Reading in heightmaps (Made with world machine) is a piece of cake in Houdini. Although you'd most likely do that in a displacement shader, a VOP Point SOP is a fun and quick way to experiment.
All you need is a displace by normal and a texture op.

Looks like an album cover!


At first. a simple elevation gradient. A neat trick is to fuzz the ramp lookup with a noise function, so it won't totally look like Y=Position on color ramp.
A lot can be achieved by carefully crafting a gradient which mimicks your vegetation. There are even companies selling satellite-derived gradient data for world machine.

The next significant steps to improve the looks is slope-dependent distribution (more or less for free) and the usage of maps sourced from the by-product of the erosion operator in world machine.

A note on obtaining elevation data:
The DLR has processed data from the NASA SRTM project, 1-3 arcseconds resolution. It is available for free here:
https://centaurus.caf.dlr.de:8443/eoweb-ng/template/default/welcome/entryPage.vm
dt2 (military elevation data) can then be converted to tif using gdal (install via package manager/brew):
for t in *.dt2 do ; gdal_translate -of GTiff $t $t.tif ; done

Base shading algorithm:
I went for an 3-Layer approach:

  1. A base gradient that defines the vegetation
  2. Slope-based "rocks" (steep slopes have less vegetation) - those have some extra displacement and coloring to spice up the elevation model
  3. If desired: snow on top, controllable via ramp, only where terrain is not too steep

Up-rezzing / LOD
The higher the base grid, the higher the detail. Images below are real-time openGL viewport renders.





Resolution as above, offline rendered with one HDR:

Up until now, these results are fully procedural without computed maps from Worldmachine.

The HDR is available at
http://adaptivesamples.com/2015/11/09/hdri-mpumalanga-veld/
and made by Greg Zaal, (blender) cycles developer, with full dynamic range of the sun, giving nice sharp shadows. He has a very technical and good approach to creating HDR, so you should definitely check out and support his work.

Displacement has been moved into the shader now:

Some early forest growth.