This is a project I've been chipping away at for the past little while in my spare time.
All the vegetation was modeled exclusively in SpeedTree, from high to low poly. The assets and textures are built using my own personal scanned data, processed in Agisoft, while certain elements such as rocks, clutter and terrain materials were built using a mix of my scans, Substance, ZBrush, and traditional modeling & texturing techniques.
All assets were roughly built to the higher end of current video game budgets, minus the LODs, but since I was just having fun with the project I didn't bother to optimize it further. (For example, the trees range from 13k - 30k triangles, rocks range from 2k - 5k triangles.)
I've uploaded the project files in case it might be helpful see how things were constructed and put together.
Before you can open and navigate the project, you will need to activate LPV in your Unreal Engine directory.