Adding Trees and Surface Water to your Terrain
As you might have guessed, it is not possible to use the editor to add features such as trees and surface water. These need to be stored using SimCity 3000's native file format, which Maxis has not publicly released.
However, if you would like to add trees to your terrain, it is very simple to do.
If you choose to add other features (eg infrastructure), you can still use this method; however in this case it is probably more appropriate to save your game in the Starter Towns folder (usually C:\Program Files\Maxis\SimCity 3000\Cities\StarterTowns) and rename it so that it ends in .st3.