After I had sculpted my environment in Unreal I created a base blockout so I could see which areas needed to be clear for my animation shot.

To populate the scene I painted the foliage using various brush densities onto my landscape and then went back and edited the placement of some of the assets. 

To begin, I added a bp skysphere, exponential fog (turned to volumetric fog). I also changed the sun's position in the bp skysphere and star brightness to create a night sky even though not very visible here. I then added the moon with a directional light and a skylight to help lift the shadows in the forest further. 

I then added a slight blue tinge to the moon and slightly lowered the brightness, played around with the volumetric fog and turned down the lux for the directional light a lot to make the scene much darker.