Not really, Mue. They do affect the level of detail and thus the number of objects displayed, but you can't increase the maximum number of objects for a TOD. That value is hardcoded I think. Each TOD has the same limit.
In order to add more trees, the only way is to use custom shapes. Instead of using the stock shape of a single tree as offered by the TerrainEditor, one must introduce custom shapes where instead of a tree you have a forest model, whose graphics are determined by a .tga file. However, I don't know how to do this in practice. If I'm not mistaken, Stary is the only modder around here who knows how to create such TODs with custom shapes.