Despite appearances, trees are social beings. For a start, they talk to each other. They’re also sensing, co-operating and collaborating, even across species boundaries. Peter Wohlleben, the German forester-turned-tree-whisperer and author of The Hidden Life of Trees, also says they are suckling their young, that juveniles are learning, and that some elders are sacrificing themselves for the sake of the next generation.
While Wohlleben’s view is a step too far towards anthropomorphism for some scientists, the traditional view of trees as isolated, non-sensory beings has been shifting for some time. For example, the phenomenon known as “crown shyness”, in which similarly-sized trees of the same species appear to be respecting each other’s space was recognised almost a century ago. Sometimes, instead of interlacing and jostling for light, the branches of immediate neighbours stop short of one another, leaving a polite gap. There’s still no consensus on how this is managed – abrasion of growing branch tips is one possibility, another is that growth is inhibited when leaves sense infrared light scattered by other leaves close by.
If trees can be shy at their branch-tips, more recent research shows they are anything but at their roots. In a forest, the hair-like tips of individual root systems not only overlap, but can interconnect, sometimes directly via natural grafts, but also extensively via networks of underground fungal threads, or mycorrhizae. Through these connections, trees can share water, sugars and other nutrients, and pass chemical and electrical messages to one another. In addition to serving in communications, the fungi extract nutrients from the soil and convert them into forms that trees can use. In return they receive sugar – as much as 30% of the carbohydrate produced by photosynthesis in a woodland goes to pay for mycorrhizal services.
Much of the current research on this so called “wood wide web” stems from the work of the Canadian biologist Suzanne Simard. Simard describes the largest individual trees in a forest as hubs or “mother trees”. Mothers have the deepest, most extensive roots, and are able to supplement smaller trees with water and nutrients, allowing saplings to thrive even in heavy shade. Further experiments suggest that individuals are able to recognise close relatives and favour them when it comes to transferring the good stuff. Thus healthy individuals can support damaged neighbours, even leafless stumps, keeping them alive for years, decades, even centuries. This connectivity and collaboration makes much more sense when you realise that the dependent trees may not be random strangers, but old friends, or more persuasive still, close relatives.
The discernment shown by trees extends to their enemies as well as allies. Scientists have known for more than 40 years that if a tree is attacked by a leaf-eating animal, it releases ethylene gas. On detecting the ethylene, nearby trees prepare to repel boarders – boosting production of chemicals that make their leaves unpalatable, even toxic. The strategy was first discovered in a study of acacias, and it seems giraffes understood it long before humans: having finished browsing one tree, they habitually move more than 50 metres in an upwind direction before starting on another that is less likely to have sensed the emergency.
More recently, however, it’s become clear that not all enemies trigger the same response. When elm and pine trees (and probably others) are first attacked by caterpillars, they respond within seconds to distinctive chemicals in caterpillar saliva, releasing an additional scent that attracts specific varieties of parasitic wasp. The wasps lay eggs in or on the bodies of the caterpillars, and then the emerging wasp grubs devour their host from the inside. If the damage to leaves and branches is caused by something for which the tree has no means of counterattack – such as the wind or cutting with an axe – the chemical response is focused on healing rather than defence.
A sobering aspect of recent revelations is that many of these newly recognised “behaviours” are limited to natural growth. In plantations, there are no mother trees, and there is very little connectivity. This is partly because of the way young trees are transplanted and partly because when they are thinned to prevent competition, what little underground connectivity neighbours have established is severed. Seen in this light, modern forestry practices begin to seem almost monstrous: plantations are not communities but crowds of mute, factory-farmed individuals, felled before they have ever really lived. None of the scientists involved are seriously suggesting trees have feelings, or that the newly discovered sociability of trees is driven by anything other than natural selection. The transactions being observed may be utilitarian rather than altruistic, they might be accidental. Nevertheless, the result is that in supporting each other, trees build sheltered, humid microcosms in which they and their future offspring will stand the best chance of surviving and reproducing. We know these places as forests. The trees, if they “know” anything, know them as home.
Yorkshire Tea and trees
Yorkshire Tea is planting a million trees over five years – with a bit of help from the Woodland Trust, UK schoolchildren and Kenyan tea farmers. Learn more at yorkshiretea.co.uk/yorkshire-tree
.png?w=600)
