Just a test... not even a wip...
A plane 600X600m, converted to editable poly, trasformed to a mountain with noise brush and mesh smooth...
One forest object with 3 trees (2nd image) rendered in Maxwell, initial image at 2000X1500 for 30min...
You can understand that there are many trees there... Only 1.9 Gb RAM used as I can see from task manager...perfect!