How to properly generate forests?

I’ve been playing around with both Cesium and Unreal for couple of weeks now. What I’ve been trying to do is generate forests over my own terrain. I’ve got shp files that contain the information of the areas where the trees should be. I’ve used FME to extrude those areas slightly, converted the results to citygml and then brought it to Cesium and from Cesium to Unreal. Once in Unreal I’ve used folliage system to generate trees over those extruded planes, which from birds eye perspective look fine, but on ground level the trees’ are obviusly floating a bit as they were planted on a surface that was brought up from the ground level.

What I’m asking has anyone else been trying to do this, and come up any better way to add forest areas to their terrains? Or does anyone know is there a way after the folliage system has generated the trees, if it’s possible to clamp to the terrain bellow once the extruded area has been removed?