Mountain weight painting

I would like to weight paint mountain so I can put trees on it.
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I would add more weight at indentations of the mountain, you know how a real mountain looks like.
These are the effects I want to get
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Look at where the trees are.

Hi!
Look at https://www.gamedev.net/blogs/entry/2254908-creating-a-forest-scene-in-blender/
Or search for “weight paint particle system” in blender to obtain the effect that you are aiming at
Cheers!
Sparazza