Well I don't use CAD, rather vector drawing software called Xara Photo & Graphic Designer 9 which is $89 software, but I use it for all my maps (for real and imagined locations) and I'm convinced it would be a great tool for landscape artists. If you access to photo images of trees and such from various CAD applications they should be able to import fine, otherwise there are various sources for plant objects to place in your map. I actually use the Pro version, but you won't need that for doing just maps. You can look at all my various photo-realistic maps to get a feel for what you can do.

Here's a link to the free 30 day trial software: Xara Photo & Graphic Designer 9 (note this is a link to a affiliate link).