the planting of lines of trees to stop desertification problems in the plains is pretty widespread. A lot of it was done in America and Russia in the past, now China seems to be doing it, e.g. “Green Wall of China” Great Green Wall (China) - Wikipedia
Well, can somebody explain to me, how does it work? What is so special about the trees? E.g. suppose we use a bulldozer to make a big dirt wall of the same height as a tree, would that work the same way?
Or, suppose the tree fence has to be wide to work. (BTW, how wide is it supposed to be? How many rows of trees?) Well, so let’s make two or three parallel dirt walls whose combined air-dynamic properties will be similar to the requisite tree fence.
Incidentally, yes, I do know about how the trees’ roots are holding the soil or something like that. But, the tree roots from a tree fence are only taking up so much space - basically the space under the fence. Meanwhile, the declared purpose of the fence (AFAIK, at least) is to deal with the problem of the wind destroying the soil far away from the fence and its roots. In fact, maybe that’s why it is even called “windbreak” Windbreak - Wikipedia
Ok, so what’s wrong with my reasoning? Why do they plan tree fences that will grow up maybe in 10 years if they could just go run a bulldozer and build an equivalent wall right now?
There are a bunch of reasons, and I don’t know them all. I can give you three, though;
The term “windbreak” means exactly what its constituent parts would suggest – a tree row breaks the wind. This prevents the windborne movement of soil on a large scale, reducing erosion of topsoil and the formation of desert surfaces. Even though there may be windborne transport on both sides of the tree line, it is reduced by the barrier to surface winds posed by the trees, causing less soil to be moved by the wind and more to be dropped by it.
In the typical types of trees chosen for such basrriers, the root system is extensive, spreading several feet from the tree. Hence, direct soil retention by root system is not “just under the tree” but in a multi-foot strip the length of the windbreak. Combined with the wind breaking discussed in (1), this helps to stabilize a much larger area than might be expected from just noting the tree line.
There is a real microclimatic effect as well. The net effect of a tree is to draw water up, owing to transpiration and osmotic pressure. Net effect is that the subsoil surrounding the tree is relatively wetter (perhaps ‘somewhat less arid’ would be more accurate) than the surrounding area, the air surrounding the tree is somewhat higher in humidity than air further away, and in general you have a small area that is relatively more humid. Presuming an ongoing subsurface water supply, this acts as an ongoing barrier to desertification.
A very minor but present additional benefit is, quite simply, shade. Sunlight absorbed by leaves does not heat and dry the soil in the shaded area, providing an area where plants with semiarid tolerance find it easier to flourish.
Someone who understands this more thoroughly can expand on and correct this, but I hope it’s some help.
Another non-expert here, but I think another reason is that trees don’t simply block or re-direct the wind as a solid barrier would. The wind still goes through the trees, just at decreased intensity and speed. A full-on solid barrier could act like a mountain range, concentrating the wind, potentially doing more damage than leaving well enough alone.
First of all, that’s a hell of a lot of volume dirt to move. Where are you getting the dirt from? And no, it won’t work the same way. The dirt wall will have sloped sides at the angle of repose. The wind will simply blow over it with little turbulence, slowing down relatively little. The wind will erode the dirt from the top and blow it away. Occasional heavy rains will also cause erosion.
A tree, because of its complex shape, will have much more surface area. It will break up the wind stream and cause turbulence, slowing the wind much more than a sloping surface. The tree’s roots will hold the soil against erosion by both wind and rain.
A tree costs very little to plant compared to building a wall, and its growth after that is free. Why go to such great expense when you can have a good windbreak much more cheaply.
Actually, they generally choose trees that have high growth rates. The trees will become more and more effective every year as they grow. A wall will be vastly more expensive while being less effective. It will require maintenance as it erodes, and will be a continual expense.
Trees also provide shade and coolness. I lived in the desert, and when I worked at the AFB I’d walk through the wind-break trees at lunch. It was much cooler walking there than it was in the sun.
Another advantage to planting trees is that they repair themselves. If a tree dies, seeds will sprout and you’ll have more trees filling in no time. Compare that to a dirt (or any other man-made) wall and there’s a huge advantage in the long run.
Also, in regards to roots… it isn’t just a few feet that they spread. Many trees have amazing root systems that can extend out a hundred feet or more. (Here’s a gardening-related link). So the bulk of the tree, by weight, may be above ground, but in terms of area covered, you only see the tip of the iceberg.
I haven’t been personally involved with anti-desertification efforts, but I know a lot of people who were.
Trees are good because they fix the soil locally, and prevent erosion further down the line by slowing not only wind, but water. One of the bigger problems in arid climates is that the water from the rare rainstorms runs off immediately if there are not enough plants to catch it. The rushing water can cause massive erosion very quickly. Erosion in one area has a domino effect on other areas, so the theory is that if you can slow or stop it where it starts, you can protect large areas. You have to understand the rate that this erosion is happening- once it gets started, an area can go from farmland to wasteland is just a few seasons.
Usually, they choose tree species that contribute to the fertility of the soil, making it more likely that more ground cover and crops will grow. Trees can also provide benefits to the local population, such as firewood and marketable products (in particular, gum arabic.) This makes it more likely that the local population will value and maintain the windbreak.