The idea that plants communicate in an unperceivable way has existed for decades. Literature is teeming with examples of enchanted forests where trees impede or aid humans in mysterious ways. It may not come as a surprise then, that trees and plants are constantly communicating with each other through an underground network of fungi.
Through the affectionately named ‘Wood-wide web’, trees and plants are connected to each other by mycorrhizal fungi. This is the term given to all fungi that symbiotically associate with plants. The mycorrhiza extends underground and either lay on top of plant roots or, in some cases, completely penetrate them. In return for carbon rich sugars – a by-product of photosynthesis – the fungi provide nitrogen and phosphorous nutrients to the plants. Without access to these, the plant’s growth is extremely limited.
Mycorrhizal relationships are vital for plants to grow healthily. In fact, orchids can’t germinate without them. The fungi provide orchid seeds with the necessary nutrients to begin their lifecycles.
As well as providing the plants with important nutrients, the mycorrhiza can also connect two otherwise solitary plants. This phenomenon can occur anywhere with plant life, from arid deserts to snow-covered tundras. When two plants are connected, they can communicate chemically with each other and ‘talk’.
This process was first observed by Suzanne Simard. By using carbon isotopic analysis, they proved that trees were exchanging carbon between them. In the canopy where only 3% of sunlight reaches, older ‘matriarch’ trees do this to help younger, shaded trees grow. Without these extra resources, it’s likely they would die.
Plants can also ‘warn’ others about incoming attacks. When broad bean plants are attacked by aphids, a sap-sucking insect, they release a chemical that both attracts wasps and repels the aphids. Neighbouring plants also begin releasing this chemical, but only when they’re connected to the original one by a mycorrhizal network.
Tomato plants show a similar behaviour. Ren Sen Zeng, a researcher at South China Agricultural University, grew pairs of tomato plants where only some were allowed to form mycorrhizal networks. In each pair, one plant was then sprayed by a fungus that causes blight disease. A few days later, the second plant in each pair was sprayed with the same fungus. When a mycorrhizal network had formed, the 2nd plants were significantly more resistant to the blight. They had been ‘warned’ by the plant that was initially sprayed.
Whilst the relationships described so far seem ‘good-natured’ there is also a more hostile side to the mycorrhizal networks. For example, the phantom orchid has no naturally occurring chlorophyll meaning it can’t create its own carbon sugars. Instead, it steals from the mycorrhizal network to obtain carbon in a parasitic way. This type of plant is known as a ‘myco-heterotroph’.
There is also evidence to suggest that the fungal networks are used to damage other plants. Allelopathy is the process where a plant attempts to inhibit other surrounding plant’s growth by secreting chemicals into the surrounding area. Recent literature suggests that the plants also use the mycorrhizal network to expand their range, sending chemicals directly to nearby plants.
This underground network isn’t the only way plants ‘talk’. Acacias protect themselves from giraffes by releasing tannins – a class of chemical that causes sickness in herbivores. When one begins to be eaten, they release ethylene gas into the air, alerting neighbouring acacias to start pumping out tannins alongside them. Many plants adopt similar means to defend themselves, for example, when lima bean plants suffer tissue damage, they release green leaf volatiles (GLVs). These travel through the air and cause neighbouring plants to grow faster and resist damage from herbivores.
Some use this as evidence to say that forests are massive communities and even possess some form of intelligence. Scientists are sceptical of this terminology when discussing plant communication. They feel that other researchers are forcing the idea of consciousness onto plants and forests and that it does nothing for our understanding of how they work. In 2007, 33 plant scientists published a critique on this emerging idea, explicitly refuting that plants possess intelligence.
From one perspective, older trees are transferring carbon to younger, disadvantaged saplings. From a different angle, the younger trees are simply feeding off the older ones parasitically. Instead of the forest being a conscious community, maybe it’s just plants chemically interacting with each other in random and selfish ways - put into place by natural selection.