I was watching a war movie when there was a scene where the enemy walks out of the tree line and into the field for a battle. This got me thinking.
What causes there to be natural groupings of trees like forests and natural areas with relatively nothing like meadows? What is the natural cause for a relatively straight tree line that leads to a wide clearing?
In: Biology
Soil depth and lack of water are common reasons for a natural break, in Europe most of the treelines are man made.
Natural tree lines and open areas are rarely straight, and they happen for a variety of reasons. The default for an area is usually forest, but trees won’t grow well if there’s to much water in the soil (swampy meadows), natural fires are too frequent (grasslands/prairies), no soil to anchor the roots (rocky outcrops), not a long enough growing season (arctic and alpine tree lines) and a bunch more. Most trees have a dependent relationship with soil fungus too, so if the soil isn’t healthy enough to support the fungus, the trees won’t do well either. There’s always exceptions though.
Aside from different soil conditions, grazing animals can cause that.
New trees can’t grow in a meadow that is regularly “mowed” by grazing animals. On the other side, grass can’t grow between trees that are standing close to each other. Add to that that both spread their seeds in a relatively close radius, and you get a system that naturally forms boundaries instead of intermixing.
That boundary will move with time, but that may not be observable to humans easily, given how slow trees grow. When an old tree on the boundary falls and lands on the meadow side, it will create an area that has some protection against grazing, and where new trees have a better chance of getting old enough to not be grazed. If it falls in the other direction, it has the opposite effect. The small area it was standing in before is now open soil where grass will rapidly grow.
The latter effect can also create new meadows when it happens at another type of boundary that lets sunlight through to the floor. You often have natural meadows along not too small streams. They create a light gap and make the soil softer (so trees fall easier). Perfect “seeds” for meadows.
Once that meadow has formed, the beach area is quickly overtaken by medium-high plants because grazing animals keep some distance (footing is treacherous there). But the meadow still grows.
You get a similar effect when a really old and big tree falls inside the forest. Suddenly there’s a light gap and open soil. Grass grows much faster than trees and takes the new ground first. Add some grazers, and trees will have a hard time taking that patch back. On the other hand, if there are enough meadows around for all the grazing animals, they will ignore such tiny spots.
Some meadows, I believe, are where a beaver pond (since vanished) killed the trees by drowning their roots.
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