Savannas and Grasslands: Holobionts Adapting to a Changing Earth

 

The Savanna of the Tarangire Park, in Tanzania (image from Wikipedia)

A recent paper by Carla Staver and Carolyne Stromberg on Savannas, recently appeared on "Science" -  It is a fascinating, although brief, review of what we know about savannas and grasslands. The interest in this kind of studies lies, in my opinion, in their "deep time" perspective. We are used to the existence of savannas and grasslands, but we often tend to forget that they are a relatively recent innovation in the biosphere. Staver and Stromberg estimate that they appeared "just" 20 million years ago. To compare, the forest biome is at least 400 million years old.

In evolutionary terms, if something exists, it is because it has a reason to exist. Savannas and Grasslands are mostly a reaction of the ecosystem to the profound changes that occurred during the Cenozoic, the past 66 million years. Earth emerged out of the End-Mesozoic disaster, the one that destroyed the dinosaurs, as a hot and lush planet. But, some 50 million years ago, a phase of cooling started, and it is lasting in our times (except for the recent human perturbation). 

My personal interpretation of this cooling phase is that the outgassing of CO2 from the mantle could not compensate for the carbon sequestration operated by the biosphere and that the cooling is the result of the gradually lower CO2 concentrations in the atmosphere. (image from Zhang et al.)

At some moment, plants had to adapt to a CO2 concentration as low as 170 ppm -- never seen before in Earth's history. This adaptation led to the appearance of the "C4" mechanism of photosynthesis that uses less water and less CO2. The result was a major rebalancing of the ecosystem: for some reasons, probably linked to the water pumping mechanism from roots to the leaves, trees are not happy to use the C4 mechanism, preferring instead the older "C3" one. That led to the widespread appearance of savannas and grasslands, better adapted to a CO2-poor climate. 

From then on, two different holobionts have populated Earth: Forests and grasslands. The main difference is that a Forest has a closed canopy, whereas a savanna has an open one. The effects on the water cycle management are profound: the forest can trigger the biotic pump mechanism to carry water vapor from the oceans, while the savanna, probably, cannot. Both biomes are adapted to the conditions that they themselves create: forests thrive in humid environments and they tend to create it using the biotic pump. The savannas prefer a dry environment: they create it to keep forests away. Savannas also tend to thrive in the presence of mega-herbivores, which instead may be a cause of damage to forests. We may see this situation as a tug of war between the two biomes, although it is also true that the ecosystem knows no "war," only adaptation. Those holobionts that adapt best, survive. It may be possible that grass and trees are two sides of a single, large holobiont that includes savannas, grasslands, and forests. The concept of holobiont is fractal. 

And now? The savanna monkeys (aka "humans") have changed everything. They have methodically razed the forests but, at the same time, they recently engaged in a major re-forestation effort. They have destroyed forests by fires but have also done incredible efforts to suppress forest fires. They have also damaged grassy ecosystems turning them into pastures and removing the large herbivores, but they are also trying to preserve the remaining herbivores. In short, they can't decide what they want to do! The only sure thing is that they have been raising the CO2 atmospheric concentration by burning fossil carbon, gradually returning it to the levels of the early and mid-Cenozoic. That favors trees against grasses. Indeed, we are seeing a remarkable defined reforestation trend all over the Earth. 

So far, we cannot say how this heavy intervention of the savanna monkeys will affect the ecosystem in the long term. The pumping up of the CO2 levels in the atmosphere may be a short-lived pulse, or it may affect the planet for millions of years. Whatever the case, Gaia has been around for a few billion years, and she surely knows what to do. She can deal with these monkeys as they deserve. 

h/t Mara Baudena. To know more of the evolution of forests and savannas, see this post on "The Proud Holobionts" If you cannot access the paper by Staver and Stromberg, ask me for a copy at ugo.bardi(thingamajig)unifi.it

   

Forests: Holobionts that Dominate the Land's Ecosystem

 

The beech ("Fagus") forest of Abbadia San Salvatore, in Tuscany, Italy. A living holobiont in all its splendor. (photo by the author).

Not many people, today, have a chance to see a fully grown, mature forest. Of course, trees are common even in cities, and there are many places where trees grow together in sufficient numbers that they can be termed "woods." But mature forests have become rare in our urbanized environment. 

One such mature forest survives on the slopes of the Amiata mountain (Monte Amiata), an ancient volcano located about at the center of Tuscany, Italy. Not really a "pristine" forest, but managed by humans with a sufficiently light hand so that it can grow according to its tendency of forming a "monodominant" forest. It is composed nearly completely of a single species of trees: the Fagus sylvatica the European beech. In the photo, below, you can see the east side of the Amiata mountain seen from the valley. 

Our remote ancestors were, most likely, savanna creatures: they weren't used to forests. We may only imagine the awe they felt when they migrated north, from their original African home, to walk in the great forests of Eurasia, by then emerging out of the last ice age. It is a sensation that we can still feel, today. Not many of us are acquainted with the subtleties of a forest ecosystem, but we can recognize that we are looking at something gigantic: an enormous creature that dominates the land. 

A forest is much more than just trees -- it is the true embodiment of the concept of "holobiont." (at least in the version called "extended holobiont" by Castell et al.).  It is an assembly of different creatures that live in symbiosis with each other. The beauty of the concept is that the creatures that form a holobiont are not altruistic. Individual trees don't care about the forest -- they probably don't even know that such a thing as a "forest" exists. They all act for their own survival. But the result is the optimal functioning of the whole system: a forest is a holobiont is a forest.

It has been only in recent times that we have been able to understand part of the intricate network of relations that create the forest holobiont. You may have heard of the "mycorrhiza," the association between tree roots and fungi -- a concept known since the 19th century. It is a typical symbiotic relationship: the plant provides food (carbohydrates) to the fungus, while the fungus provides minerals for the plant. The intricate network of tree roots and fungi has been termed the "Wood Wide Web" since it connects all trees together, exchanging sugars, nitrogen, minerals, and -- probably -- information. 

But trees also get together above ground to support each other. A monodominant forest, such as the beeches of Monte Amiata forms a relatively uniform canopy that provides several advantages. It shades the ground, maintaining it humid, and avoiding the growth of competing species. The trees also shield each other from the gusts of wind that may topple an isolated plant. 

The canopy is the interface between the ground and the atmosphere. Trees evaporate enormous amounts of water in a process called "evapotranspiration." Trees do not do that to favor other trees -- it is their way to exploit the sun's heat to pull water and nutrients all the way from the roots to the leaves of the crown. Evaporated water is a byproduct of the process and, yet, it is fundamental for the survival of the forest. 

It is a complex story that sees water being transferred from the ground to the atmosphere, where it may condense around the particles of volatile organic compounds (VOCs), also emitted by the trees. The result is the formation of low-height clouds that further shield the ground from solar heat and that will eventually give back the water in the form of rain. 

But it is not just a vertical movement: the condensation of water droplets above the canopy of a forest creates a depression that generates wind. This wind may transport inland humid masses of atmosphere from the oceans, where the water had evaporated. It is the mechanism of the "biotic pump" that guarantees abundant rain whenever forests exist. Cut the forest, and you lose the rain. It is not enough to plant trees to have the rain back. You have to wait for the forest to mature and form a full canopy to trigger the biotic pump. 

So, we have all the reasons to be awed at the sight of a fully grown forest. And we have all the reasons to keep it the way it is. The whole planetary ecosystem depends on healthy forests, and we have only recently learned how important forests are. Yet, we keep cutting and burning them. Is it too late to remedy the damage done? Maybe not, but we'll have to see. 

To learn more

Holobionts: https://theproudholobionts.blogspot.com/2022/08/holobionts-new-paradigm-to-understand.html

About the biotic pump: https://www.bioticregulation.ru/pump/pump.php

About the role of forests on climate: https://www.nature.com/articles/s41467-021-24551-5, see also https://theproudholobionts.blogspot.com/2022/08/forests-do-they-cool-earth-or-do-they.html

For a more detailed discussion of forests as holobionts: https://theproudholobionts.blogspot.com/2022/02/the-greatest-holobiont-on-earth-old.html

Below: one of the beeches of the Monte Amiata, shown with Ugo Bardi's wife Grazia, and his Grand-Daughter, Aurora