Why Trees ‘Scream’ and Other Fascinating Facts

Posted on Dec 6, 2016

leaves on a tree in the new forest, dorset

When trees are really thirsty, they begin to “scream”.  But, 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.

Trees communicate with each other

Think that trees cannot communicate? Scientists from the University 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.

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 behavior is astonishing. The trees that were being eaten gave off a warning gas (specifically, ethylene) that signaled to neighboring 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.

Trees live in communities

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.

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 photosynthesis 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.

Trees store carbon

Over the course of their lives, trees store up to 22 tons of carbon dioxide in their trunks, branches, and root systems.