Can Holobionts Love Each Other?

Two holobionts enjoying each other's company. Sara peacefully sleeps in the "arms" of a beech tree on the Amiata mountain. She is obviously happy to be embraced in this way; we are tempted to think that the tree thinks the same.

Do trees perceive the world around them? Absolutely yes, but in a very different way from how we humans do. Trees have no eyes, no muscles, no above-ground nerves. But they perceive chemical signals, light signals, vibrations, and, probably, things that we don't even imagine could be perceived. So, the tree gently holding Sara cannot "see" her. But it can perceive her presence in the form of vibrations and chemical signals. The tree perceives Sara more or less how we could perceive a ghost. 

And what does the tree think of Sara? The brain of the tree is below ground; it is the vast network of connections of the root system, boosted by the help of fungi. It is called the mycorrhizal system. Maybe this alien brain can form an image of the strange creature resting near its trunk, although, for us, it is nearly impossible to understand in which form. Maybe the tree is asleep, too. And what does it dream? We can't know, just as we can't know what Sara is dreaming. The only thing we can say is that there is no reason to think that holobionts can't be in love with each other. 

h/t Sara

A Stand, a Grove, a Wood, or a Forest? A Discussion with Louise, the Natural Biologist

A "faggeta", a forest of beech trees in Abbadia San Salvadore, central Italy. 

By Mark Haubner

A Discussion with Louise, the Natural Biologist

Way back in 2021, my Drawdown East End teammates and I got together and, based on the
science of Drawdown, decided to embark on a program to plant trees in the five towns of our
Peconic Bioregion. Our imaginations took hold of us, and we decided that we would encourage
planting one tree for every man, woman, and child on our two forks—170,000 all told.

Gathering information and lining up our list of benefits brought me to asking a professional
environmental person for some input, so I met with Louise at a local coffee shop in Riverhead.

The conversation went something like this:

Louise, we’re very excited—we’re starting a program to plant 170,000 trees.

Where? she asked.

All over the entire region, some 2-3 trees on every home’s property, I replied.

That’s nice, she said.

But, I asked, isn’t this a great way to increase our trees and forests?

Not really, she said.

Why not? I asked.

They’re not all in the same place, came the answer.

What if we planted 100 trees together?

You’d have a stand.

What about 1000?

You’d have a grove.

What about 10,000.

You’d have a wood.

And 170,000 in one place?

Well, she admitted, that would be a forest. But you’d have to get back to me in 300 years to see
how you did.

_______________________________________________________________

Note: this post originated from a discussion on the Republican plan of planting a trillion trees (!!) to fight global warming (see NBC news). A desperate plan that would do nothing except damage. 

About Mark Haubner

Drawdown East End, Steering Committee
North Fork Environmental Council, President
North Fork Civics, Moderator
Riverhead Neighborhood Preservation Council, Member
Town of Riverhead Environmental Advisory Committee, Chair
Town of Riverhead Comp Plan Steering Committee, Member
Suffolk County Council on Environmental Quality, Member
Taking A Lead on Zero Waste, Co-convener
 
UCAL San Diego, Sustainability & Behavior Change Certificate
MIT En-ROADS, Ambassador
Inspiring Transition, Catalyst
Creating the Future, Integrity Board, Member
Capra’s Systems View of Life, Alumni
CBSM Programs, Designer

Trees Come from Air

 

Richard Feynman, 1983. He didn't have the concept of "holobiont" -- but in this clip, he shows that he clearly understood the metabolism of the biosphere. (the part about trees starts at 3:50).

Rain is Life, and Holobionts Create it

 

Buying vegetables at a stall in Florence on a rainy day. My wife, Grazia, is the one with the pink umbrella. 

By Ugo Bardi

After three months of drought during the summer, Florence is now drenched in rain. It has been raining for two months and people are complaining that it is too much! But it is how things have to be: I recently discovered that rain is an "autocatalytic" phenomenon. A more humid atmosphere creates more rain, and rain creates a more humid atmosphere. And that creates long periods of static weather -- too little rain and then too much. 

I learned that from the work of Anastassia Makarieva, Mara Baudena, and several others who have studied the relationship between atmospheric humidity and rain. Look at this figure, from a paper by Makarieva et al., to be published.

The y-axis is the amount of rain, in mm/hour. The x-axis is the amount of water vapor in the "air column." Note the strongly non-linear relationship. It is a typical power law: a small increase in atmospheric humidity causes a large change in rainfall. The three red points indicate the boundary of the "abiotic regime" (no rain) and the power law region.  

As I said, it is an autocatalytic phenomenon, Rain tends to generate more rain, at least as long as it wets the land and it generates moisture transpiration, which increases the water vapor content in the atmosphere. This has very practical consequences in many senses. One is the role of forests in weather and climate. Forests generate strong evapotranspiration, that is they pump water from the soil to the atmosphere. And, also, forests tend to keep water in the soil, slowing down the runoff.

So, not only do forests generate rain, but they also tend to maintain the rain pattern. Without forests, and with the land covered with buildings, you have the typical desert climate: dry most of the time, then with short periods of heavy rain. Disasters ensue, now a common pattern in areas such as California or Italy, where deforestation has taken place. 

So, we need our fellow holobionts, the trees. Onward, fellow holobionts!

(below, some rain-loving holobionts pictured together)

The Return of the Ents: The Tribe of the Trees

Image by VargasNi

The idea of trees moving and fighting humans is old, it goes back to Shakespeare's Macbeth and the prophecy of the witches

Macbeth shall never vanquish'd be until

Great Birnam Wood to high Dunsinane Hill
Shall come against him.

You find the same idea again in Tolkien's "Trilogy of the Ring," with the creatures called "Ents," which attack the city of the orcs at the battle of Isengard. Both Shakespeare and Tolkien express a similar idea, that at some point too much is too much and nature rebels against human evil with all its force. To the point of seeing trees taking their roots out of the ground, and marching against human cities. 

In modern times, the idea that trees and humans are in conflict is gaining attention. The concept of "biotic regulation of the environment" proposed by Makarieva, Gorshkov, and others, is gaining ground in the world. It basically says that if we destroy the world's forests, we destroy ourselves. Not an easy position to take in a world where forests are considered "natural resources" and where the standard economic theories say that a tree has no monetary value unless it is cut down and sold as wood.

Would "humanizing" trees in fiction help people to have a more gentle attitude toward trees? Maybe, but it is not so clear. Personally, I always found depressing Tolkien's walking trees. Their representation in the 2002 movie "The Two Towers" didn't change my opinion of them. They are clumsy, ugly, and not really believable, not even in a fantasy movie. 

Recently, another take on presenting the world from the viewpoint of trees was tried by Stefano Mancuso, well-known botanist at the University of Florence, and an expert in plant neurobiology. If there is a human being on this planet who can know something about how plants think, he is the one! So, he published a novel titled "the tribe of the trees" (la tribù degli alberi). (so far available only in Italian).

I have mixed feelings about this novel. For one thing, it is a well-written story, nice to read, captivating, and with delightful characters.  The story moves onward smoothly, one event after the other, leading to the conclusion when trees discover the problem of global warming -- even though they don't know anything about atmospheric physics and have never seen a human being.  

Mancuso's trees have many "tree-like" characteristics, and they are far from being as clumsy and ugly as Tolkien's Ents. And note that there are no human beings whatsoever in the novel: it is only trees! But Mancuso's trees are, in my opinion, a little too humanized. They can move, speak to each other, and, in many ways, behave like human beings. Mancuso's forest looks very much like a modern university, with its various departments (=tribes) and their researchers, librarians, technicians, etc.

An expert in plant neurobiology, such as Mancuso, could have told us much more about how trees "think," if they do (I think they do!). But I can also understand that in novel terms it is not easy to build a story about creatures whose brain is located underground, cannot move, and perceive the external world mainly as a combination of chemical signals. The power of human imagination is immense, but it would be a truly alien novel, one that maybe only trees could read!

For me, the best human fiction piece that tries to understand trees is "The Secret of the Old Wood" (Il segreto del bosco vecchio) by Dino Buzzati (1935). It is, however, one of those masterpieces that go beyond the mere concept of narrative and touch the very fabric of the universe. If you can understand Italian, read the book or watch the movie (or both). It is a humbling experience that will make you reflect on what it means to be human. Or a tree. 

Humans and Trees: They like each other, they hate each other

 

The Wochecha Mariam church in Addis Ababa. In a generally dry landscape, wherever you see a green circular area in Ethiopia, it is often around a Church. The Ethiopians have recognized long ago the value of trees as part of the spiritual experience of being human.

Humans and trees have had a difficult relationship over the past few tens of thousands of years, when humans started using stone tools and they discovered that, with some patience, a stone axe could take down a large tree. The age of deforestation is still ongoing, actually accelerating. It was not so long ago, in this extended time perspective, that the English landlord Jonah Barrington who lived in Ireland during the early 19th century, uttered a sentence that summarizes the ruthless kind human attitude toward trees: "Trees are stumps provided by nature for the repayment of debt." Somehow, we have created a frame of thought that sees no value in a tree until it is felled and sold on the market.

And yet, humans have a more complicated relationship with trees than simply cutting them down with chainsaws. It is a minority view, often disparaged with the term "tree-hugging," but it is there. Trees are something that we cannot ignore. They have been with us from the remote origins of humankind. And, sometimes, we truly feel that we need to hug a tree. Sometimes, we do, and I think it can't be so bad for one's health. In the picture, you see Grazia (Ugo Bardi's wife), hugging a Cupressus sempervirens in Italy.

Do trees perceive the presence of humans? A good question. They have a complex and sophisticated sensory system that perceives light, chemicals, and vibrations. It may well be possible that a tree can sense a human walking nearby as a combination of sounds and smells. Does a tree like to be hugged? Some questions are beyond our understanding, but they are still worth asking. And, who knows? Can we exclude that the trees of the sacred forest around a church in Ethiopia pray to God just like humans do? 

Forest Recovery: A quote by Anastassia Makarieva

 

You see, there is a succession process for forest recovery. We first have shrub grasses after some disturbance like fire, then it takes time for that to be replaced by trees. So if we are lucky, our grand grandchildren will be walking in such a forest, so this dimension should also be stressed. We are working for the future we are not just securing for ourselves some two dozen years of better comfort. Rather, we send a message through centuries such that people will remember us, and walking into this forest along the brooks and rivers they will remember us with gratitude for our care and dedication.

Anastassia Makarieva  

Can HolobiontsThink?

 

My wife, a holobiont called Grazia, hugs another holobiont, a Cupressus Sempervirens, in the hills near Florence, Italy

This is an excerpt from the chapter I am writing for a multi-author book 

I started this chapter by examining trees and forests as holobionts, then looking at human beings. Trees and humans are as alien to each other as we could possibly imagine. Humans are mobile creatures with an extravagantly powerful metabolism that makes them able to sustain protracted efforts longer than any other living animal. That turbo-charged metabolism is also used to maintain their large brains, of which they are very proud. They use their brains to control their muscles and their sophisticated sensory apparatus, as well as to deal with each other in complicated social rituals.

Trees are the opposite in almost all respects: they are immobile, their metabolism is slow: and they can’t even control their internal temperature. They don’t have eyes, nerves, brains, and not even muscles. Yet, they move, they sense their environment mainly by chemical signals, but also visual and mechanical ones -- including vibrations in the acoustic frequency. They "know" what's going on around them, but in ways that are mostly alien to mobile mammals, including humans.

Nevertheless, humans and trees are both holobionts at their core, and they share more than it would seem at first sight. It is not even forbidden to ask whether trees and other plants might be “conscious” in some way. This is a subject of wide debate, nowadays, and it would be out of the scope of the present text to enter into the details of a question whose answer depends primarily on the definition of the entity being debated. For what we are concerned, we can rather ask the question of whether some creatures store somewhere a schematic representation of at least some elements of the outside world, and modify their behavior depending on the sensor input they receive. That implies a certain level of “consciousness.”

In the case of human beings, there is no doubt that this capability exists. Assuming that most of the readers of this text are human, they should be familiar with the typical sensation of being encased in a bodily container. We have no direct perception of having a brain, but somehow we perceive that we are “inside” a body, that we are a sort of "homunculus" that resides someplace behind the eyes. And, surely, we do keep representations of the outside world, sometimes even too much, as when our political leaders claim that they can “create their own reality.”

How about a tree, then? Where would a tree have its representation of the outside world? As I said, trees have no brains and no nervous system, but they can transmit electric signals from cell to cell. It is a still scarcely known field, but it is known that the phloem and xylem cells form a network to transmit electrical signals long-distance within the plant. At the root level, the mycorrhizal system shares chemical signals within single plants and also from one plant to another. Would such a network be able also of storing information, just like a neural network does in animals? There is no reason to deny that it could. In this case, the representation of the outside world would be stored in the plant as a configuration of the network, continuously changed by sensorial inputs, and leading to signals being transmitted to the various parts of the plants instructing them, for instance, to release volatile organic compounds to fight an insect attack.

If that were the case, apart from the slippery concept of consciousness, a plant would not have the sensation of being encased in a bone cage that humans have. Its intelligence would be delocalized all over the structure. The plant would “feel” the conditions of the leaves, and the presence of sunlight. It would “smell” chemicals floating in the air and perceive the sound of living creatures moving in the vicinity. It would also be actively sending and receiving chemical signals through the mycorrhizal system. In short, it might have a representation of the external world of complexity comparable to the one that humans can build in from their sensorial input. Whether plants could also “create their own reality,” that is, dream, is impossible to say. Communicating with trees is a challenge that was never met, at least in terms compatible with the scientific method. Nevertheless, there seems to be a certain empathy between trees and humans. In the photo, the author’s wife, Grazia, communicates with a specimen of Cupressus sempervirens, in Tuscany.

About this encounter of these two holobionts, we may speculate about their reciprocal sensorial experience. For the human, the tree holobiont is perceived mainly as a visual entity -- but her sensorial system has no capability of detecting the underground root system. Nor she can detect the complex chemical signaling that the tree is operating inside and outside itself. For the tree, instead, the human cannot generate as a visual image, but it is possible that the tree perceives the human from the vibrations she generates and, maybe, detecting the chemical signals she produces. Whether the tree knows that it is being hugged is impossible to say, but we cannot completely discount this possibility. As a further note, both humans and trees use sunlight for chemical processing on their surfaces. Trees use it for photosynthesis, while humans need it to synthesize the compound called "Vitamin D" that they need for their survival.   

We can gather from this discussion that creating a representation of the outside world is a fundamental survival element of holobionts. And since holobionts are the main form that life on Earth takes, we should admit that all holobionts have this kind of capability. In other words, holobionts can “think.” Not in the same way as humans think, of course. But the process of thinking is part of the homeostatic adaptation that all living beings tend to attain. To accomplish that, they need to process information: it is the basic idea of the “dissipation structures” as defined by Prigogine. These structures process entropy and dissipate it, and entropy is basically information. So, holobionts are structured in such a way as to modify their internal structure to obtain homeostasis and maintain it despite changes in the structure of their environment. The holobiont itself is the holobiont’s “brain” and its internal structure stores a representation of the outside world.

Seen in these terms, the hypertrophic brain of which humans are so proud is not an exception to the rule that holobionts store information in their networked structure. All the neurons in the human brain are the same: there is no “super-neuron” that controls the other neurons. In a sense, you could say that the brain is a holobiont, possibly the biggest one known in the ecosystem in terms of the units it contains, with a total of some 86 billion neurons. It is still a small number if compared to the genetic information stored by the whole biosphere has been estimated as Thus, the total amount of genetic information stored in the natural biota is of the order of 1016 bit (Gorshkov et al., 2000) and coincides as an order of magnitude with the information stored in a human brain. This similarity may give us some interesting insights about the idea that the world in which we live is a single, extremely large, holobiont to which we sometimes give the name of Gaia, the Earth Goddess (Castell et al., 2019)

My Neighbor Holobiont

The most beautiful scene ever created. From Miyazaki's "Tonari no Totoro" (My neighbor Totoro). The true spirit of the holobiont is caught in this scene.

Thinking like a Tree. Old-Growth Forests as Holobionts

A "holobiont" is a living creature formed of independent, but cooperating, organisms. It is a wide-ranging concept that can explain many things not just about the ecosystem of our planet, but also about human society, and even more than that. Photo courtesy of Chuck Pezeshky. This post was modified and improved thanks to suggestions received from Anastassia Makarieva.

When was the last time that you walked through an old-growth forest? Do you remember the silence, the stillness of the air, the sensation of awe, the feeling that you are walking in a sacred place? The inside of a forest looks like a cathedral or, perhaps, it is the inside of a cathedral that is built in such a way to resemble a forest, with columns as trees and vaults as the canopy.  If you don't have a forest or a cathedral nearby, you can get the same feeling by watching the masterful scene of the forest-God appearing in Miyazaki's movie, "Mononoke no Hime" (The Princess of the Ghosts). 

In a way, when you walk among trees, you feel that you are at home, the home that our remote ancestors left to embark on the mad adventure of becoming human. Yet, for some humans, trees have become enemies to be fought. And, as it is traditional in all wars, they are demonized and despised. It was the English landlord Jonah Barrington who commented about the destruction of Ireland's old forests that "trees are stumps provided by Nature for the repayment of debt." And, as it is traditional in all wars of extermination, not a single enemy was left standing. 

The war metaphor is engrained in our minds of primates, the only mammals that wage war against groups of their own species. So much that sometimes we imagine trees fighting back. In the "Trilogy of the Ring" by Tolkien, we see walking trees, the "ents," standing in arms against humanoid enemies and defeating them. Clearly, we feel guilty for what we have been doing to Earth's forests. A sensation of guilt that goes back to the time when the Sumerian King Gilgamesh and his friend Enkidu were cursed by the Goddess for having destroyed the sacred trees and killed their guardian, Humbaba. From that remote time, we have continued to destroy Earth's forests, and we are still doing that. 

Yet, if there is a war between trees and humans, it is not obvious that humans will win it. Trees are complex, structured, adaptable, tough, and resourceful creatures. Despite the human attempts to destroy them, they survive and even thrive. The most recent data indicate a greening trend of the whole planet [3], probably the result of humans pumping carbon dioxide (CO2) into the atmosphere (this greening is not necessarily a good thing, neither for trees nor for humans [4], [5]). 

But what are trees, exactly? They have no nervous system, no blood, no muscles, just as we have no capability of doing photosynthesis, nor of extracting minerals from the soil. Trees are truly alien creatures, yet they are made of the same building blocks as we are: their cells contain DNA and RNA molecules, their metabolism is based on the reduction of a molecule called adenosine triphosphate (ATP) created by mitochondria inside their cells, and much more. And, in a certain sense, trees do have a brain. The root system of a forest is a network similar to that of a human brain. It has been termed the “Wood-Wide Web” by Suzanne Simard and others [1]. What trees “think” is a difficult question for us, monkeys but, paraphrasing Sir. Thomas Browne [2], what trees are thinking, just like what song the Sirens sang to Ulysses, though puzzling questions are not beyond all conjecture. 

Whether trees think or not, they have the basic characteristics of all complex living systems: they are holobionts. "Holobiont" is a concept popularized by Lynn Margulis as the basic building block of the ecosphere. Holobionts are groups of creatures that collaborate with each other while maintaining their individual characteristics. If you are reading this text, you are probably a human being and, as such, you are also a holobiont. Your body hosts a wide variety of creatures, mostly bacteria, that help you in various tasks, for instance in digesting food. A forest is another kind of holobiont, vaster but also structured in terms of collaborating creatures. Trees could not exist alone, they need the all-important "mycorrhizal symbiosis." It has to do with the presence of fungi in the soil that collaborate with plant roots to create an entity called the “rhizosphere,” the holobiont that makes it possible for a forest to exist. Fungi process the minerals that exist in the soil and turn them into forms that plants can absorb. The plant, in turn, provides the fungi with energy in the form of sugars obtained from photosynthesis. 

So, even though trees are familiar creatures, it is surprising how many things are scarcely known about them and some are not known at all. So, let’s go through a few questions that disclose whole new worlds in front of us. 

First: wood. Everyone knows that trees are made of wood, of course, but why? Of course, its purpose is the mechanical support of the whole plant. But it is not a trivial question. If wood serves for mechanical support, why aren’t our bones made of wood? And why aren’t trees, instead, made of the stuff our bones are made of, mainly solid phosphate?

As usual, if something exists, there is some reason for it to exist. Within some limits, evolution may take different paths simply because it has started moving in a certain direction and it cannot move back. But, as things stand on Earth, wooden trunks are perfectly optimized for their purpose of support of a creature that doesn't move. Tree trunks (not palms, though) grow in concentric layers: it is well known that you can date a tree by counting the growth rings in its trunk. As a new layer grows, the inside layers die. They become just a support for the external layer called the “cambium” which is the living part of the trunk, containing the all-important “xylem”, the ducts that bring water and nutrients from the roots to the leaves. The cambium also contains the "phloem," another set of ducts that move water loaded with sugars in the opposite direction, toward the roots. The inner part of the trunk is dead, so it has no metabolic cost for the tree. Yet, it keeps providing the static support the tree needs. 

The disadvantage is that, because the internal part of the wood is dead, when a branch or a trunk is broken, it cannot be healed by reconnecting the two parts together. In animals, instead, the bones are alive: there is blood flowing through them. So, they can regrow and rebuild the damaged parts. It is probably a necessary feature for animals. They jump, run, fly, fall, roll, and do more acrobatic feats, often resulting in broken bones. Of course, a broken bone is a major danger, especially for a large animal. We don’t know exactly how many animals suffer broken bones and survive, but it seems that it is not uncommon: live bones are a crucial survival feature [6], [7]. But that's not so important for trees: they do not move and the main stress they face is a heavy gust of wind. But trees tend to protect themselves from wind by shouldering against each other – which is, by the way, another typical holobiont characteristic: trees help each other resisting wind, but not because they are ordered to do so by a master tree. It is just the way they are.

That's not just the only feature that makes wood good for trees but not for animals. Another one is that bones, being alive, can grow with the creature they support. They can even be hollow, as in birds, and so be light and resilient at the same time. If our bones were made of wood, we would have to carry around a large weight of deadwood in the inner part of the bone. That's not a problem for trees which, instead, profit from a heavier weight in terms of better stability. And they do not have to run unless they are the fantasy creatures called "ents."  Spectacular, but Tolkien would need to perform some acrobatic feats of biophysics to explain how some trees of Middle Earth can walk around as fast as humans do.

So, there is plenty of logic in the fact that trees use wood as a structural material. They are not the only creatures doing that. Bamboos (bambusoideae), are also wooden, but they are not trees. They are a form of grass that appeared on Earth just about 30 million years ago, when they developed an evolutionary innovation that makes their "trunk” lighter, being hollow. So, they can take much more stress than trees before breaking and that inspired many Oriental philosophers about the advantages of bending without breaking. Among animals, insects and arthropods use a structural material similar to wood, called "chitin." They didn't solve the problem of how to make it grow with the whole organism, so use it as an exoskeleton that they need to replace as they grow.

Now, let's go to another question about trees. How does their metabolism work? You know that trees create their own food, carbohydrates (sugar), by photosynthesis, a process powered by solar light that works by combining water and carbon dioxide molecules. One problem is that sunlight arrives from above, whereas trees extract water from the ground. So, how do they manage to pump water all the way to the leaves? 

We animals are familiar with the way water (actually, blood) is pumped inside our bodies. It is done by an organ called "heart," basically a "positive displacement pump" powered by muscles. Hearts are wonderful machines, but expensive in terms of the energy they need and, unfortunately, prone to failure as we age. But trees, as we all know, have no muscles and no moving parts. There is no “heart” anywhere inside a tree. It is because only the feverish metabolism of animals can afford to use so much energy as it is used in hearts. Trees are slower and smarter (and they live much longer than primates). They use very little energy to pump water by exploiting capillary forces and small pressure differences in their environment. 

"Capillary forces" means exploiting interface forces that appear when water flows through narrow ducts. You exploit that every time you use a paper towel to soak spilled water. It doesn't happen in human-made ducts, nor in the large blood vessels of an animal body. But it is a fundamental feature in the movement of fluids in heartless (not in the bad sense of the term) plants. But capillary forces are not enough, by far. You need also a pressure difference to pull the water high enough to reach the canopy. That you can attain by evaporating water at the surface of leaves. The water that goes away as water vapor creates a small difference in pressure that can pull more water up from below. This is called a "suction pump." You experience it every time you use a straw to drink from a glass. It is, actually, the atmospheric pressure that pushes the water up the straw. 

Now, there is a big problem with suction pumps. If you studied elementary physics in school, you learned that you cannot use a suction pump to pull water higher than about 10 meters because the weight of the water column cannot exceed the atmospheric push. In other words, you wouldn't be able to drink your coke using a straw longer than 10 meters. You probably never made the experiment, but now you know that it won't work! But trees are far higher than ten meters. You just need to visit your local park to find trees that are far taller than that. 

That trees can grow so tall is a little miracle that even today we are not sure we completely understand. The generally accepted theory for how water can be pumped to such heights is called the “cohesion-tension theory” [8].  In short, water behaves, within some limits, as a solid in the live part of a tree trunk, the “xylem.” The ducts do not contain any air and water is pulled up by a mechanism that involves each molecule pulling all the nearby molecules. The story is complicated and not everything is known about it. The point is that trees do manage to pump water to heights up to about 100 meters and even more. There is a redwood tree (Sequoia sempervirens), in California, that reaches a height of 380 feet, (116 m). It is such an exceptional tree, that it has a specific name “Hyperion.” 

Could trees grow even higher? Apparently not, at least not on this planet. We are not sure of what is the main limiting factor. Possibly, the cohesion-tension pumping mechanism that brings water to the leaves ceases to work over a certain height. Or it could be the opposite problem: the phloem becoming unable to carry sugar all the way down to the roots. Or, perhaps, there are mechanical limits to the trunk size that can support a crown large enough to feed the whole tree. 

Nevertheless, some works of fiction imagined trees so huge that humans could build entire cities inside or around the trunk. The first may have been Edgar Rice Burroughs, known for his "Tarzan" novels. In a series set on the planet Venus, in 1932, he imagined trees so big that an entire civilization had taken refuge in them. Just a couple of years later, Alex Raymond created the character of Prince Barin of Arboria for his "Flash Gordon" series. Arboria, as the name says, is a forested region and, again, trees are so big that people can live in them. More recently, you may remember the gigantic "Hometrees" of the Na'vi people of planet Pandora in the movie "Avatar" (2009).  In the real world, some people do build their homes on trees -- it seems to be popular in California. The living quarters must be cramped, to say nothing about the problems with the static stability of the whole contraption. But, apparently, a section of our fantasy sphere still dreams about the times when our remote ancestors were living on trees. 

But why do trees go to such an effort to become tall? If the idea is to collect solar light, which is the business all plants are engaged in, there is just as much of it at the ground level as there is at 100 meters of height. Richard Dawkins was perplexed about this point in his book “The Greatest Show on Earth” (2009), where he said:

“Look at a single tall tree standing proud in the middle of an open area. Why is it so tall? Not to be closer to the sun! That long trunk could be shortened until the crown of the tree was splayed out over the ground, with no loss in photons and huge savings in cost. So why go to all that expense of pushing the crown of the tree up towards the sky? The answer eludes us until we realize that the natural habitat of such a tree is a forest. Trees are tall to overtop rival trees - of the same and other species. … A familiar example is a suggested agreement to sit, rather than stand, when watching a spectacle such as a horse race. If everybody sat, tall people would still get a better view than short people, just as they would if everybody stood, but with the advantage that sitting is more comfortable for everybody. The problems start when one short person sitting behind a tall person stands, to get a better view. Immediately, the person sitting behind him stands, in order to see anything at all. A wave of standing sweeps around the field, until everybody is standing. In the end, everybody is worse off than they would be if they had all stayed sitting.”

Dawkins is a sharp thinker but sometimes he takes the wrong road. Here, he reasons like a primate, actually a male primate (not surprising, because it is what he is). The idea that trees “compete with rival trees – of the same and other species” just doesn’t work. Trees can be male and female, although in ways that primates would find weird, for instance with both male and female organs on the same plant. But male trees do not fight for female trees, as male primates do with female primates. A tree would have no advantage in killing its neighbors by shadowing them -- that wouldn't provide "him" or "her" with more food or more sexual partners. Killing the neighbors would perhaps allow a tree to grow a little larger, but, in exchange, it would be more exposed to the gust of wind that could topple it. In the real world, trees protect each other by staying together and avoiding the full impact of gusts of wind. 

It doesn’t always work and if the wind manages to topple a few trees, then a domino effect may ensue and a whole forest may be brought down. In 2018, some 14 million trees were destroyed in Northern Italy by strong gales. The disaster was probably the result of more than a single cause: global warming has created winds of a strength unknown in earlier times. But it is also true that most of the woods that were destroyed were monocultures of spruce, plantations designed for wood production. In the natural world, forests are not made of identical trees, spaced from each other like soldiers in a parade. They are a mix of different species, some taller, some less tall. The interaction among different tree species depends on a number of different factors and there is evidence of complementarity among different species of trees in a mixed forest [9], [10]. The availability of direct sunlight is not the only parameter that affects tree growth and mixed canopies seem to adapt better to variable conditions. 

As a further advantage of being tall, a thick canopy that stands high up protects the ground from sunlight and avoids the evaporation of moisture from the soil, conserving water for the trees. When the sun makes the canopy hotter than the soil, the result is that the air becomes hotter higher up, technically it is called "negative lapse rate" [11].  Since the cold air is below the hot air, convection is much reduced, the air stays still, and water remains in the soil. If that's not completely clear to you, try this experiment: on a hot day, scorching if possible, stand in the sun while wearing a thick wool winter hat for several minutes. Then wear a sombrero. Compare the effects. 

So, you see that having a canopy well separated from the ground is another collective effect generated by trees forming a forest. It doesn't help single trees so much, but it does help the forest in conserving water by generating something that we could call a "holobiont of shadows." Each tree helps the others by shadowing a fraction of the ground, below. And that creates, incidentally, the "cathedral effect" that we experience when we walk through a forest. Again, we see that this point was missed by Dawkins when he said that "That long trunk could be shortened until the crown of the tree was splayed out over the ground, with no loss in photons and huge savings in cost." Another confirmation of how difficult it is for primates to think like trees. 

That doesn’t mean that trees do not compete with other trees or other kinds of plants. They do, by all means. It is typical for a forest especially after an area has been damaged, for instance by fire. In that area, you see growing first the plants that grow faster, typically herbs. Then, they are replaced by shrubs, and finally by trees. The mechanism is generated by the shadowing of the shorter species created by the taller ones. It is a process called "recolonization" that may take decades, or even centuries before the burned patch becomes indistinguishable from the rest of the forest.

These are dynamic processes: fires are part and parcel of the ecosystem, not disasters. Some trees, such as the Australian eucalypti and the African palms seem to have evolved with the specific purpose of burning as fast as possible and spreading flames and sparks around. Have you noticed how palms are “hairy”? They are engineered in such a way to catch fire easily. So much, that it may be dangerous to prune a palm by using a chainsaw while climbing it. A spark from the engine may set on fire the dry wood filaments and that may be very bad for the person strapped to the trunk. It is not that palms could have evolved this feature to defend themselves from chainsaw-yielding monkeys, but they are fast-growing plants that may benefit from how a fire cleans a swat of ground, letting them re-colonize it faster than other species. Note how palms act like kamikaze: single plants sacrifice themselves for the survival of their seed. It is another feature of holobionts. Some primates do the same, but it is rare. 

Other kinds of trees adopt the opposite approach. They optimize their chances for survival when exposed to fire by means of thick bark. The ponderosa pine (Pinus ponderosa) is an example of a plant adopting this strategy. Then there are more tricks: have you ever wondered why some pinecones are so sticky and resinous? The idea is that the resin glues the cone to a branch or to the bark of the tree and keeps the seeds inside. If a fire burns the tree, the resin melts, and the seeds inside are left free to germinate. More evidence that fires are not a bug but a feature of the system. 

In the end, a forest, as we saw, is a typical holobiont. Holobionts do not evolve by the fight for survival that some interpretations of Darwin’s theory had imagined being the rule in the ecosystem. Holobionts can be ruthless when it is necessary to eliminate the unfit, but they aim at an amicable convivence of the creatures that are fit enough. 

The “holobiontic” characteristic of forests is best evidenced by the concept of “biotic pump,” an example of how organisms benefit the holobiont they are part of without the need for hierarchies and planning.



The concept of biotic pump [11] was proposed by Viktor Gorshkov, Anastassia Makarieva, and others, as part of the wider concept of biotic regulation [12]. It is a profound synthesis of how the ecosphere works: it emphasizes its regulating power that keeps the ecosystem from straying away from the conditions that make it possible for biological life to exist. From this work comes the idea that the ecosystemic imbalance we call "climate change" is caused only in part by CO2 emissions. Another important factor is the ongoing deforestation. This is, of course, a controversial position. The general opinion among climatologists in the West is that growing a forest has a cooling effect because it removes some CO2 from the atmosphere. But, once a forest has reached its stable state, it has a warming effect on Earth’s climate because its albedo (the light reflected back into space) is lower than that of the bare ground. But studies exist [13] that show how forests cool the Earth not only by sequestering carbon in the form of biomass but because of a biophysical effect related to evapotranspiration. That is, the water evaporates at low altitudes from the leaves, causing cooling. It returns the heat when it condenses in the form of clouds, but the heat emissions at high altitudes are more easily dispersed towards space because the main greenhouse gas, the water, exists in very small concentrations. It may be a minor effect compared to that of the albedo, but it is a point not very well quantified. 

The concept of biotic pump states that forests act as "planetary pumping systems," carrying water from the atmosphere above the oceans up to thousands of kilometers inland. It is the mechanism that generates the “atmospheric rivers” that supply water to lands that are far away from the seas [14]. The biotic pump mechanism depends on quantitative factors that are still little known. But it seems that the water transpired by trees condenses above the forest canopy and the phase transition from gas to liquid generates a pressure drop. This drop pulls air from the surroundings, all the way from the moist air over the sea. This mechanism is what allows the inner areas of the continents to receive sufficient rain to be forested. It doesn’t work everywhere, in Northern Africa, for instance, there are no forests that bring the water inland, and the result is the desert region we call the Sahara. But the biotic pump operates in Northern Eurasia, central Africa, India, Indonesia, Southern, and Northern America.

The concept of the biotic pump is an especially clear example of how holobionts operate. Single trees don’t evaporate water in the air because they somehow “know” that this evaporation will benefit other trees. They do that because they need to generate the pressure difference they need to pull water and nutrients from their roots. In a certain sense, evapotranspiration is an inefficient process because, from the viewpoint of an individual tree, a lot of water (maybe more than 95%) is "wasted" in the form of water vapor and not used for photosynthesis. But, from the viewpoint of a forest, the inefficiency of single trees is what generates the pull of humidity from the sea that makes it possible for the forest to survive. Without the biotic pump, the forest would quickly run out of water and die. It often happens with the rush to "plant trees to stop global warming" that well-intentioned humans are engaged in, nowadays. It may do more harm than good: to stabilize the climate, we do not need just trees, we need forests. 

Note another holobiontic characteristic of trees in forests: they store very little water, individually. They rely almost totally on the collective effect of biotic pumping for the water they need: that's because they are good holobionts! Not all trees are structured in this way. An example is the African baobab, which has a typical barrel-like trunk, where it stores water more or less in the same way as succulent plants (cacti) do. But baobabs are solitary trees, 

Incidentally, evapotranspiration is one of the few points that trees have in common with the primates called "homo sapiens." The sapiens, too, "evapotranspirate" a lot of water out of their skins -- it is called "sweating." But the metabolism of primates is completely different: trees are heterothermic, that is their temperature follows that of their environment. Primates, instead, are homeotherms and control their temperature by various mechanisms, including sweating. But that doesn't create a biotic pump! 

The concept of "biotic pump" generated by the forest holobiont is crucial the correlated one of "biotic regulation," [12] the idea that the whole ecosystem is tightly regulated by the organisms living in it. Natural selection worked at the holobiont level to favor those forests that operated most efficiently as biotic pumps. Plants other than trees and also animals do benefit from the water rivers generated by the forest even though they may not evotranspirate anything. They are other elements of the forest holobiont, an incredibly complex entity where not necessarily everything is optimized, but where, on the whole things move in concert. 

It is a story that we, monkeys, have difficulties in understanding: with the best of goodwill, it is hard for us to think like trees. Likely, the reverse is also true and the behavior of monkeys must be hard to understand for the brain-like network of the tree root system of the forest. It does not matter, we are all holobionts and part of the same holobiont. Eventually, the great land holobiont that we call “forests” merges into the greater planetary ecosystem that includes all the biomes, from the sea to land. It is the grand holobionts that we call “Gaia.” 

References

[1] S. W. Simard, D. A. Perry, M. D. Jones, D. D. Myrold, D. M. Durall, and R. Molina, “Net transfer of carbon between ectomycorrhizal tree species in the field,” Nature, vol. 388, no. 6642, pp. 579–582, Aug. 1997, doi: 10.1038/41557.

[2] T. Browne, “Hydriotaphia,” in Sir Thomas Browne’s works, volume 3 (1835), S. Wilkin, Ed. W. Pickering, 1835.

[3] Shilong Piao et al., “Characteristics, drivers and feedbacks of global greening,” | Nature Reviews Earth & Environment, vol. 1, pp. 14–27.

[4] D. Reay, Nitrogen and Climate Change: An Explosive Story. Palgrave Macmillan UK, 2015. doi: 10.1057/9781137286963.

[5] A. Sneed, “Ask the Experts: Does Rising CO2 Benefit Plants?,” Scientific American. https://www.scientificamerican.com/article/ask-the-experts-does-rising-co2-benefit-plants1/ (accessed Jun. 23, 2021).

[6] S. Hoffman, “Ape Fracture Patterns Show Higher Incidence in More Arboreal Species,” Discussions, vol. 8, no. 2, 2012, Accessed: Jun. 26, 2021. [Online]. Available: http://www.inquiriesjournal.com/articles/799/ape-fracture-patterns-show-higher-incidence-in-more-arboreal-species

[7] C. Bulstrode, J. King, and B. Roper, “What happens to wild animals with broken bones?,” Lancet, vol. 1, no. 8471, pp. 29–31, Jan. 1986, doi: 10.1016/s0140-6736(86)91905-7.

[8] Pi. Cruiziat, “Plant Physiology and Development, Sixth Edition,” in Plant Physiology and Development, Oxfprd University Press, 2006. Accessed: Jun. 24, 2021. [Online]. Available: http://6e.plantphys.net/essay04.03.html

[9] L. J. Williams, A. Paquette, J. Cavender-Bares, C. Messier, and P. B. Reich, “Spatial complementarity in tree crowns explains overyielding in species mixtures,” Nat Ecol Evol, vol. 1, no. 4, pp. 1–7, Mar. 2017, doi: 10.1038/s41559-016-0063.

[10] S. Kothari, R. A. Montgomery, and J. Cavender-Bares, “Physiological responses to light explain competition and facilitation in a tree diversity experiment,” Journal of Ecology, vol. 109, no. 5, pp. 2000–2018, 2021, doi: 10.1111/1365-2745.13637.

[11] Gorshkov, V.G and Makarieva, A.M., “Biotic pump of atmospheric moisture as driver of the hydrological cycle on land,” Hydrology and Earth System Sciences Discussions, vol. 3, pp. 2621–2673, 2006.

[12] V. G. Gorshkov, A. Mikhaĭlovna. Makarʹeva, and V. V. Gorshkov, Biotic regulation of the environment : key issue of global change. Springer-Verlag, 2000. Accessed: Sep. 24, 2017. [Online]. Available: http://www.springer.com/it/book/9781852331818

[13] R. Alkama and A. Cescatti, “Biophysical climate impacts of recent changes in global forest cover,” Science, vol. 351, no. 6273, pp. 600–604, Feb. 2016, doi: 10.1126/science.aac8083.

[14] F. Pearce, “A controversial Russian theory claims forests don’t just make rain—they make wind,” Science | AAAS, Jun. 18, 2020. https://www.sciencemag.org/news/2020/06/controversial-russian-theory-claims-forests-don-t-just-make-rain-they-make-wind (accessed Jun. 25, 2021).

The Word for "Forest" is "Holobiont:" Reviewing a few Books on the Subject

 

The idea of "holobionts" is making inroads just about everywhere, although sometimes people describe the concept without using the term. But we are learning from what we see around us, and the best example from which to learn about holobionts is the forest. 

Just like no man is an island, a single tree doesn't make a holobiont. Not even several trees planted together make a good holobiont. They are a plantation, a completely different thing. And the naive faith that planting trees will solve all our climate problems is just that: a naive idea that plays straight into the hands of the logging companies which get to be paid twice: once for planting trees (to save the environment) and once for cutting the trees (selling the wood on the market). 

A true holobiont is not a single tree, not many trees stuck together in nice rows. It is a forest. And a forest needs to be born, to live, to grow, to mature, and to become a full-fledged holobiont. A living being. A creature of the planetary ecosystem that lives with it and gives life to much more than itself. Forests are part of the "biotic regulation" of the whole system, which includes controlling climate and managing the atmospheric rivers by means of the biotic pump mechanism. We cut forests at our own risk. Forests may not survive humans, but humans will surely not survive without forests. 

So, a quick review of 4 books about the forest as a holobiont, none of them mentions the term, but all of them are infused with it. 

The Hidden Life of Trees. Peter Wohlleben.(2016). A popular science book that had a remarkable success. It deserves it. It is timely, well done, and overall understandable, although at times a little too long. Wohlleben cites the biotic pump and one of its discoverers, Anastassia Makarieva, although, curiously, he doesn't use the term, always saying "water pump." But it is a fine book, by all means recommended. 

Finding the Mother Tree, Suzanne Simard, (2021). I had big expectations for this book, but I must say I was disappointed. There is nothing wrong with enlivening a text with personal stories about the author but, if one exaggerates, the book is no more a scientific text, but an autobiography of the author. This book should have been titled "An Autobiography of Suzanne Simard." Some people like biographies, personally I found this one completely flat. The stories about discovering the way trees communicate with each other are interspersed in a mass of uninteresting details about Simard's personal life told in a style that reminds that of a second-rate novel. Too bad, because Simard surely has interesting things to say. But, here, the idea just didn't work. 

The word for world is Forest, by Ursula K. Le Guin (1972). Not the best novel by Ursula Le Guin, it has an ideological overtone that harms the story, but it is still a gem -- as just about everything that Le Guin wrote. The Forest in the book is a metaphor for human consciousness: truly an anticipation of the concept of holobiont that came much later, but that Le Guin clearly had in mind. This novel also was at the basis of the later movie "Avatar" which also used some concepts by Suzanne Simard in describing inter-tree communication in forests.

Il Segreto del Bosco Vecchio, (The Secret of the Old Woods). By Dino Buzzati (1935). This is not a science book, it is a novel. But if you want to read about a true holobiont, do read it. The story of someone who inherits a wood and plans to cut it to make some money. Then, he discovers that the forest is inhabited by invisible spirits. In my modest opinion, it is one of the best novels ever written in the world. Too bad it doesn't seem to have been ever translated into English. A splendid movie was made out of it but, again, only in Italian.

 

And if you have some time, take a walk in the woods and discover the great holobiont yourself! Humans can be friends with trees even in difficult conditions. Here is a photo taken in Florence a few months ago, this lady made a little garden for herself and for everybody out of a small patch around a tree growing in front of her house, near a busy avenue. 

The Secret of Holobionts: How Excessive Efficiency can Kill

 

Five minutes are enough to take a look at this amazing video. It is extremely well done and it tells you about things that you would never have suspected. How can it be that trees exist? Well, it turns out that their metabolism is truly alien and it exploits physical phenomena that you wouldn't have imagined could be used in that way. But Mother Gaia has many tricks!

One point that has fascinated me most is how this behavior of threes highlights a fundamental characteristic of holobionts: the individual organisms that form the holobiont do not act with a purpose, they do not have "in mind" to benefit the group. But, if it is true that what's good for the hive is good for the bee, also the reverse is often true. And especially in this case. 

Trees pump water by evaporating water, which means they lose most of it. From the video, you'll learn that just about 5% of the pumped water is used by the tree for its needs. The rest evaporates away -- it is the process of evapotranspiration. 

So, trees are highly inefficient pumping machines. But, curiously, this inefficiency is what benefits the forest. This huge evaporation is what puts in motion another pump: the biotic pump. It is a mechanism that generates a depression that, in turn, pulls water from the humid atmosphere near the sea all the way to the inner areas of the forest. Without this mechanism, forests could hardly exist inland. 

If trees were 100% efficient, they would evaporate nothing and the forest would die. I think there is a deep message here, not valid just for forests: too high efficiency can kill. Living is sharing, and if there is no sharing there is no life.

 

 

 

Can Plants Perceive Us? The Opinion of Professor Suzanne Silard

 

What could be more holobiontic than this? Words of Suzanne Simard, as reported by the New York times.

 

“I think these trees are very perceptive,” she said. “Very perceptive of who’s growing around them. I’m really interested in whether they perceive us.” I asked her to clarify what she meant. Simard explained that trees sense nearby plants and animals and alter their behavior accordingly: The gnashing mandibles of an insect might prompt the production of chemical defenses, for example. Some studies have even suggested that plant roots grow toward the sound of running water and that certain flowering plants sweeten their nectar when they detect a bee’s wing beats. “Trees perceive lots of things,” Simard said. “So why not us, too?”