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

Science, Forests, Bears, and Clouds

 From "Kelebek," Miguel Martinez's blog

12/12/2021

by Miguel Martinez

Last night at sunset, a big chill, dark red clouds to the west. And the half moon, above the black cypress trees to the right, silhouetted against the fire of the dying sun.

Below  Bellosguardo, that silent little wall where sometimes a black cat walks, and to the left you can see the snow on the mountains, and, below, the city of the old enemy, Fiesole....

We go up towards Marignolle, and Marco recites to me the verses of the mad poet, Dino Campana:

To the ghostly garden of mute laurels

To the green garlands

To the autumnal earth

A final greeting!

Walking between the silent walls that hide the secrets of an occult city, we arrive at the villa of the ancient family.

From one end of the great hall, Abraham looks at us in an eighteenth-century painting, as he prepares to sacrifice Isaac; from the other end, the ancestor of the family looks at us in a portrait, and has the same beard and the same look (and faith) as Abraham. And between the two, the menorah, on the wooden sideboard that bears the date MDCXXXVII engraved on it.

We gathered to hear Anastasija Makarieva, black hair, almond-shaped blue eyes and high cheekbones, from the Institute of Nuclear Physics in St. Petersburg. An institution heir to that other half of the world, which not only managed to build Soviet atomic bombs from scratch, but explored worlds unknown to Westerners.

Anastasija (with the accent on the "i") doesn't deal with atomic bombs at all, but with forests.

We've all heard of the forests of the Amazon, but we never talk about the perhaps even larger ones that stretch from the Baltic to the Pacific.

Now, as an oriental language major who has a hard time telling an ash tree from an elm, who hasn't taken notes, and is going from memory, I'm going to try to report to you what Anastasiya said, any nonsense is just my own.

It is said that we are living an immense environmental crisis, linked to CO2 emissions with related global warming; and that we must therefore reduce these emissions.

Which however has a huge consequence: if the problem is too much CO2, we reduce CO2 even at the cost of an extermination, end of the problem. The war against climate change is all there.

The scientists in St. Petersburg do not deny the issue of emissions at all, but they say that there is another factor, which is perhaps even more important, that is leading us towards climate catastrophe.

If life exists, it exists because the biosphere exists; and the biosphere is intimately linked to something the Russians call the biotic pump.

Trees are apparently remarkably incompetent machines: they disperse 90% (I'm quoting from memory) of the water they absorb into the atmosphere.

But coastal trees catch what little water the sea sends down to the earth; they alone, through evapotranspiration, are able to make what by its nature goes down go up. Emitting not only water, but also other substances that allow the water to condense, they form clouds, and through various very complex mechanisms - which complement those known to meteorology - they create winds, which bring moisture inland.

And they therefore allow life on the continents, and generate rivers. So, life on earth depends on the forest world. But it's not enough to plant millions of trees at random, as the technogreens would like to do.

Anastasija tells us about the fir trees, planted en masse at the beginning of the twentieth century, in Bohemia, which today have been infested and destroyed in a short time by pests, because there is no variety; about the problem of coeval trees - the biotic pump really works only when there is the set of trees of many generations, with the whole surrounding ecosystem.

And an American artist listening to us tells of a friend of his, who in order to recreate a forest, very slowly took the humus of a still intact forest, with all its variety.

Siberia and Amazonia are the two forest poles of the world, in their immense diversity. But for some reason, the Siberia they're selling off to the multinational timber industry doesn't seem to interest anyone.

"I've only been to Siberia twice," Anastasija admits. "But every year my partner and I camp in a tent on the coast of the White Sea."

Once we saw a bear. From very far away... so we got closer.

We found him in front of us, and then I felt inside me, what the bear was thinking inside his head: the end had come!

He looked one last time at the sea, then turned around, trying to show as little as he could of his profile. And then suddenly, he gathered all his strength, and ran off into the woods!"

And she gives us the picture of the owl, seen in a tree far, far north, with which we open this story.