What I want to to is use trees at the base of my image from a lake to near a snowline in my image but is there a way to constrain the trees to a certain layer in the composition ?
Take a distribution shader (can be found under color shaders), and set the altitudes (and slope max 50 degrees or so for trees). Blend your trees by this distribution shader. If you use the fractal coming with it you can adjust the distribution a little more. Then set coverage to 0.5 and fractal breakup to 2 for a distribution totally dependent upon the fractal. And you can change the fractal's seed or colors, etc.
Check this quick setup out.
I still can't get it right ... I do everything you say but the trees still go over the top of the mountains :(
Maybe the max height needs to be lowered? Otherwise put your tgd up here, for some adjustment. I'll have a look at it.
Here's the .tgd file :) Hope you can understand it :)
The distribution shader shouldn't be the one for the population to 'sit on'. Check this setup out. I also changed the angle of the 5000x5000 population square to be right in front of the camera, which is more efficient, as well as some other population settings. If you want to change the pattern of distribution have a look at the fractal attached to the distribution shader. That determines the patches of wood. Good luck.
Thank you :D