Eli5 Why can’t we just slowly plan trees out in the desert to turn it into a forest?
You can. Sorta. In some places.
Deserts aren’t just places with few plants, they’re places with little rain.
Trees, like all plants, need water to live. Deserts don’t have a whole lot of water to support them, so just planting trees don’t help because the trees simply won’t be able to get enough water to survive.
There is a process called “desertification” where the destruction of plants results in less water retention in the soil which results in insufficient water to support the amount of the plants that it used to have because the water supplied by the rain doesn’t stick around long enough to support those plants. This ultimately results in a desert-like environment where there’s less plant life than you would expect from the amount of precipitation received.
This can happen both naturally and through human (usually agricultural) activity. You can reverse desertification by planting trees and other appropriate plants.
Because deserts aren’t an inherently bad thing. [In fact, that only reason that the Amazon rainforests exist is because of the Sahara deserts.](https://www.youtube.com/watch?v=ygulQJoIe2Y) Even if we could easily and cheaply convert a desert to a forest, we would be wiping out all of the life that has adapted to live in that environment.
Think of it another way:
Plants/Trees grow where they can.
If they aren’t growing somewhere it’s prob because they can’t- too steep, too cold, too dry, too wet, etc.
There are some efforts to reverse desertification, including hearding cattle and other livestock through the edges in order to condition the soil, but the basic problem is that deserts are deserts because the local weather doesn’t supply enough water via rain to support a lot of plants.
Simply planting trees will most likely just result in the trees dying from dehydration, though planting trees might be combined with other efforts to affect the local climate to mitigate or reverse desertification.