How to soften/lighten very dark shadows?

I’m really struggling to integrate my speedtree asset into a scene. I’ve been playing with multiple lighting settings with very little progress - I have a standard outdoor scene with a skylight and a directional light source with default settings, and the only setting that made any real difference was changing the environment colour in the world settings (set it to a lightish blue colour instead of black). That seemed to fix the shadows that are cast onto the ground by the trees, but the shadows that are on the inner part of the tree are absolutely too dark for my liking. I’ve been going in circles trying to fix this issue, can anyone point me to the right settings? Also (see screenshot) what is happening to the trunk shadows here? This is a standard export from speedtree and production quality lighting with no other assets in the scene.