Open your Oblivion\Data directory and find the folder Distant LOD. Deleting everything in it will remove all custom distant trees/rocks/etc, but since Oblivion and the official DLCs as well as some unofficial mods use .bsa for their LOD you don't have to worry about deleting those.
If, on the other hand, you have multiple mods that use distant LOD, make sure you delete the correct trees, related to the mod that was once there.