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Be more tree

We admire trees for their solitary strength, but it is their remarkable facility for collaboration and sharing that provides lessons for humankind.

By Mark Cocker

Notable trees, be they very old, large or even misshapen, have long held a fascination for people. Such attachments appear to be more widespread than ever. One thinks of the 1.2 million tourists who stand every year before a giant sequoia in California called General Sherman, or, on a more modest scale, the 600,000 visitors who pay their respects to the Major Oak of Sherwood Forest.

This arboreal popularism might be a recognition of the processes of photosynthesis to which trees substantially contribute, and which are the bedrock on which the entire biosphere stands. Yet it can’t only be gratitude for our breath that sends people to visit trees. Marine phytoplankton are thought to deliver half the oxygen we inhale, and I don’t see anyone rushing to the seashore to honour the kindness of algae.

Trees, in contrast, are viewed as a universal good and now a panacea for many human-induced ills. Before the last general election, British political parties – one of which even has a tree motif as its symbol – indulged in a hilarious bidding war to tell us how many new saplings they were going to plant. Behind this positive, if instrumental, advocacy I suspect there is a further, less healthy element at play in our preoccupation.

[See also: The quiet persistence of the orchid]

Through its very architecture, a tree can seem to be set apart in a landscape. This standing alone in the physical sense is central to what we might call the “inner tree”, the notion that trees symbolise qualities intrinsic to humans. We too have long thought of ourselves as a species apart, uniquely endowed with morals or consciousness. Yet this separateness readily bleeds into exceptionalism, from which flows our presumed rights to total dominance over nature.

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If this is even remotely so, then the best way to counter that myth would be to read James B Nardi’s glorious study of the interlocking multitudes that participate in tree-dom. One would then understand how trees are not only in a state of perpetual communion, but also – and here’s the strongest political message in Nardi’s studiously apolitical book – that trees are themselves utterly dependent on a sharing process. There is nothing solitary about a lonely tree.

The most fundamental of these symbiotic relationships, which has itself been mythologised as a form of arboreal socialism, is the “wood-wide web”. It was discovered as long ago as 1888 that the fine hair-roots in trees and the thread-like hyphae emanating from fungi can fuse and interpenetrate. Plant physiologists have now proved that the two partners in these so-called mycorrhizal associations pass nutrients back and forth to one another. Yet they can also transmit subterranean chemical signals, tree to tree, until they are broadcast across entire forest communities.

It is a remarkable story and Nardi rehearses it thoroughly, but he also draws on more recent research that shows how these relations extend beyond fungi to bacteria, as well as insects and nematodes. All of those organisms independently manufacture the very same hormones used by trees in their own communication. Essentially, there is a common woodland language, albeit chemical in character, and by producing metabolites identical to their arboreal hosts, the others aid trees in suppressing bacterial pathogens and in stimulating tree health and growth.

The interactions that the author particularly loves to explore involve what fellow American naturalist Edward Wilson called “the little things that run the world”. In fact, the book is largely about insects. Its principal characters are dozens of wasps, beetles, moths and flies, and Nardi finds space for a rich array of crustacea, arachnids, tardigrades, rotifers, protozoa, and even bacteria and viruses. He is steeped in this microscopic world and his excellent 2007 book, Life in the Soil, was itself a championing of creatures often overlooked by a wider public.

Throughout their slow-motion lives, trees suffer assault from herbivores of many kinds intent on the carbohydrate-rich wood and foliage. The novelist Willa Cather suggested that her own tree attachments were inspired by the apparently heroic resignation that trees showed in “the way they have to live”. If only she knew what Nardi explains here.

Trees are not at all passive. Their inner world involves a veritable seethe of volatile compounds. The instant their leaves come under attack, they release defensive chemicals such as salicylic acid. These hormones operate deep within the tree’s own tissue but also through the atmosphere. They warn neighbours to mount their own defence against predators, but the same airborne chemicals have a further function as a recruitment mechanism, attracting insects that are themselves predatory upon herbivorous caterpillars.

[See also: Spring reflection: What tadpoles taught me]

Despite his academic role (research scientist at the University of Illinois), Nardi has a knack for expressing his nature stories in the clearest, simplest terms. He has personally prepared the book’s hundreds of precise line-drawings that add immeasurably to his tale-telling. He also gives the same loving attention to details that are so macabre they could easily receive horror-film treatment. Some of the creatures lured in by a tree’s advertising campaign are called parasitoid wasps, but they don’t kill leaf-devouring insects directly. Rather, they inject their own eggs into the living tissues of the host, with sometimes as many as 100 squirted into a single caterpillar. The wasp larvae then embark on a gradual devouring of the live victim, eating their way from the inside outwards. As a further measure to protect her unhatched embryos, a mother wasp can also inject the same caterpillar with a virus that enhances the parasitoid’s success. The pathogen suppresses any immune response from the victim and fools the caterpillar’s own blood into not recognising an ultimately fatal foreign body lodged among its cells.

Nardi shows that trees, in calling on the services of wasps, are capable of building the broadest alliances. Some insects and fungi can do much the same but with a reverse outcome. Dutch elm disease is perhaps the best example of an anti-tree confederation. Since it arrived in the UK in the 1960s it has removed most of the mature elms from the British landscape. Yet this much-lamented blight entails its own kind of exquisite complexity. The story involves a pathogenic fungus named Ophiostoma (“the snake-mouthed”). It works in harmony with a bark beetle, which spreads the fungus spores from tree to tree. In return, the fungus detoxifies the elm’s own defensive chemicals, which helps to break down tree tissue and prepare it as food fit for a small beetle. In addition to working in collaboration with one another, their alliance turns the tree upon itself. The fungus somehow encourages an elm to produce chemical signals that are attractive to yet more bark beetles. The tree thus recruits its own nemesis.

It may seem a brutal business. Yet tree death is itself an opportunity for wider networks of decomposers that ultimately deliver for the health of the whole system. What the author says of these agents of decay could serve as a leitmotif for his book: “Their complex web of interactions defies a facile understanding of how all these lives are intertwined.”

A tree may be bound to a single spot for the whole of its term. It may endure for centuries and seem heroically resigned, doomed even, to isolation. Yet its entire existence is one prolonged process of sharing. It is a lesson from trees we should heed. Nardi’s book is the perfect place to learn it.

The Hidden Company That Trees Keep: Life from Treetops to Root Tips
James B Nardi
Princeton University Press, 320pp, £25

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[See also: Consider the noble octopus]

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This article appears in the 24 May 2023 issue of the New Statesman, The Tory Crack-Up