The green, grassy material provides a visual ‘anchor' for the trees, and they don' t seem to be arbitrarily sprouting from the soil.

For more information about Altitude Fractalizer, see the WorldBuilder Basics: Part 3 tutorial.

Now let' s create a vapor plume.  This is often an occurrence with real volcanoes; superheated gases, steam and ash escape through surface vents and form huge overhead clouds.

Add a Cumulonimbus cloud.  Place it within the caldera (the summit ‘crater' ). 

In the Cloud Shape property, reduce the X and Y settings for Voxels Number. 

This will make our initial cloud shape tall and relatively skinny.  Using only the Scale manipulator tool in this case might produce unwanted compression or stretching of detail within the cloud.