If you are looking for foliage specifiacaclly, you could look at the Tree Party addon, which is designed for doing exactly what you are looking for with trees. It even animated them and comes with a tree editor.
Or there is also a pretty cool little library that Dark Coder wrote, its in the WIP section under "Foliage Library" and it does a some of what tree party does(though not nearly all, they are 2 different things), but it comes with the source, so if you wanted to, you could modify it to work with whatever kinds of objects you wanted to give it.
Link to dark coder's Foliage Library :
http://forum.thegamecreators.com/?m=forum_view&t=144892&b=8
Otherwise, its all down to you to come up with and design efficient sets of code to do what you want it to. Looking at the source of Dark Coder's foliage library, which actually does trees, grass, rocks, weeds and clutter(tree stumps, fallen branches, misc objects etc)pretty well, will give you a really good idea of how you would do something like you want.
I think there are a fair few threads in these forums and the dbpro ones too that deal with generating various things in various ways, from terrains and heightmaps to textures and numbers. Most DBPro programs and ideas can be ported to GDK quite easily, so its always a good place to look aswell when you need a solution to something.
If it ain't broke.... DONT FIX IT !!!