I actually already used baked in AO and cavity maps, but it is insufficient.
Again, i just want to iterate that 31k lightmapped trees is not insane nor is it pushing the engine in any way; as Kalle-H points out - 31k trees with 32x32px lightmaps only amounts to 186mb - that’s the equivalent of approximately 7 2048 textures. To put that into perspective, it’s equal to two current-gen character models (with diffuse, normals, pbr textures, etc).
I shipped an Unreal 3 title for the last gen consoles that had well over 100k lightmapped objects in one level with no problems - it’s all down to how well you pack your lightmaps and stream your levels.
The fact here is that Unreal 4’s foliage lightmap atlasing is fundamentally broken - if I were to bake 31k individual tree static meshes instead I would have no issues (other than in increase in draw calls).
I had a similar idea to this but it seemed to still try to pack everything into one atlas - I’m going to try again though, perhaps splitting it into 4-5 copies of the same model will work? I will try it out and report back.