We once thought that plants were just standalone entities, much like us, the key difference being our ability to interact with the world and each other. But research over the past few decades has found that plants, especially trees, may not be as inert as once thought. Granted, plant communication isn't majorly advanced, but it has been likened to communicating via the internet.
A common theme in the evolutionary race has always been the overcompensation of species; organisms would adapt to dampen or negate the effects predatory species had on them, and in turn said predators would counter-adapt, and so on. This veritable arms race is prevalent in most organisms on Earth, but in regards to plants and their phytophagous enemies, the race has gone a bit sci-fi.
Originally formed to protect the individual against insect damage or disease, systemic response chemicals (SRCs) would be released as an immediate defence, but also be distributed to unaffected parts of the plant, bolstering against the threat. These chemicals would increase the production of lignin, the hard outer lining of plant cells to make the flesh less digestible, and tannins to give an acrid taste.
This is a pretty standard counter-adaptation to survive stresses, but a 1983 paper sparked intrigue when undamaged Sitka Willow trees were found intercepting these chemicals from adjacent Sitkas under attack from tent caterpillars. The lack of root connection between individuals meant that leaves damaged by the caterpillar infestation would release SRCs pheromonaly across the stand of trees, triggering pre-emptive defences in their neighbours. Some scientists speculated this kind of response was incidental; that SRCs leeched into the air were not a true communication and more like 'overhearing' the distress. However, the plant-to-plant interaction story goes further down the rabbit hole (literally).
Flickr | foam
Symbiotic fungi are another example of incredible adaptation right beneath our feet. Specifically tailored for mutually beneficial relationships, they can increase the mineral uptake for the host in return for some of the host's sugars. These fungi physically work their way into the roots forming a mycorrhiza, which then branches out far beyond the root range of the host. These mushroom roots, or mycelium, grow so big they eventually form a network with other trees and their mycorrhizae. Plants that share a mycorrhizal network can indeed send defensive SRC messages via this connection, far exceeding the range of pheromonal SRCs, safeguarding a larger stand of trees. But more impressively can also actively manage resources across the network!
Say if a tree were lacking nutrients a nearby 'donor' tree would redistribute its surplus supply. This was tested in Simard's 1997 paper by supplementing trees with 13CO2, a naturally occurring traceable isotope. Once the donor tree was exposed to 13CO2 recipient trees were deprived of nutrients. The isotope was observed to have been transferred from the donor's leaves to recipient roots through their shared mycorrhizal network. In a naturalistic setting this is theorised to assist struggling saplings as Nitrogen and Phosphates can also be transferred.
No one is absolutely sure what benefits the donor trees gain from this exchange, but logic would dictate it as the age-old strategy of self-preservation; by ensuring the development of neighbours, the more nutrients they'll produce and therefore give back in the long run. If the mycorrhizal fungus is obligate to the tree species, it would surely benefit them both to facilitate these exchanges, as it would influence their standing against competitors, as well as increase the overall nutrient pool for both symbionts.
Flickr | scottks
However, it's not always sunshine and rainbows. The defensive strategies that unite each tree can also be used to insidiously carry out attacks on competitors. Species of tree, such as American Black Walnut and some Hickory sp., produce a carcinogenic compound called Juglone. These trees use this compound to monopolise their surroundings by sending the toxin through their mycorrhizal connections, killing off all competitors.
All of this research has changed the way we look at plant life and given insight into how they've dominated the Earth; these communications showcase the interconnectedness of species. No one knows exactly why pheromonal or mycorrhizal exchanges occur, but recent studies found that air pollution adversely affects the transmission and interception of airborne SRCs and that intensification of farming, and changes in land use affect the establishment of mycorrhizae. Given there is still so much to discover on this relatively new subject we can only hope there's enough time to get to the bottom of this mystery.
By Thomas Phillips - Online Journalism Intern