Well, I'm not a mental ray expert, but switching the Sunlight Mode to Skylight: Illumination from FInal Gather (FG)
might work for your scenario better than actually chosen IBL. Optionally, when using the Skylight: Illumination from IBL
use the Opaque
Shadow mode (to be honest I'm not sure, if there is any advantage of using that "Image Based Lightning" mode as you're using a procedural mr Physical Sky
map in the Environment map and not a High Dynamic Range Image).
Feel free to double check by using the Forest Tools
utility to export all scattered geometry to instances, that's not a Forest Pack related issue (you'll face longer render times even after that conversion to "standard Max's instances"). You can find it under Utilities - More ... - Forest Tools
. For more information about it, check our on-line reference guide