Unity Tree Billboards on Terrain have incorrect lighting!

Might be an easy solution but I can’t find out what to do to remedy this:


I’ve made my billboard distance quite short so you can see the difference; the lighter coloured trees/bushes (in contrast to the darker ones at the bottom) are the billboard versions of the speedtrees that populate my terrain.
Why are they so bright? I’ve messed around with the Ambient Occlusion, scene lighting, etc. etc. but nothing fixes this. And yes, it looks the same in game mode and in the build.

This ISN’T using speedtree, but the built-in Unity tree creator.
Any ideas??

Probably too late for you, but you simply need to put the trees in a folder named “Ambient-Occlusion” and reimport them.