This was a bit worrying so I tried it out. My first render was OK so I tried various things to try and reproduce the problem... another productive train journey
Load population, smooth normals on by default, renders OK.
Turn off smooth normals, renders OK
Turn on smooth normals, render is black
Turn off smooth normals, render remains black.
Exit and load with smooth normals off, renders OK.
Turn smooth normals on, render is black
Save with smooth normals on, restart TG2 and reload, renders OK
Repeated with the same results.
I was only using a small population (100x100m, 30m spacing). Test objectwas the XFrog sequoia. This doesn't really provide a definintive answer on how to get it to work but it may provide some clues.
While on objects. One thing that I've found with most imported models is that many textures need to be tiled to be scaled correctly (tree bark textures and bump maps in particular). Rather than use a colour image in the default shader I add the texture as an image map specifying the size and repetition and plug this into the colour function of the default shader.