Heard it on the grapevine: the secret society of plants

The botanical underground is a social network of powerful alliances and nepotism. Decoding its messages could lead to radical change in farms and forests

Every autumn swarms of dusty grey moths engulf the mountainside birch forests of northern Scandinavia, laying their eggs on twigs so that, come springtime, the newly hatched larvae can feast upon budding leaves. It looks like a battle that the trees, with no natural defences, are doomed to lose, but some have a secret weapon. They form an alliance with a neighbouring plant, a kind of rhododendron, borrowing wafts of its volatile insecticides as a sort of olfactory camouflage. “This kind of interaction has never been observed in the field before,” says Jarmo Holopainen at the University of Eastern Finland in Kuopio, who made the discovery ( New Phytologist, vol 186, p 722). His study is one of the latest to demonstrate the unexpectedly complex relationships between plants.

We’ve known for some time that

plants respond to one another, but only now are we realising how subtle and sophisticated their interactions can be. Plants continually eavesdrop on each other’s chemical chatter – sometimes sympathetically, sometimes selfishly. Some plants, like the Scandinavian rhododendron, assist their neighbours by sharing resources. Others recognise close relatives and favour them over strangers. And at least one parasitic plant homes in on its host’s telltale chemical scent (see “Scent of a victim”).

“Plants don’t go out to parties or to watch the movies, but they do have a social network,” says Suzanne Simard, a forest ecologist at the University of British Columbia in Vancouver, Canada. “They support each other and they fight with each other. The more we look at plant signalling and communication, the more we learn. It’s really incredible.”

Since the development of time-lapse photography, it has been possible to document the dances and scuffles in densely populated plant communities: saplings on the forest floor compete for space to stretch their roots and shoots; fallen trees provide young ones with nourishment; vines lash around desperately searching for a trunk they can climb to reach the light; and wildflowers race each other to open their blooms in springtime and compete for the attention of pollinators. To truly understand the secret social life of plants, however, you must look and listen more closely.

A good place to start is underground in the rhizosphere – the ecosystem in and around plant roots. Beneath the forest floor, each spoonful of dirt contains millions of tiny organisms. These bacteria and fungi form a symbiotic relationship with plant roots, helping their hosts absorb water and vital elements like nitrogen in return for a steady supply of nutrients.

Now closer inspection has revealed that fungal threads physically unite the roots of dozens of trees, often of different species, into a single mycorrhizal network. These webs sprawled beneath our feet are genuine social networks. By tracing the movement of radioactive carbon isotopes through them, Simard has found that water and nutrients tend to flow from trees that make excess food to ones that don’t have enough. One study published in 2009, for example, showed that older Douglas firs transferred molecules containing carbon and nitrogen to saplings of the same species via their mycorrhizal networks. The saplings with the greatest access to these networks were the healthiest (Ecology, vol 90, p 2808).

As well as sharing food, mycorrhizal associations may also allow plants to share information.

Heard it on the grapevine: the secret society of plants