Garden Notes: The “Wood Wide Web”
By Seth Hamby
The evolution of life on Earth has fascinated and perplexed people for ages. Our search for meaning and understanding has led us down a myriad of paths, some fruitful and some fateful. Arguably, it has been through science that humans have learned the most about the world in which we live. However, evolution remains an abstraction unless we embrace the fact that we are intimately connected to the long and beautiful history of life on Earth. In the words of the famous Evolutionary Biologist Stephen Jay Gould, "variation itself is nature's only irreducible essence...I had to place myself amidst the variation." In other words, the trajectory of the evolutionary history of life on this planet has made our existence possible, and it, therefore, behooves us all to appreciate and protect our planet and its systems that sustain us. Nothing could demonstrate this interconnectedness more eloquently than what scientists are now calling the "Wood Wide Web," a network of fungal threads that connect plants to each other. Before we delve into the subject at hand, we must first go way back to its ancient roots.
The prevailing consensus is that life originated in the oceans. Three and a half billion years ago, photosynthetic cyanobacteria evolved and began emitting oxygen as a waste product. According to new evidence, “simple” eukaryotic (non-bacterial) life was present in terrestrial environments as early as 1-1.2 billion years ago, perhaps much earlier. Approximately 600 mya, enough oxygen accumulated in the atmosphere to form the ozone layer, which reduced the amount of ultraviolet radiation from the sun to levels conducive to complex life on land. This is where our story begins.
Even before the terrestrial environment was colonized by life, the ancestors of fungi and plants had been interacting in the aquatic environment for ages. Studies of fungal carbohydrate-active enzyme genes that target plant cell walls indicate fungi were predating on plants long before venturing onto land. We cannot know for sure at this time, but this genetic ancestral trait may have helped facilitate the first plant-fungal symbioses in the harsh new conditions of dry land. It is not difficult to imagine that two organisms who team up to tackle a novel environment could be more successful than a single organism going at it alone.
According to genetic sequence data, all land plants (Embryophytes) evolved from a freshwater, multicellular green alga, most likely a Charyophyte algae. The transition from saltwater to freshwater likely evolved first in brackish estuarine environments. As the Charyophyte algae adapted to freshwater, it moved further from the marine environment. Once adapted, it likely colonized seasonal freshwater bodies where it was exposed to intermittent periods of wet and dry, facilitating adaptations that helped make possible the eventual transition to land.
Among living Embryophytes (land plants) today, the Bryophytes (mosses, liverworts, and hornworts) are considered to be most closely related to these early "land plants." In fact, the first known non-vascular plant fossil, from ~470 mya during the Devonian was that of a liverwort (Marchantiophyta). When we look at the community of microorganisms (microbiome) living in association with extant Streptophytes (Embryophytes and Charyophytes, collectively), most notably the liverworts, we see strong symbiotic relationships with nitrogen-fixing bacteria and mycorrhizal fungi.
Therefore, there is strong evidence to make the case that the invasion of plants into the terrestrial environment was likely facilitated through symbiotic relationships with fungal partners. In addition, these early terrestrial environments were nutrient-poor and prone to desiccation, so having a partner that could send out "roots" to anchor you and help feed you would be critical to survival.
The first known fossil to definitively show a symbiotic relationship between a land plant and fungi, that formed tree-like fungal roots (arbuscular mycorrhizas), was the 410 million-year-old Aglaophyton major, discovered in the Devonian-age Rhynie chert deposits of Scotland.
What are mycorrhizae? Mycorrhizae is the name of the association created when fungi infect the roots of plants (singular is mycorrhiza). The mycorrhiza is a non-pathogenic, root invader, bent on absorbing plant nutrients, and verging on parasitic. I say it ominously to demonstrate the fact that most relationships in nature are based on a complex calculus of cost versus benefit, not on selfless cooperation. In this case, the fungi get much-needed carbohydrates and the plants get much-needed mineral nutrients, among other things.
In fact, roughly 90 percent of land plants participate in mycorrhizal associations, especially in temperate regions where winter conditions make nutrient acquisition difficult for plants. In certain environments where conditions are right, namely bottomland forests, but by no means exclusively, the soil contains an abundance of mycorrhizal fungi so dense that networks are created connecting the trees together. The trees and their seedlings and saplings can then use these “common mycorrhizal (mycelial) networks” (CMN) to exchange nutrients and chemical messages. This network is often referred to as the “Wood Wide Web.” We will get back to the Wood Wide Web, but let us segue for a moment and talk about plant chemistry.
If you would have asked anyone other than the most open-minded of biologists and ecologists fifty years ago if plants could “talk” to each other, you would likely be dismissed as a fanciful or unrealistic person. In recent years, however, the reality of inter-plant communication is now widely accepted in the mainstream scientific community. Plants accomplish this amazing feat by means of volatile organic compounds (VOCs). In an effort not to belabor the profoundly complex chemistry, I will give a very broad explanation. All plants produce metabolites that control their energy budgets and make life possible. The metabolites responsible for normal growth, development, and reproduction are called “primary metabolites.” The metabolites that have evolved throughout time for specific ecological purposes are called “secondary metabolites.” Volatile organic compounds are a class of secondary metabolites.
Not only do plants communicate with other plants via airborne chemicals, but plants can also communicate with different parts of themselves in the same way. To anthropomorphize, plants are capable of performing a chemical “soliloquy” with themselves. A famous and gruesome example of chemical communication within and between plants of the same species occurred in South Africa, in the 1990s, during an extreme drought. It was discovered that kudu antelope were inexplicably dying en masse. After careful observation and study, scientists determined that at a certain level of browsing, acacia trees begin creating high levels of tannin-C in their leaves. The trees also began emitting ethylene gas, which prompted other acacia trees to follow suit. The high tannin-C content reacted with the guts of the kudu antelopes causing them to die and therefore reduce herbivory on the trees. In this case, the trees were responding to grazing pressures due to drought, but other examples have shown trees reacting similarly to the removal of apex predators and the subsequent proliferation of their herbivore prey.
Probably the most familiar example of plants releasing signaling chemicals is when we mow our lawns. While we humans might find the smell of fresh-cut grass delightful, it is actually a chemical warning to other plants that danger is imminent. This is called bunkering, and within minutes your lawn can respond by moving valuable resources from its leaves to its roots. You may also notice that a myriad of predatory insects such as wasps start showing up looking for a meal. These predators are attracted by these distress chemicals, which are also released when herbivorous insects attack plants.
The Wood Wide Web is capable of integrating multiple plant species as well as fungal species that interact, exchange information, and respond to changing conditions, creating a complex adaptive social network. It is likely that plants with greater access to sunlight and other resources can share the bounty with other plants who have less access to these resources, and that this sharing is not confined to members of the same species. Also, chemicals that discourage other species from growing (allelopathic) can be sent to areas where certain plants are unwanted within the network. These networks can also “facilitate defense against insect herbivores and foliar necrotrophic fungi by acting as conduits for interplant signaling.”
There are even dominant “mother trees,” who act as the “servers” in the forest network, controlling the flow of resources and information and ensuring the success of their offspring. Without these mother trees, experiments have shown that forest regeneration attempts often fail and that seedling survivorship is greatly reduced. Forest management, as well as sustainable agriculture, have already adopted techniques that limit disturbance to mycorrhizal networks and keep mother trees intact, therefore preserving forest health, while increasing carbon sequestration, crop yield, and biodiversity.
There is much yet to be discovered about the amazing and complex interactions within the Wood Wide Web. This has been only a basic introduction, but you can see how easy it is to venture off on a tangent when it comes to ecology. This is because literally everything is connected to everything else in some way.
“Man does not weave this web of life. He is merely a strand of it. Whatever he does to the web, he does to himself.” –Chief Seattle (c. 1786 – June 7, 1866, Suquamish and Duwamish Chie.)
