I never said AO=GI… I said that you could fake some cavity shadows, as in places like the base of the tree or branches near the trunk, with AO/cavity-like mapping. Many 3d programs have options for these. Zbrush has it built right into the render options. Even with other programs, like 3dsmax/maya, you could just throw in a skybox with a light that is at a similar angle to your scene and bake out some “AO/GI/cavity” style maps. I was simply giving you a way to fake in some shadows. You are complaining about the engine not caving to your insane needs for the amount of light mapped models you have. That’s the engine’s way of saying “Hey… You should consider some alternative options for your game’s lighting needs…”
Your trees don’t look like they have many polygons in them. Vertex painting one tree and instancing that would definitely be cheaper. You might even be able to find a way to convert the “AO/GI/cavity” style maps into vertex color information. It would need some interpolation, but it would work. Programs like Zbrush make this a breeze though where you can take a texture and “drop” it down onto the mesh, in the form of polypaint, and vice versa. That way you would have one generalized tree that could be distributed across the scene. If you needed a bit of variety for them, you could make a few variations of shadowing. Regardless, it would probably be astronomically easier than dealing with 31k lightmapped trees…
And again, you can still bake the tree’s shadows into the terrain’s lightmapping.