By Doyle Irvin, American Forests

Photo credits to Flickr users Gerben van Heijningen (desert), Grempz (ocean), and Arne (forest)

Every day, scientists are discovering fascinating new aspects of how everything on our planet intertwines. Working at American Forests means that we are constantly learning new ways in which trees are integral to the functionality of our ecosystem. I’d like to share with you something new that I’ve only recently come across.

People have known for a long time that desertification can be fought with trees. We know that vegetation helps to anchor critical soils, keeping the ground where it is, allowing its nutrients to recycle again and again in the cycle of life. Many rapidly expanding countries have experienced sudden desertification, after they extensively cut down their forests in order to jump start their industrialization and economic expansion. Without the shade that forests provide, ground temperatures increase, drying the soil and speeding up the desertification process. Places like India and Kenya are now trying to combat these developments by planting millions of trees.

What has also been clear for a long time is that trees and forests are a great source of existing moisture, through their evapotranspirative properties. Their “tree sweat” provides water for both their surrounding vegetation and the clouds above. The clouds then move with the wind to new areas. Scientist Dominick Spracklen of the University of Leeds found “this really strong impact – air that travelled over a lot of forest brought a lot more rain than air that didn’t.”

A new development that is starting to gain steam in the scientific community is the theory that forests anchor and attract rainfall for their regions, instead of simply serving as a source of moisture for the air. Russian scientists Dr. Anastassia Makarieva and Professor Victor Gorshkov of the Petersburg Nuclear Physics Institute have proposed that forests act as what they call a “biotic pump.”

To explain this process, picture three distinct bio-zones: desert, ocean and forest. Forests actually evaporate more moisture than oceans, and deserts of course evaporate the least of the three.

Evaporation creates low-pressure air zones by reducing the temperature of the air. Remember, heat causes expansion, meaning that a hotter quantity of atmosphere will expand and push itself into a colder area (this is why city streets and deserts provide such great updrafts for birds to fly with). This transference of atmosphere from one area to another will bring whatever moisture is on the wind as it rushes in to fill low-pressure areas. This means that whatever area is evaporating more — creating a low pressure zone by lowering air temperatures — is going to attract moisture from adjacent lower-evaporation areas.

Now, these aren’t hard and fast rules you can apply everywhere — there are always exceptions. But they are general principles. What they tell us is that, generally, if you have a desert next to an ocean, the ocean will be evaporating more moisture than the desert, and the wind will blow out into the ocean. If you have a forest next to an ocean, the wind will blow in from the ocean and bring the moisture with it.

If you have an unbroken forest reaching far into the mainland of the continent, it acts as a “biotic pump,” sustaining the water cycle (and the direction of the moisture-laden wind) as a positive feedback loop deep into the interior.

Diagrams courtesy of CIFOR.

This also means that if you have no trees – or decide to cut them all down – you are staring right down the barrel of drought season. Think of it as similar to the phrase, “you need some money to make some money.” You need trees in order to have the moisture to grow new trees: if you cut them all down, you might not be able to get them back.

If this theory withstands the tests of time and scientific scrutiny, it could lead to a deeper understanding of how to conserve and protect our natural world. To read more in-depth explanations of the processes involved, check out the biotic pump website and this article in Scientific American.

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