Smart Ancient Trees pack was released today.
I hope you will find it useful for your projects.
There is also rigged versions of trees, roots and stumps.
You can open FBX file in any program like Blender, 3D MAX, Maya or game engine like Unity or Unreal and adjust roots, brunch or trunk accordingly to terrain and envelopment objects.
Unreal Technical Details:
Physically Based Rendering: Yes
Number of Textures: 9, diffuse, normals, and specular.
Texture Size: 4096x4096,
Collision: Yes, automatically generated
Number of Meshes: 24
Number of Materials and Material Instances: 6 Materials
Intended Platform: PC, PS4, XBOX ONE
Platforms Tested: PC