If you don't care about the specific shapes (like little tree icons in a line), you can just hit Cancel or Ignore when the box pops up. AutoCAD will usually substitute a simple line.
After digging around, here is what I found: aeccland.shx
In short, is the "Rosetta Stone" for site design drawings inside AutoCAD Architecture and Civil 3D. If you don't care about the specific shapes
The name is a dead giveaway if you break it down: aeccland.shx