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

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