Trees should be part of your forest brush/unit, unless you individually place them as objects (the later goes against your 4096 object limit). For Textures and meshes added in, I always used Notepad++ and scripted them in manually (nothing like doing it yourself so it's done right, eh?). I have NEVER added anything in via the beam map editor. Never. I always used scripting, and with the magic of copy-paste-a in the script editor, it may actually be faster. I actually don't even know how to add stuff like that to the level data base (beyond adding it out of art object folders in the f1 section of the editor once I've added it to the base files via scripting). Want to make a house with the same mesh and different textures? Give it a new scripting reference in your materials.cs, with a unique name, using the same .dae mesh and different textures. Unique names are IMPORTANT. At this point when editing scripts, you'll need to name your new textures in your appropriate materials.cs file in the directory with all the textures. Materials files are scanned on map-load. For best result, don't do this with the map running. Modifying textures while the map is loaded can cause a CTD. Working with materials.cs is not something I can help you with, if you do not already know about these. You should by now, know about it. If you do not, I am sure the map-making tutorials have that covered already as it's beyond the scope of this post. Oh, and being in my mid-30's with over 20~21 years of PC experience, and then building my own pc's since late 96... yeah let's just say 80~150wpm is no big deal to type in provided my nerves, circulation and blood sugar are fine at the moment. OC'ed my 1st cpu in feb/97. I will pop in to this thread now and then to give a fair, friendly but otherwise unbiased bit of advice now and then. Good luck with your map. You don't need to copy base-game-objects & textures into your map dir to use them unless they're custom, just make a reference to them in the script. Uploading base-game objects in your mod, that are part of the base game isn't a good thing(if you checked your EULA it's forbidden). Base-game objects mean something included with BeamNG.Drive. This is due to the fact the game is in development, and things may change, and also copyright law. Do not get scared, just know these things off the bat from the get-go and you'll be off to more of a running start than you realize. On the plus side, you'll save on download size (my maps are ~430mb in size, they'd be +1gb if I used lots of base-game objects and textures in the map structure). Again, good luck.
In a post, that says "First map", trying to give advice, from one map developer who's made game maps for 20 years and counting, to one just starting out in this game... Well put it this way, you can never give too much advice. I am at a loss for why I must explain this, other than the fact that documentation for this pre-release game is quite spartan to say the least.
THIS MAP IS NOT DEAD! i've been working on some things... UPDATES: revamped forest! added streetlamps: new suburbs: new lakeside town! added a quarry! added a port and an air port! (some WIP textures) finished mountain village and beach town! added lighthouse island! (wip textures) added logging area! added a rally course! added Easter eggs!