What do you mean exactly? Build tree walls or add some more trees? Tree walls are just walls standing vertical with some material from texture representing a picture of some trees, lanscape (transparent "sky" above of course). Sometime they look flat, so you can put some more objects before them to make it more interesting. These walls can be really low-poly, like panel lenght around 30-70, even 100 meters (maximum for btb), then Material Change can be even 500 m
This is how I make forests - little too much for your case maybe, but it can be an idea how it works.
And settings for these particular walls are:
Green ovals - panel lenght shorter to avoid too sharp corners
Red ovals - unless you use vertical textures, you have to rotate them 90 or -90 degrees. The X offset must be there because of some line appearing above material, but also
chub_pearson said you can leave some transparency at the bottom of the picture - good idea!
http://www.racedepartment.com/bobs-track-builder/38312-showing-background-scenery-hard-easy-way.html
That looks dense enough for me, still not too many objects yet
You can watch the video from my WIP (RBR) track also - enjoy.