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.