I remember falefee showed how to do this in one of his twitch livestreams, but i don't remember what exactly it was.
LODs: https://wiki.beamng.com/Exporting_static_objects#Export_layout a130, a420, etc. are the various lods. Those numbers represent the amount of pixels on screen for that lod to be used. a130 means the object must take around 130 pixels on screen before that lod is used (it would be small, that lod is very likely a low poly model). Bigger numbers means the pixel take more space on screen, means that's the model that is used when the player is close to them. If you want to make your object disappear, make an empty lod basically. Another way around that is using billboards. It can be done directly in-game (shape editor). I use that on my simple_map level to hide the 'skycurve' track when you are far from it. I basically set up the billboards so they would not generated, so when the game switches to them, it's just invisible.
Not differently as if you have different object. Just assign the material and UV map your LODs and voilà (LODs are self-made by you, not automatically generated). Billboards are automatically generated, and you can tune the parameters from the shape editor (resolution, steps, distance, etc.)
I changed the LODs to a single object and it works fine, but it doesn't seem to apply to forest elements
