As per your request (PM).
Explanation of the Region editor functionality (CS Wiki).
I created the Forgotten Forest (FF) region area and a FF test area (screenshots, esp file). Use the test area to experiment.
I copied the Steadhelm preset (an example) to the FF test area.
As there are no high mountains the values in this preset could work (*) for the FF.
Go to the FF test area in the region editor (Silgrad Tower land).
Check every texture and write down the values for the data (density and clustering).
Check every object (e.g. trees) and write down the values for the data (density, clustering, radius, radius wrt Parent, sink value etc.).
The sink value determines how far the object sinks into the land (to avoid floating).
Delete everything from the preset.
Now add the FF textures (drag and drop) and use the data you wrote down (recreate the same (as in directory) tree). The same applies to the other objects (FF trees, FF rocks).
For the tree use the same number of objects (textures, trees, rocks etc.)
If you reduce the number it will affect the generation process (and you have to readjust the values).
When done, generate the test area and check the generated content.
"Obliterate Now" after checking and change the things you didn't like. Generate again etc.
You have to repeat this process until completely satisfied.
If so, go to the FF area and "Copy Objects from Other region" (select the FF test area). Delete the FF test area.
Finally, generate the FF area.
(*) The trees for the FF are new (custom) so you have to check the sink values and these are prob. different from the Steadhelm preset.
Check also the the presets that were used in Oblivion (helpful).