When caterpillars chomp on a tasty oak leaf, the oak senses the damage and produces bitter tannins that not only discourage the caterpillars but also attract parasitic wasps to lay their eggs inside the caterpillar’s soft body. These quickly hatch and the grubs eat the caterpillar from the inside out, a scenario reminiscent of the sci-fi horror franchise, Alien.
What has this to do with IBS, you might wonder. Well, to be explicit, not a lot, though the more I learnt from Peter Wohlleben’s new book, ‘The Hidden Life of Trees‘ , the more parallels I could see with the secret life of the gut. Science advances more quickly by being aware of what is happening in other, often quite disparate, fields and seeking to apply it.
Trees would seem to have all the components of a primitive ‘nervous’ system, consisting of sensors, a system of communication and intelligent responses. So just as leaves sense damage by caterpillars and signal the production of toxins, so roots move through the soil to find optimal growing conditions, avoiding areas where the soil is badly drained or where it is too dry or too acidic; finding their way between stones. It is too much to suggest that trees have a brain; they don’t even have nerves. Cell to cell communication occurs slowly – very slowly, by electrical impulses and chemical signals. The most one can say is that they possess a kind of vegetable intelligence.
Trees are sociable organisms. They grow better together. In the wild, most trees grow in woods or forests, co-existing in a complex ecosystem relying on each other for physical support, shelter, protection and nourishment.
It is for purposes of nourishment that the hidden life of trees seems most bizarre. Trees are intimately dependant on fungi. Under the floor of the forest an intricate network of tiny tubes, thousands of miles in length attaches to the root hairs of trees and makes connections with other trees and plants.
Trees produce their own energy high up in the leaf canopy by converting carbon dioxide and water into sugars by a unique chemical process catalysed by light. Without sufficient light, a tree can starve. The sugars are conveyed throughout the trees and down to the roots via tiny channels called phloem in the living layer of the tree trunk between the bark and the hard wood and find their way into the fungal mycelium. In this way, the fungi are sustained by sugars produced by photosynthesis high up in the canopy. They, in turn, digest rotting matter in the soil and extract nitrogen and minerals which they convey to the tree via the same root hairs. In this way the fungi and trees support each other in a truly symbiotic relationship.
It is through this ‘wood-wide-web’ of fungal mycelia that nutrients are shared between the trees and other plants in a locality so that damaged or ill trees are sustained and saplings nourished until they can access enough light to produce sufficient sugar to thrive independently. Thus trees can communicate with each other, sustaining each other in times of drought, supporting the growth of new trees when a mature tree collapses, and even acting like a primitive immune system to quarantine infection. In this way, the forest is a single organism, reminiscent of but more complicated than a colony of ants that harvest fungi and milk aphids.
Plants and fungi also have symbiotic relationship with other micro-organisms in the soil, not only other species of fungi, but springtails, beetle mites, mini-centipedes and vast populations of a seemingly infinite diversity of soil bacteria. These teeming populations of microscopic organisms assist fungi in breaking down organic compounds in the soil to provide the raw material for plants to take up through their root hairs. They also perform a complex variety of functions within individual plants. One notable example is the nitrogen fixing bacteria in the root nodules of legumes.
Although the soil microbiome is for the most part beneficial, plants need to protect themselves against harmful fungi and bacteria. These can invade through wounds in the bark caused by injury or insect pests. They gain access not only to sugars in the living phloem but also break down the structural elements of the plant, the cellulose and lignin. The tree might fight back with antimicrobial substances such as terpenes or tannins or by sealing off the wound with resins or new growth, but by the time the fungi have begun to digest the hard wood, the tree is weakened and its demise inevitable. The Honey Fungus, which can cover as much as 2000 acres, weigh 660 tons and live for thousands of years, is a pervasive saprophyte that lurks under the soil of woods awaiting an opportunity to invade sick trees and feed on the rotting wood.
The ecology of the gut may be seen as similar to woodland ecology only inside out. Trees and other plants a grow in soil composed of a diversity of chemicals and micro-organisms; we have the soil inside our colon. The implications are much the same. If you sterilise the soil, then the tree will not grow. Germ free animals fail to thrive. Plants need manure and compost. We need the same only we call them probiotics and prebiotics. Like the soil in the forest, the contents of the gut is a complete ecosystem. Bacteria produce chemicals that restrain the growth of other bacteria while at the same time protecting and nourishing the delicate layer of cells that line the root hairs or colonic epithelium and promoting the health of the whole organism. Most antibiotics have been extracted from heavily manured soil and manure, of course, comes from the gut of animals. Some antibiotics, like Penicillin and cephalosporins, come from fungi or moulds. Others like Streptomycin, Gentamicin, and tetracycline come from bacteria.
Both the tree and the gut interact with the relevant microbiome to regulate its composition and function by a combination of nourishment and antibacterial chemicals. Changes in the local environment of the gut caused by alterations in diet, stress, infection or injury are detected by primitive sense organs, which act via an enteric nervous system to bring about alterations in gut movements, absorption or secretion and activation of the gut immune system.
Trees can look after themselves quite well and protect themselves against insects and harmful fungi and bacteria as long as they are not stressed. The same applies to the gut. Stress for a tree includes drought, inundation or erosion; anything that alters the composition and health of the soil, the pH, the degree of oxygenation, the temperature or the just the amount of soil. Drought will dry out the soil and kill off the fungi, limiting the availability of water and micronutrients and reducing sugar production. Inundation will reduce the soil pH and redox potential, damaging the roots and encouraging consumption by saprophytic fungi. Conversely a plant may be killed by overfeeding in the same way as obesity weakens human beings. Similarly, the gut bacteria are compromised by changes in diet, antibiotics and other drugs. In both trees and human beings, a depleted or unstable microbiome may render the organism vulnerable to invasion by harmful species.
The gut, like the forest, is a vast ecological system. We are only too aware of rainforests damaged by climate change and soil erosion. And in a parallel system, too many people are suffering from ill defined malaise and vulnerable to epidemics of low grade inflammation affecting different parts of the body and probably related to invasion by micro-organisms from the gut. We are only just beginning to wake up to the dangers of undermining our gut bacteria with antibiotics and restrictive diets and destabilising them with the gut reactions of stressful life styles.
Viewed from the analogy of the wood-wide-web, just feeding probiotics seems hopelessly inadequate, rather like spraying the forest with fungal spores and hoping that some may reach the forest floor and establish themselves. Epidemics of illness in a forest system imply environmental damage, caused by climatic change or forest clearance. They require custodial care. So, confronted with epidemics of ill-defined human illness that defy specific diagnosis and treatment, we need to care for the microenvironment within our guts in a way that allows micro-organisms to grow and establish healthy connections with the absorptive cells and immune system. This involves a healthy prebiotic diet of poorly absorbed carbohydrate and a life style that prevents damaging physiological changes in the climate of the colon.
Apart from the obvious, there is an enormous temporal difference between that way a tree or our gut reacts. Everything a tree does, it does slowly – very slowly. Time is on their side. Trees can live for thousands of years. There are trees surviving in Britain that were saplings at the time of the Norman conquest. And in America, the longest lived trees, the giant Redwoods were present when man first crossed the Siberian land bridge and spread throughout the continent. So trees can bide their time, lying dormant while they wait for the danger to pass and the right conditions to germinate and grow. We do not have the same luxury.
An insightful, moving and lovely analogy, thank you.
Thank you, Carly
Fantastic post Nick. Really well written and full of great ideas, I love the idea of the Wood Wide Web and of humans having their soil on the inside!
Thanks Joan. I really enjoyed writing this one. Lots of fascinating connections.