@ Juggernaut
Hi, I wrote my own editor to suit my own needs when I started this project. When I am placing an object in the editor it will save a text file containing all the objects info, i.e. object number, rotation, position, scale or what ever info you want. When I am loading these objects back into the engine I load the text file giving me all the info that object needs.
I treat different object in different ways as far as LoD is concerned. For the like of larger objects that are visible farther away I use different object models of a lower poly count but the smaller things like grass I just fade in at a particular distance from the camera.
As for the trees they are static I would use light mapping but certainly shadow mapping only the trees that are close to the players view would keep the frame rate up.
I hope some of this helps as I know how much work is needed for big open landscapes, I am fours years into this project and I am just scratching the surface.

Lead programmer ULIDIA