I´m also experimenting with very large amounts of trees. I may not have the final answer but I can share some experience.
Attached are some Images of 130k trees.
The tree used was not really an optimal tree from an old tree library wich was around 20mb in size.
I would also suggest Speedtree btw.
I created 2 LODs.
LOD0 was was the mesh as I got it from the Library. Cast Shadow and Collision is enabled.
LOD1 are 4 Bilboards, (well more or less they don´t align to the camera) rendered from the Mesh. 3 from the sides and one from top wich are mapped on simple planes.
LOD1 blendes at 0.02 Screensize.
Materials. I made the experience that turning off mipmaps on the billboard textures give the best results. As long as temporal AA is used there´s no big problem
with flickering. With Mipmaps they start to brighten up in the distance.
I use a Two Sided Foliage shader for nice transluceny effects.
Lighting and Shadows.
I use a Moveable direct light as sun and a stationary skylight. There is no way to get acceptable results with static lightmaps from this amounts of trees. I think I even was not able to turn
the lightmap size that low that I would not run out of memory. Btw. the indirect lighting from the skylight is just not that visible in my scene as I made a pretty
high controst with a post process volume in case you wonder.
I tried all possible shadows. The best looking shadows are raytraced shadows. Also the performance is acceptable. The problem with them is this
So I use cascade shadow maps with a dynamic shadow distance of roughly 60k. Just so large it covers my trees. As larger the value as worse the shadow quality.
Every shadow beyond the trees on the terrain is handled by the far shadow cascades.
the performence is acceptable at the moment. It´s a pretty hybrid test. There is just a terrain and trees. Don´t know if this is still possible if grass, stones and other meshes are involved.
Also I´m not yet sure how to mix baked with dynamic stuff… Like a house interior that would be placed in this scene.But well I´m still experimenting with this scene.