stones” were arranged in a circle. I had stumbled on the gnarled remains of an enormous ancient tree stump. The interior had rotted long ago – a clear indication that the tree must have been felled at least four or five hundred years earlier. But how could the remains have clung on to life for so long? It was clear that the surrounding beeches were pumping sugar to the stump to keep it alive.
Trees share food with their own species for the same reasons as human communities: there are advantages to working together. On its own, a tree cannot establish a consistent local climate. It is at the mercy of wind and weather. But together, many trees create an ecosystem that moderates extremes of heat and cold, stores a great deal of water and generates a great deal of humidity. And in this protected environment, trees can live to be very old.
To get to this point, the community must remain intact. If every tree were looking out only for itself, then quite a few would never reach old age. Regular fatalities would result in many large gaps in the tree canopy, which would make it easier for storms to get inside the forest and uproot more trees. The heat of summer would reach the forest floor and dry it out. Every tree, therefore, is valuable to the community and worth keeping around for as long as possible.
Trees warn each other of danger
Four decades ago, scientists noticed something on the African savannah. Giraffes there were feeding on umbrella thorn acacias, and the trees didn’t like this. It took the acacias mere minutes to start pumping toxic substances into their leaves. The giraffes got the message and moved on to other trees – but they didn’t start nibbling until they were about 100 yards away. The reason for this behaviour is astonishing. The trees that were being eaten gave off a warning gas (specifically, ethylene) that signalled to neighbouring trees of the same species that a crisis was at hand. Right away, all the forewarned trees also pumped toxins into their leaves to prepare themselves.
Similar processes are at work in our forests. Beeches, spruce and oaks all register pain as soon as a creature nibbles on them. When a caterpillar takes a bite out of a leaf, the tissue around the site of the damage changes. In addition, the leaf tissue sends out electrical signals, just as human tissue does when hurt. However, the signal is not transmitted in milliseconds, as human signals are; instead, the plant signal travels at the slow speed of a third of an inch per minute.
When to shed your leaves is a question of character
On the country road near my home stand three oaks. They grow unusually close together: mere inches separate the 100-year-old trunks. That makes them ideal subjects to study because the environmental conditions for all three are identical. This means that if the oaks behave differently, it must be because of their own innate characteristics. And they do, indeed, behave differently.
In winter, when the trees are bare, or in summer, when they are in full leaf, the driver of a car speeding by wouldn’t even notice three separate trees. However, with autumn comes a different story. When the oak on the right is already turning colour, the other two remain completely green. It takes a couple of weeks for the two laggards to follow their colleague into hibernation. But if their growing conditions are identical, what accounts for the differences in their behaviour? The timing of leaf drop, it seems, really is a question of character.
As we know, a deciduous tree has to shed its leaves. But when is the optimal moment? Trees cannot anticipate the coming winter. They don’t know whether it is going to be harsh or mild. All they register are shortening days and falling temperatures. If temperatures are falling, that is. There are often unseasonably warm days in the autumn, and now the three oaks find themselves in a dilemma. Should they use these mild days to photosynthesise a while longer and quickly stash away a few extra calories of sugar? Or should they play it safe and drop their leaves in case there’s a sudden frost that forces them into hibernation? Clearly, each of the three trees decides differently.
Young trees are kept in check by their mothers
Young beech trees are so keen on growing quickly that it would be no problem at all for them to grow about 18in taller per season. Unfortunately for them, their own mothers do not approve of rapid growth. They shade their offspring with their enormous crowns, and the crowns of all the mature trees close up to form a thick canopy over the forest floor. This canopy lets only three per cent of available sunlight reach the ground and, therefore, their children’s leaves. With that amount of sunlight, a tree can photosynthesise just enough to keep its own body from dying.
But what purpose does this restriction serve? Don’t parents want their offspring to become independent as quickly as possible? Trees, at least, would answer this question with a resounding no, and recent science backs them up. Scientists have determined that slow growth when the tree is young is a prerequisite if a tree is to live to a ripe old age.
Dr Suzanne Simard, who helped discover maternal instincts in trees, describes “mother trees” as dominant trees widely linked to other trees in the forest through their fungal-root connections. These trees pass their legacy to the next generation and exert their influence in the upbringing of the youngsters. “My” small beech trees, which have by now been waiting for at least 80 years, are standing under mother trees that are about 200 years old. The stunted trees can expect another 200 years of twiddling their thumbs before it is finally their turn. The wait time is, however, made bearable. Their mothers are in contact with them through their root systems, and they pass along sugar and other nutrients. You might even say they are nursing their babies.
Why trees ‘scream’ and other fascinating facts
- Whereas it is generally accepted that we know less about the ocean floor than we know about the surface of the moon, we know even less about life in the soil. There are more life forms in a handful of forest soil than there are people on the planet.
- Think that trees cannot communicate? Scientists from theUniversity of Western Australia have registered the roots of grain seedlings crackling at a frequency of 220 hertz. When other seedlings’ roots were exposed to crackling at this frequency, they oriented their tips in that direction.
- When trees are really thirsty, they begin to “scream”. If you’re out in the forest, you won’t be able to hear them, because this all takes place at ultrasonic levels. Scientists at the Swiss Federal Institute for Forest, Snow and Landscape Research explain these sounds as vibrations occurring in the trunk when the flow of water from the roots to the leaves is interrupted.
- Over the course of their lives, trees store up to 22 tons of carbon dioxide in their trunks, branches, and root systems.
- Walnuts have compounds in their leaves that deal so effectively with insects that garden lovers are often advised to put a bench under a canopy of walnuts if they want a comfortable place to relax, because this is where they will have the least chance of being bitten by mosquitoes.
- To grow its trunk, a mature beech needs as much sugar and cellulose as there is in a 2½-acre field of wheat. Every five years, a beech tree produces at least 30,000 beechnuts. Assuming it grows to be 400 years old, it can fruit at least 60 times and produce a total of about 1.8 million beechnuts.