Yes, thats why I was wondering if they could not just cast shadows using the static light maps rather than dynamic shadows.
I also found that using high detail trees (20k triangles+) with lods down to 8 triangles in the distance , uses huge amounts of memory for lighting. My scene was using about 4gb ram with lightmass, and took about 15mins to light, but with 500 distant trees it was 18gb+ and almost 2 hours to light. Even though those trees were only 8 tris each in the lod object displayed, i`m assuming it was lighting as if they were the lod0 mesh at 20k tris each?
So I created a tree mesh just for distance that was just a single 8 tri mesh and it fixed that issue.