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
No comments:
Post a Comment