How Forests Create Rain: a New Study on the Effect of Evapotranspiration

From the "Proud Holobionts" blog

Image created by Dall-E

The idea that forests create rain has been known by peasants for hundreds, perhaps thousands, of years. The first scientific studies go back to Alexander von Humboldt (1769–1859), but the subject remains controversial. Nevertheless, we are starting to understand the deep and complex interactions between the atmosphere and the biosphere. They form a true "holobiont," a system of connected elements that affect each other in non-linear ways. A recent paper published by a research group led by Anastassia Makarieva shows how evapotranspiration, the evaporation of water by trees, modifies the water vapor dynamics and may generate high moisture content regimes that provide the rain needed by the land ecosystem. There is still much that we need to understand about these mechanisms, but one point is clear: forests are a crucial element of the stability of Earth's climate, and they must be preserved as much as possible (U.B.)

Press Release, 14/03/2023

As water scarcity globally grows, and deforestation threatens the remaining natural forests, understanding how vegetation impacts the water cycle becomes increasingly important.  In their new paper, “The role of ecosystem transpiration in creating alternate moisture regimes by influencing atmospheric moisture convergence” published in Global Change Biology ( https://doi.org/10.1111/gcb.16644), an international and interdisciplinary team led by TUM demonstrated the existence of two potential moisture regimes – one drier, with additional moisture decreasing atmospheric moisture import, and one wetter, with additional moisture enhancing atmospheric moisture import. In the drier regime, water vapor behaves as a passive tracer following the air flow. In the wetter regime, it modifies atmospheric dynamics.

The team based their analysis on the previously established non-linear dependence of precipitation on atmospheric moisture content – increasing absolute humidity leads to a negligible precipitation increment if the atmosphere is dry, but to a large increment when the atmosphere is sufficiently wet. Combining this dependence with a full consideration of the water budget, the researchers showed that an increase in precipitation in humid conditions facilitated by increased evapotranspiration, should lead to enhanced moisture import. They illustrated these patterns with the data from the Amazon basin and the Loess Plateau in China.

Dr. Anja Rammig (TUM School of Life Sciences and study author) considers these results as having profound implications for the ongoing studies of the resilience of the Amazon forest in the face of the danger of deforestation and climate change. Dr. Scott Saleska (University of Arizona, study author) believes that the new results are in agreement with the profound role of leaf phenology in the Amazon forest for water cycle regulation. By forcing a decline in forest evapotranspiration, deforestation can dehumidify the atmosphere and thus drive the forest into the drier regime where transpiration of the re-growing vegetation would further aggravate aridity by decreasing moisture import. Getting out of this landscape trap could be impossible. Dr. Ruben Molina (University of Antioquia, Colombia, study author) hopes that the study findings will raise the awareness of the importance of tropical forest conservation.

Dr. Andrei Nefiodov (Petersburg Nuclear Physics Institute, Russia) participating in the study says that the new results corroborate the concept of the biotic pump of atmospheric moisture that emphasizes the dominant role of natural forests in transporting moisture inland. Dr. Antonio Nobre (INPE, Brazil, study author) compares this biotic moisture pumping to a beating heart, and highlights the good news: even in arid lands, by restoring the vegetation one should be able to enhance the atmospheric moisture convergence and streamflow. To achieve that, the ecological restoration strategy should be carefully designed to guide the ecosystem transition from the dry to wet regimes.

“I suspect that natural vegetation will be best for maintaining a moist and productive environment as these systems kept the world green and productive long before people got involved” – emphasizes Dr. Douglas Sheil (Wageningen University, author), collaborating on the research. “We do need to take into account the holobiontic relationships among all ecosystem elements that allow for an efficient regulation of the water cycle,” adds another author Dr. Ugo Bardi (Club of Rome, University of Florence).

Anastassia Makarieva (Institute for Advanced Study, TUM, lead author) emphasizes the need for a broad international cooperation in the studies of the ecology of the water cycle: “We have shown that the non-linear precipitation dependence on atmospheric moisture content, first noted by our co-author Dr. Mara Baudena (CNR-ISAC, Italy) and her colleagues, has widely ranging implications. The atmospheric water flows do not recognize international borders, thus deforestation disrupting evapotranspiration in one region could trigger a transition to the drier regime in another. Our results indicate that natural forests of the Earth, in both high and low latitudes, are our common legacy of pivotal global importance as they support the terrestrial water cycle. Their preservation should become a widely recognized priority for our civilization to solve the global water crisis.”

Makarieva, A. M.,  Nefiodov, A. V.,  Nobre, A. D.,  Baudena, M.,  Bardi, U.,  Sheil, D.,  Saleska, S. R.,  Molina, R. D., &  Rammig, A. (2023).  The role of ecosystem transpiration in creating alternate moisture regimes by influencing atmospheric moisture convergence. Global Change Biology,  00,  1– 21. https://doi.org/10.1111/gcb.16644

Earth's Past and Future: a Long-Term View

 

For the Orthodox Easter day of 2022, I thought to abandon the daily noise of the news and take a long-term view. So, here is a post, republished from "The Proud Holobionts," that presents the history of our planet during the past 400 million years and some hypotheses on what could happen in the next billion years or so. Easter is a time of rebirth and of hope, so let's hope for a future of peace for these poor savanna monkeys, so noisy and so unruly. 

Gaia and the Savanna Monkeys. 

The Great Cycle of Earth's Forests

From "The Proud Holobionts" April 19, 2022 - By Ugo Bardi

 Forests appeared on Earth some 400 million years ago, and they have been thriving over that long period. But, during the past 150 million years, they started to show signs of distress, reacting to the decline in atmospheric CO2 concentrations and to the competition with grasslands. As Earth changes, will forests be able to cope and survive? It is an extremely slow trend, but we cannot rule out that forests will conclude their cycle and disappear in a geologically short time. This text is an attempt to reconstruct the story of forests, and not just of foreste, and to imagine what their future could be in deep time. (image courtesy of Chuck Pezeshky) 

A forest is a magnificent, structured, and functional entity where the individual elements -- trees -- work together to ensure the survival of the ensemble. Each tree pumps water and nutrients all the way to the crown by the mechanism called evapotranspiration. The condensation of the evaporated water triggers the phenomenon called the "biotic pump" that benefits all the trees by pumping water from the sea. Each tree cycles down the carbohydrates it manufactures using photosynthesis to its mycorrhizal space, the underground system of roots and fungi that extracts mineral nutrients for the tree. The whole "rhizosphere" -- the root space -- forms a giant brain-like network that connects the trees to each other, sometimes termed "the Wood Wide Web." It is an optimized environment where almost everything is recycled. We can see it as similar to the concept of "just in time manufacturing" in the human economy. 

Forests are wonderful biological machines, but they are also easily destroyed by fires and attacks by parasites. And forests have a competitor: grass, a plant that tends to replace them whenever it has a chance to. Areas called savannas are mainly grass, although they host some trees. But they don't have a closed canopy, they don't evapotranspirate so much as forests, and they tend to exist in much drier climate conditions. Forests and grasslands are engaged in a struggle that may have started about 150 million years ago when grass appeared for the first time. During the past few million years, grasses seem to have gained an edge in the competition, in large part exploiting their higher efficiency in photosynthesis (the "C4" pathway) in a system where plants are starved for CO2.

Another competitor of forests is a primate that left its ancestral forest home just a couple of million years ago to become a savanna dweller -- we may call it the "savanna monkey," although it is also known as "Homo," or "Homo sapiens." These monkeys are clever creatures that seem to be engaged mainly in razing forests to the ground. Yet, in the long run, they may be doing forests a favor by returning the atmospheric CO2 concentration to values more congenial to the old "C3" photosynthetic mechanism still used by trees. 

Seen along the eons, we have an extremely complex and fascinating story. If forests have dominated Earth's landscape for hundreds of millions of years, one day they may disappear as Gaia gets old. In this post, I am describing this story from a "systemic" viewpoint -- that is, emphasizing the interactions of the elements of the system in a long-term view (it is called also "deep time"). The post is written in a light mood, as I hope to be able to convey the fascination of the story also to people who are not scientists. I tried to do my best to interpret the current knowledge, I apologize in advance for the unavoidable omissions and mistakes in such a complex matter, and I hope you'll enjoy this post. 

The Origin of Forests: 400 million years ago

Life on Earth may be almost 4 billion years old but, since we are multicellular animals, we pay special attention to multicellular life. So, we tend to focus on the Cambrian period (542-488 million years ago), when multicellular creatures became common. But that spectacular explosion of life was all about marine animals. Vascular plants started colonizing the land only during the period that followed the Cambrian, the Ordovician, (485 - 443 million years ago). 

To be sure, the Ordovician flora on land was far from impressive. As far as we know, it was formed mainly by moss and lichens, although lichens may be much older and precede the Cambrian. Algae may have eked a precarious life on rocks even much earlier. Mosses, just like lichens, are humble plants: they are not vascularized, they don't grow tall, and they surely can't compare with trees. Nevertheless, they could change the planetary albedo and perhaps contribute to the fertilization of the marine biota -- something that may be related to the spectacular ice ages of the Ordovician. It is a characteristic of the Earth system that the temperature of the atmosphere is related to the abundance of life. More life draws down atmospheric CO2, and that cools the planet. The Ordovician saw one of these periodic episodes of cooling with the start of the colonization of the land. (image from Wikipedia)

There followed another long period called the "Silurian" (444 – 419 My ago) when plants kept evolving but still remained of the size of small shrubs at most. Then, during the Devonian (419 -359 million years ago) we have evidence of the existence of wood. And not only that, the fossil record shows the kind of channels called "Xylem" that connect the roots to the leaves in a tree. These plants were already tall and had a crown, a trunk, and roots. By the following geological period, the Carboniferous (359 - 299 My ago), forests seem to have been widespread.  

A major feature of these ancient trees was the development of an association with fungi. Their roots formed what we call a "mycorrhizal" symbiotic system. The fungi receive carbohydrates that the tree manufactures using photosynthesis, while the tree receives from the fungi essential minerals, including nitrogen and phosphorous. We don't know the details of how this symbiotic relationship evolved over hundreds of millions of years but, below, you can see a hypothesis of how it could have happened. (Source) (in the figure, "AM" stands for "arbuscular mycorrhiza" - the oldest form of symbiotic fungi).

Another major evolutionary innovation that may have been already operating in the Paleozoic forests is the "biotic pump." As an effect of the pressure drop created by the condensation of evapotranspirated water, forests can create pump water vapor from the ocean and create the "atmospheric rivers" that bring water inland. That, in turn, creates the land rivers that bring that water back to the sea. As forests create their own climate, they can expand nearly everywhere. The image shows clouds created by condensation over the modern Amazon rainforest (source).  

If we could walk in one of those ancient forests, we would find the place familiar, but also a little dreary. No birds and not even flying insects, they evolved only tens of million years later. No tree-climbing animals: no monkeys, no squirrels, nothing like that. Even in terms of herbivores, we have no evidence of the kind of creatures we are used to, nowadays. Grass didn't exist, so herbivores may have subsisted on decaying plant matter, or perhaps on ferns. Lots of greenery but no flowers, they had not evolved yet. You see in the image (source) an impression of what an ancient forest of Cladoxylopsida could have looked like during the Paleozoic era.

The Paleozoic forests already had one of the characteristics of modern forests: fires. There had never been fires on Earth before for at least two good reasons: one was that there was not enough oxygen, and the other was that there was nothing flammable. But now, with the oxygen concentration increasing and plants colonizing the land, fires appeared, lighting up the night. They would remain a characteristic of the land biosphere for hundreds of millions of years.

Image Source. The "fire window" is the region of concentrations in atmospheric oxygen in which fires can occur. Note how during the Paleozoic, the concentration could be considerably larger than it is now. Fireworks aplenty, probably. Note also how there exist traces of fires even before the development of full-fledge trees, in the Devonian. Wood didn't exist at that time, but the concentration of oxygen may have been high enough to set dry organic matter on fire. 

Wildfires are a classic case of a self-regulating system. The oxygen stock in the atmosphere is replenished by plant photosynthesis but is removed by burning wood. So, fires tend to reduce the oxygen concentration and that makes fires more difficult. But the story is more complicated than that. Fires also tend to create "recalcitrant" carbon compounds, charcoal for instance, that are not recycled by the biosphere and tend to remain buried for long times -- almost forever. So, over very long periods, fires tend to increase the oxygen concentration in the atmosphere by removing CO2 from it. The conclusion is that fires both decrease and increase the oxygen concentration. How about that for a taste of how complicated the biosphere processes are? 

 The Mesozoic: Forests and Dinosaurs

At the end of the Paleozoic, some 252 million years ago, there came the great destruction. A gigantic volcanic eruption of the kind we call "large igneous province" (sometimes affectionately "LIP") took place in the region we call Siberia today. It was huge beyond imagination: think of an area as large as modern Europe becoming a lake of molten lava. (image source)

It spewed enormous amounts of carbon into the atmosphere in the form of greenhouse gases. That warmed the planet, so much that it almost sterilized the biosphere. It was not the first, but it was the largest mass extinction of the Phanerozoic age. Gaia is normally busy keeping Earth's climate stable, but sometimes she seems to be sleeping at the wheel -- or maybe she gets drunk or stoned. The result is one of these disasters.  

Yet, the ecosystem survived the great extinction and rebounded. It was now the turn of the Mesozoic era, with forests re-colonizing the land. Over time, the angiosperms ("flowering plants") become dominant over the earlier conifers. With flowers, forests may have been much noisier than before, with bees and all kinds of insects. Avian dinosaurs also appeared. They seem to have been living mostly on trees, just like modern birds. 

For a long period during the Mesozoic, the landscape must have been mainly forested. No evidence of grass being common, although smaller plants, ferns, for instance, were abundant. Nevertheless, the great evolution machine kept moving. During the Jurassic, a new kind of mycorrhiza system evolved, the "Ectomycorrhizae" which allowed better control of the mineral nutrients in the rhizosphere, avoiding losses when the plants were not active. This mechanism was typical of conifers that could colonize cold regions of the supercontinent of the time, the "Pangea."  

During the Mesozoic, the dinosaurs appeared and diffused all over the planet. You surely noted how the Jurassic dinosaurs were often bipedal (See the illustration showing an early form of Iguanodon). They are also called "ornithopods," it is a body plan that allows herbivorous creatures to eat leaves on the high branches of trees. A bipedal stance makes the creature able to stand up, balancing on its tail. Some dinosaurs chose a different strategy, developing very long necks for the same purpose: the brontosaurus is iconic in this sense even though, traditionally, it was shown half-submerged in swamps (the illustration is from the New York Tribune of 1919). That's a bit silly, if you think about that. Why should a semiaquatic creature need a long neck? Think of a hippo with the neck of a giraffe: it wouldn't work so well. 

A much better representation of long-necked dinosaurs came with the first episode of the "Jurassic Park" (1993) movie series, when a gigantic diplodocus eats leaves. At some moment, the beast rises on its hind legs, using the tail as further support. 

If you are a dinosaur lover (and you probably are if you are reading this post) seeing this scene must have been a special moment in your life. And, after having seen it maybe a hundred times, it still moves me. But note how the diplodocus is shown in a grassy environment with sparse trees: that's not realistic because grass didn't exist yet when the creature went extinct at the end of the Jurassic period, about 145 million years ago. 

To see grass and animals specialized in eating it, we need to wait for the Cretaceous (145-66 million years ago). Evidence that some dinosaurs had started eating grass comes from the poop of long-necked dinosaurs. That's a little strange because, if you are a grazer, the last thing you need is a long neck. But new body plants rapidly evolved during the Cretaceous.  The Ceratopsia were the first true grazers, also called "mega-herbivores". Heavy, four-legged beasts that lived their life keeping their head close to the ground. The Triceratopses gained a space in human fantasy as prototypical dinosaurs, and they are often shown in movies while fighting tyrannosauruses. You see that in Walt Disney's movie "Fantasia" (1940). It may have happened for real.

Note the heavy bone shield over the head. With so much weight on board, Trixie couldn't possibly rise on its hind legs to munch on leaves on tree branches. Note also the beak, it looks perfectly adapted for collecting grass. It means that the Cretaceous landscape was probably similar to our world. We don't know if there existed the kind of biome we call today "savanna" -- a mix of grass and trees, but surely the land was shared by forests and grass, each biome with its typical fauna. 

The Great Cooling and the Rise of C4 Grass

At the end of the Cretaceous period, 66 million years ago, a new large igneous province appeared in the Deccan region, in India. It generated another climate disaster with the associated mass extinction. Most dinosaurs were wiped out, except those we call "birds" today. A large meteorite also hit Earth at that time. It caused only minor damage but, millions of years later, it gave human filmmakers a subject to explore in many dramatic movies. 

In time, the Deccan LIP faded away, and the era that followed is called the "Cenozoic." The ecosystem recovered, forests re-colonized the land, and mammals and birds (the only survivors of the Dinosauria clade) fought to occupy the ecological niches left free by their old masters. The early Cenozoic was a warm period of lush forests that offered refuge to a variety of animals: birds made their nests in branches, while squirrels and other small mammals jumped from branch to branch, or lived at the bottom. It is during this period that primates evolved: the huge forests of those times offered refuge for a variety of species that had probably already developed sophisticated social behaviors.  

Grass also survived the end-Cretaceous catastrophe. As a result, some mammals evolved into new "megaherbivores" or "megafauna" that occupied the same ecological niche that the triceratopsides had colonized long before.  Here is a brontotherium, a large herbivorous mammal that lived some 37-35 million years ago, during the late Eocene period (image from BBC).

The megabeasts of the Cenozoic do not have the same fascination of the giant dinosaurs, but this creature has a nice-sounding name, and it looks like Shrek, the ogre of Spielberg's movie. Note how the beast is correctly shown walking on a grassy plain. The Eocene is supposed to have been mostly forested, but grass existed, too. The brontotherium was an opportunistic herbivore, apparently able to subsist on various kinds of food, not just grass. 

During the warm phase of the Cenozoic, Earth reached a maximum temperature around 55 million years ago, some 8-12 deg C higher than today. The concentration of CO2, too, was large. That is called the "early Eocene climatic optimum". It doesn't mean that this period was better than other periods in terms of climate, but it seems that Earth was mainly covered with lush forests and that the biosphere thrived.  

Then, the atmosphere started cooling. It was a descent that culminated at the Eocene-Oligocene boundary, about 34 million years ago, with a new mass extinction. It was a relatively small event in comparison to other, more famous, mass extinctions, but still noticeable enough that the Swiss paleontologist Hans Georg Stehlin gave it the name of the "Grande Coupure" (the big break) in 1910. One of the victims was the brontotherium -- too bad!

Unlike other cases, the extinction at the Grande Coupure was not correlated to the warming created by a LIP, but to rapid cooling. You see the "step" in temperature decline in the figure. 

Why the big cooling? The answer is not completely known. Surely, the cooling was correlated to a decline in the CO2 content in the atmosphere and that, in turn, may have been generated by the collision of the Indian plate with Eurasia. It was a gigantic geological event that generated the Himalayan mountain belt. It exposed huge amounts of fresh rock to the atmosphere, and the result was the removal of CO2 because of silicate erosion and weathering. The Himalaya hypothesis is one of those explanations that seem to make a lot of sense, but it has big problems. 

Another possible explanation is that Earth just outgassed less CO2 than before. The CO2 that plants need for their photosynthesis is generated mainly at the mid-oceanic ridges where the hot mantle (the molten rock layer below Earth's crust) outgasses it, as it has been doing for billions of years. It may well be that the mantle is getting a little colder over the eons, so it outgasses less CO2 than before. It may be true, but it seems to be a weak effect -- not enough to explain the CO2 decline of the Cenozoic.

In my opinion, the most likely hypothesis is that the CO2 concentration declined because of higher biological productivity not just on land, but also in the sea (as it seems to be implied in a recent study)

In other words, the early Cenozoic may have been so booming with life of all kinds that it absorbed more CO2 from the atmosphere than the mantle could replace by outgassing. The result was the cooling phase. The abrupt step at "Grande Coupure" may be related to the evolution of a specific life form: baleen whales, which changed the equilibria of the whole marine ecosystem, drawing down even more CO2 from the atmosphere.

This interpretation agrees with the fact that ice ages are often observed after LIPs. It may be one of the many cycles of the ecosphere. So, when a major LIP appears, the rise of CO2 is disastrous at the beginning but, in the long run, it gives the biosphere a chance to rebound and expand in a CO2 rich system.  Then, the rebound generates its own doom: the abundant biological productivity draws down CO2 from the atmosphere, cools the planet, and the system finds itself CO2-starved again. In this interpretation, the Eocene cooling and the Grande Coupure were long-term consequences of the Deccan LIP that had destroyed the dinosaurs, millions of years before. I hasten to note that this is just one of the several possible interpretations but, in my opinion, it makes a lot of sense.    

The Eocene cooling had profound effects on forests. First, the CO2 decline gave an advantage to those plants which utilized a more efficient photosynthesis mechanism called the "C4" pathway. Earlier on, the standard photosynthesis mechanism (called "C3") had evolved in an atmosphere rich in CO2. The C3 mechanism is efficient in processing carbon dioxide, but it is hampered by the opposite process called "photorespiration," which becomes important when the CO2 concentration is low. Using the C4 mechanism, plants can concentrate CO2 in the cells where photosynthesis occurs and avoid the losses by photorespiration. 

C4 plants appeared shortly after the Grande Coupure and diffused mainly in grasses, Trees, instead, didn't adopt the new mechanism. The explanation is subtle: photosynthesis needs water, and the process that goes on in leaves is strongly connected to the evapotranspiration mechanism. The C4 mechanism needs less water than the C3 one, so evapotranspiration is hampered. The result is that C4 trees could not be as tall as the ordinary C3 ones, and so they are not favored by natural selection in forests. In an atmosphere of very low CO2, forests are disadvantaged because of the higher photosynthesis efficiency of grasses. 

During the period that followed the Grande Coupure, temperatures and CO2 concentrations remained stable, but at relatively low levels. The result was that many forests disappeared, replaced by grasslands and savannas. Herbivorous species evolved teeth more specialized for grazing and became "mega-herbivorous" species. The landscape must have become similar to the modern one, with patches of forests alternating with savannas. 

A typical savanna ecosystem: the Tarangire national park in Tanzania. (Image From Wikipedia). Compare with the forest image at the beginning of this post. 

Despite the expansion of savannas, rainforests continued to exist in the tropical regions. Conifer forests kept a foothold in the Northern regions, helped by their Ectomycorrhiza system that avoided the runoff of nutrients in winter. The boreal forest is also called "Taiga." 

Then, a new cooling phase started, apparently a continuation of the previous trend: cooling begets more cooling. It was the beginning of the "Pleistocene," a period of unstable climate with ice ages and interglacials following each other, triggered by small oscillations in solar irradiation caused by the characteristics of Earth's orbit. These oscillations are called "Milankovich Cycles" -- they are not the cause of the ice ages, just triggers. (Image Source).

The oscillations are caused by ice having a built-in albedo feedback so that the more ice expands, the more sunlight is directly reflected into space. That causes the temperature to decline and ice to expand even more. Taken to its extreme consequences, this mechanism may lead to the "Snowball Earth" condition, with ice covering the whole planet's surface. It may have happened for real during the "Cryogenian" period, some 600 million years ago. Fortunately, there were mechanisms able to re-heat Earth and return it to the conditions we consider "normal." 

During the Pleistocene, the CO2 concentration in Earth's atmosphere plunged to very low levels, especially during the glacial periods, when it reached levels as low as around 150 parts per million (ppm). Earth may have inched close to a new snowball Earth but, fortunately, that didn't happen. In part, it may be because the sun, today, is about 5% hotter than it was during the Cryogenian. But we will never know how close Gaia got to freezing to death. 

During the Pleistocene, the advancing ice sheets swept away all plants, but even in non-glaciated areas, forests suffered badly.  Tropical rainforests didn't disappear, but they were much reduced in extension. In the North, most of the Eurasian boreal forests were replaced by the "mammoth steppe," a huge area that went from Spain to Kamchatka. where mammoths and other mega-herbivores roamed. 

It is not impossible that an ice age colder than the Pleistocene average could have led to the eventual extinction of the forests, completely replaced by grasses. But that didn't happen, and things were going to change again. 

The Rise of the Savanna Monkeys  

Primates are arboreal creatures that evolved in the warm environment of the Eocene forests. They used tree branches as a refuge, and they could adapt to various kinds of food. From the viewpoint of these ancient primates, the shrinking of the area occupied by tropical forests was a disaster. They were not equipped to live in savannas: slow on the ground, they would be just an easy lunch for the powerful predators of the time. Primates also never colonized the northern taiga. It was probably not because they couldn't live in cold environments (some modern monkeys can do that), but because they couldn't cross the "mammoth steppe" that separated Tropical forests from the Northern forests. So, "boreal monkeys" do not exist (actually, there is one, but it is not exactly a monkey!).  

Yet, during the Pleistocene, the shrinking of the tropical forests forced some monkeys to move into the savanna, leaving their comfortable living on tree branches. The Australopythecines, (image source) appeared about 4 million years ago. We may call them the first "savanna monkeys." The first creatures that we classify as belonging to the genus Homo, the homo habilis, appeared some 2.8 million years ago. They were also savanna dwellers. 

The savanna monkeys won the game of survival by means of a series of evolutionary innovations. They increased their body size for better defense, they developed an erect stance to have a longer field of view, they super-charged their metabolism by getting rid of their body hair and using profuse sweating for cooling, they developed complex languages to create social groups for collective defense, and they learned how to make stone tools adaptable to different situations. Finally, they developed a tool that no animal on Earth had mastered before: fire. Over a few hundred thousand years, they spread all over the world from their initial base in a small area of Central Africa. The savanna monkeys, now called "Homo sapiens," were a stunning evolutionary success. The consequences on the ecosystem were enormous.

First, the savanna monkeys exterminated most of the megafauna. The only large mammals that survived the onslaught were those living in Africa, where they had the time to adapt to the new predator as it evolved. For instance, the large ears of the African elephant are a cooling system destined to make elephants able to cope with the incredible stamina of human hunters. But in Eurasia, North America, and Australia, the arrival of the newcomers was so fast and so unexpected that most of the large animals were wiped out. 

By eliminating the megaherbivores, the monkeys had, theoretically, given a hand to the competitors of grass, forests, which now had an easier time encroaching on grassland without seeing their saplings trampled. But the savanna monkeys had also taken the role of megaherbivores. They used fires with great efficiency to clear forests to make space for the game they hunted. In the book "1491" Charles Mann reports (. p 286) how "rather than domesticating animals for meat, Indians retooled ecosystems to encourage elk, deer, and bear. Constant burning of undergrowth increased the number of herbivores, the predators that fed on them, and the people who ate them both."  Later, as they developed metallurgy, the monkeys were able to cut down entire forests to make space for the cultivation of the grass species that they had domesticated meanwhile: wheat, rice, maize, oath, and many others. 

But the savanna monkeys were not necessarily enemies of the forests. In parallel to agriculture, they also managed entire forests as food sources. The story of the chestnut forests of North America is nearly forgotten today but, about one century ago, the forests of the region were largely formed of chestnut trees planted by Native Americans as a source of food (image source). By the start of the 20th century, the forest was devastated by the "chestnut blight," a fungal disease that came from China. It is said that some 3-4 billion chestnut trees were destroyed and, now, the chestnut forest doesn't exist anymore. The American chestnut forest is not the only example of a forest managed, or even created, by humans. Even the Amazon rainforest, sometimes considered an example of a "natural" forest, shows evidence of having been managed by the Amazonian Natives in the past as a source of food and other products. 

The action of the savanna monkeys was always massive and, in most cases, it ended in disaster. Even the oceans were not safe from the monkeys: they nearly managed to exterminate the baleen whales, turning large areas of the oceans into deserts. On land, entire forests were razed to the ground. Desertification ensued, brought upon by "megadroughts" when the rain cycle was no more controlled by the forests. Even when the monkeys spared a forest, they often turned it into a monoculture, subjected to be destroyed by pests, as the case of the American chestnuts shows. Yet, in a certain sense, the monkeys were making a favor to forests. Despite the huge losses to saws and hatchets, they never succeeded in completely exterminating a tree species, although some are critically endangered nowadays. 

The most important action of the monkeys was their habit of burning sedimented carbon species that had been removed from the ecosphere long before. The monkeys call these carbon species "fossil fuels" and they have been going on an incredible burning bonanza using the energy stored in this ancient carbon without the need of going through the need of the slow and laborious photosynthesis process. In so doing, they raised the concentration of CO2 in the atmosphere to levels that had not been seen for tens of millions of years before. That was welcome food for the trees, which are now rebounding from their former distress during the Pleistocene and reconquering the lands they had lost to grass. In the North of Eurasia, the Taiga is expanding and gradually eliminating the old mammoth steppe. Areas that today are deserts are likely to become green. We are already seeing the trend in the Sahara desert. 

What the savanna monkeys could do was probably a surprise for Gaia herself, who must be now scratching her head and wondering what has happened to her beloved Earth. And what's going to happen now?  

The New Large Igneous Province made by Monkeys

The giant volcanic eruptions called LIPs tend to appear with periodicities of the order of tens or hundreds of million years. But nobody can predict a LIP and, instead, the savanna monkeys engaged in the remarkable feat of creating a LIP-equivalent by burning huge amounts of organic ("fossil") carbon that had sedimented underground over tens or hundreds of millions of years of biological activity. 

It is remarkable how rapid the monkey LIP (MLIP) has been. Geological LIPS typically span millions of years. The MLIP went through its cycle in a few hundreds of years. It will be over when the concentration of fossil carbon stored in the crust will become too low to self-sustain the combustion with atmospheric oxygen. Just like all fires, the great fire of fossil carbon will end when it runs out of fuel, probably in less than a century from now. Even in such a short time, the concentration of CO2 is likely to reach, and perhaps exceed, levels never seen after the Eocene, some 50 million years ago.  

There is always the possibility that such a high carbon concentration in the atmosphere will push Earth over the edge of stability and kill Gaia by overheating the planet. But that's not a very interesting scenario, so let's examine the possibility that the biosphere will survive the carbon pulse. What's going to happen to the ecosphere?

The savanna monkeys are likely to be the first victims of the CO2 pulse that they themselves generated. Without the fossil fuels they had come to rely on, their numbers are going to decline very rapidly. From the incredible number of 8 billion individuals, they may return to levels typical of their early savanna ancestors: maybe just a few tens of thousands. Quite possibly, they'll go extinct. In any case, they will hardly be able to keep their habit of razing down entire forests. Without monkeys engaged in the cutting business and with high concentrations of CO2, forests are advantaged over savannas, and they are likely to recolonize the land. We are going to see again a lush, forested planet where arboreal monkeys will probably survive and thrive. Nevertheless, savannas will not disappear. They are part of the ecosystem, and new megaherbivores will evolve in a few hundreds of thousands of years. 

Over deep time, the great cycle of warming and cooling may restart after the monkey LIP, just as it does for geological LIPs. In a few million years, Earth may be seeing a new cooling cycle that will lead again to a Pleistocene-like series of ice ages. At that point, new savanna monkeys may evolve. They may restart their habit of exterminating the megafauna, burning forests, and building things in stone. But they won't have the same abundance of fossil fuel that the monkeys called "Homo sapiens" found when they emerged into the savannas. So, their impact on the ecosystem will be smaller, and they won't be able to create a new monkey-LIP. 

And then what? In deep time, the destiny of Earth is determined by the slowly increasing solar irradiation that is going, eventually, to eliminate the oxygen from the atmosphere and sterilize the biosphere, maybe in less than a billion years from now. So, we may be seeing more cycles of warming and cooling before Earth's ecosystem collapses. At that point, there will be no more forests, no more animals, and only single-celled life may persist. It has to be. Gaia, poor lady, is doing what she can to keep the biosphere alive, but she is not all-powerful. And not immortal, either. 

Nevertheless, the future is always full of surprises, and you should never underestimate how clever and resourceful Gaia is. Think of how she reacted to the CO2 starvation of the past few tens of millions of years. She came up with not just one, but two brand-new photosynthesis mechanisms designed to operate at low CO2 concentrations: the C4 mechanism typical of grasses, and another one called crassulacean acid metabolism (CAM). To say nothing about how the fungal-plant symbiosis in the rhizosphere has been evolving with new tricks and new mechanisms. You can't imagine what the old lady may concoct in her garage together with her Elf scientists (those who also work part-time for Santa Claus). 

Now, what if Gaia invents something even more radical in terms of photosynthesis? One possibility would be for trees to adopt the C4 mechanism and create new forests that would be more resilient against low CO2 concentrations. But we may think of even more radical innovations. How about a light fixation pathway that doesn't just work with less CO2, but that doesn't even need CO2? That would be nearly miraculous but, remarkably, that pathway exists. And it has been developed exactly by those savanna monkeys who have been tinkering -- and mainly ruining -- the ecosphere. 

The new photosynthetic pathway doesn't even use carbon molecules but does the trick with solid silicon (the monkeys call it "photovoltaics"). It stores solar energy as excited electrons that can be kept for a long time in the form of reduced metals or other chemical species. The creatures using this mechanism don't need carbon dioxide in the atmosphere, don't need water, they may get along even without oxygen. What the new creatures can do is hard to imagine for us (although we may try). In any case, Gaia is a tough lady, and she may survive much longer than we may imagine, even to a sun hot enough to torch the biosphere to cinders. Forests, too, are Gaia's creatures, and she is benevolent and merciful (not always, though), so she may keep them with her for a long, long time. (and, who knows, she may even spare the Savanna Monkeys from her wrath!). 

We may be savanna monkeys, but we remain awed by the majesty of forests. The image of a fantasy forest from Hayao Miyazaki's movie, "Mononoke no Hime" resonates a lot with us. But can you see the mistake in this image? What makes this forest not a real forest? 

__________________________

Note: You always write what you would like to read, and that's why I wrote this post. But, of course, this is a work in progress. I am tackling a subject so vast that I can't possibly hope to be sufficiently expert in all its facets to avoid errors, omissions, and wrong interpretations. Corrections from readers who are more expert than me are welcome! I would also like to thank Anastassia Makarieva for all she taught me about the biotic pump and about forests in general, and Mihail Voytehov for his comments about the rhizosphere. Of course, all mistakes in this text here are mine, not theirs.

Thinking like a Tree. Understanding the Role of Forests in the Ecosystem

The "Seneca Effect" blog deals a lot with collapses and you may find it a little catastrophistic. But I am also exploring other fields in a more positive mood. One is the concept of "holobiont," how living creatures organize themselves to form complex adaptive systems. Here is a post on this subject from my blog "The Proud Holobionts". 

The Greatest Holobiont on Earth: Old-Growth Forests

Reproduced from "The Proud 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 look like 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 the way male primates for 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).