Remember that the SNES has got a limited amount of VRAM so the trees can only be loaded in the appropriate tilesets. While yes, the tree objects can be placed in the grassland tileset, they still use different graphics so you need to load the forest tileset first (this isn't unique to the grassland;nbsp/forest tileset: Cave and Castle 2 share the same objects too, so do Switch Palace and Ghost House).
In order to use trees together with the grassland tileset, use ExGFX Bypass
and put the forst graphics either in slot BG2 and 3 (doesn't matter) and recreate the tiles with the Map16 editor
. You can read more about the Map16 editor in the ExGFX tutorial
Okay, my layout looks ugly.