How do trees grow and survive in different environments? Understanding the science of tree growth has been a complex and slowly evolving process, akin to the slow growth of trees themselves. Peter Wohlleben, a German Forester has written a book “The Hidden Life of Trees: What They Feel, How They Communicate”, takes scientific research and interweaves it with personal experience to present an alternative way of understanding individual and collective tree growth.
The key message in this book is that trees do not fit the Darwinian theory of “the survival of the fittest”, but co-operate with each other to share resources, warn of certain biological dangers and grow strong in collaboration.
Examples of the complex relationship of trees are now clearly evidenced by scientific study. Trees that are long established rely on each other for the uptake of nutrients and exposure to light. They appear to exhibit, what emotively is termed collective intelligence, through the use of underground fungal mycorrhizal networks, which help in the absorption of nutrients and water and can warn of disease, drought and insect attack. The fungi absorb nutrients, which the tree requires, whilst the tree produces sugars via photosynthesis to support the expansion of the mycorrhizal network. Larger trees, with deep roots can support younger seedlings and increase nutrient flow to debilitated trees through this system. When the oldest trees in a forest are cut down unselectively, it can interrupt the mycorrhizal network in which these trees as major contributors, have an effect on the survival rate of younger trees, which are not ready to “survive on their own”.
Scientific study has revealed that through this interconnectedness trees can send out warning signals and other trees are able to respond. Via the mycorrhizal network, trees send hormonal, chemical and electrical pulses to each other, which some scientists have compared to the nervous system of animals.
They are also able to emit pheromones and scent to warn of danger. An example of this mechanism is Umbrella Acacia, Vachellia tortilis or Acacia tortilis). When it is grazed by Giraffes, it emits a warning scent of ethylene gas, which other trees downwind recognise and then start releasing tannins in to their leaves, to make them unpalatable. The Giraffes sense this and browse up wind where the gas has yet to spread. In still air Giraffes will typically walk 30 metres; as far as the distance ethylene can spread in calm air, before browsing again.
Other examples include a defensive mechanism developed by Ulmus and Pinus, which protects the trees from leaf eating caterpillars, by releasing pheromones. These attract wasps that lay their eggs inside the caterpillars, which are killed when the larvae hatch. A similar mechanism is used to reduce Deer browsing, where certain trees emit chemicals that make the leaves unpalatable.
Trees play an important role in human consciousness and culture and it may be argued that the science is easily anthropomorphised, elevating trees further in the mysticism that different cultures attach to them. Landscape Architects should read Peter Wohlleben’s book for both its science and the emotional interpretation placed on it; for to paraphrase Geoffrey Jellicoe - “Landscape Architecture is Art through Science”.