Invisible life: how trees interact with each other

Despite their appearance, trees are social creatures. For starters, trees talk to each other. They also sense, interact and cooperate – even different species with each other. Peter Wohlleben, a German forester and author of The Hidden Life of Trees, also says that they feed their young, that growing seedlings learn, and that some old trees sacrifice themselves for the next generation.

While some scholars consider Wolleben’s view to be unnecessarily anthropomorphic, the traditional view of trees as separate, insensitive beings has been changing over time. For example, a phenomenon known as “crown shyness”, in which trees of the same size of the same species do not touch each other respecting each other’s space, was recognized almost a century ago. Sometimes, instead of intertwining and pushing for beams of light, the branches of nearby trees stop at a distance from each other, politely leaving space. There is still no consensus on how this happens – perhaps the growing branches die off at the ends, or the growth of the branches is stifled when the leaves feel the infrared light scattered by other leaves nearby.

If the branches of trees behave modestly, then with the roots everything is completely different. In the forest, the boundaries of individual root systems can not only intertwine, but also connect – sometimes directly through natural transplants – and also through networks of underground fungal filaments or mycorrhiza. Through these connections, trees can exchange water, sugar, and other nutrients and send chemical and electrical messages to each other. In addition to helping trees communicate, fungi take nutrients from the soil and convert them into a form that the trees can use. In return, they receive sugar – up to 30% of the carbohydrates obtained during photosynthesis goes to pay for mycorrhiza services.

Much of the current research on this so-called “tree web” is based on the work of Canadian biologist Suzanne Simard. Simard describes the largest individual trees in the forest as centers or “mother trees”. These trees have the most extensive and deep roots, and can share water and nutrients with smaller trees, allowing seedlings to thrive even in heavy shade. Observations have shown that individual trees are able to recognize their close relatives and give preference to them in the transfer of water and nutrients. Thus, healthy trees can support damaged neighbors – even leafless stumps! – keeping them alive for many years, decades and even centuries.

Trees can recognize not only their allies, but also enemies. For more than 40 years, scientists have found that a tree that is attacked by a leaf-eating animal releases ethylene gas. When ethylene is detected, nearby trees prepare to defend themselves by increasing the production of chemicals that make their leaves unpleasant and even toxic to pests. This strategy was first discovered in a study of acacias, and seems to have been understood by giraffes long before humans: once they have finished eating the leaves of one tree, they typically move more than 50 meters upwind before taking on another tree, as it has less probably sensed the sent emergency signal.

However, recently it has become clear that not all enemies cause the same reaction in trees. When elms and pines (and possibly other trees) are first attacked by caterpillars, they react to the characteristic chemicals in the caterpillar’s saliva, releasing an additional odor that attracts particular varieties of the parasitic wasp. Wasps lay their eggs in the bodies of caterpillars, and the emerging larvae devour their host from the inside. If the damage to the leaves and branches is caused by something the tree has no means of counterattacking, such as wind or an axe, then the chemical reaction is aimed at healing, not defense.

However, many of these newly recognized “behaviours” of trees are limited to natural growth. Plantations, for example, have no mother trees and very little connectivity. Young trees are often replanted, and what weak underground connections they manage to establish are quickly disconnected. Seen in this light, modern forestry practices begin to look almost monstrous: plantations are not communities, but swarms of dumb creatures, factory-raised and cut down before they could truly live. Scientists, however, do not believe that trees have feelings, or that the discovered ability of trees to interact with each other is due to anything other than natural selection. However, the fact is that by supporting each other, trees create a protected, moist microcosm in which they and their future offspring will have the best chance of surviving and reproducing. What is a forest for us is a common home for trees.

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