Hey, I've been searching all over to try and find out how to add distance trees like those in Timberland - they appear to be just sprites on a flat plane with no collision, but I've no idea how to go about adding them to a map. If there's any relevant tutorial I'd greatly appreciate someone linking it here, I was unable to find it on my own. As far as I know they aren't detail objects, but they're definitely needed to be added to the BSP itself. I'm trying to stay away from having to recompile the BSP from a 3D modeling program, but I'm not sure if that's possible. Any help would be great! Thanks

You need to add the faces in a 3d modelling program, and you will have to recompile. The faces don't need to be sealed, however, because a two-sided shader will be applied to them. You need to add a special character to the material name in order for tool to recognise this two-sided shader. I can't remember which symbol offhand, but the good old official HEK documentation has the answer.

Also, tip for new map makers - download both the complete single player and multiplayer tagsets. This way, if you see something you like in an SP or MP map, rip the BSP and see what the shader is called. Then all you have to do is name your shader accordingly (plus any special shader symbols) and when tool compiles the map it'll use the shader from the other map's directory.
Edited by stunt_man on Dec 4, 2013 at 11:23 PM

If you don't want to reexport the map, you can simply make scenery tag with those trees but you would need to make shader_model. Their original shader should be in timberland's folder.