When my shard did it years ago, we left them black. You can tile underneath it with floor tiles or whatnot at a negative z underneath and then have stairs going up to the hole.
For your forests, you need to go into your dragon scripts folder and go into the items folder. Then find the file 'forest' and open it up.
This is an example of what you'll see in there.
Quote:
//Tree (Grey) + green leaves
0001 0ccd 0000 0000 0000 0000 0cce 0000 0000 0000 0000
The first number, which I have bolded, is the frequency of the item to appear. If you want to change the density of how many of that item will appear, you need to change that number. I can't really tell you what to change it to though because I am going to have to do some experimenting with it myself to get the right amount.