The suggestion above is right, you define the right material under SURFACE and the trees should glue down to that surface.Background not good, trees take off again...
AC Foliage tool is fast, easy, precise you can draw (point cloud) large areas of workplace forests in one go..
available here :
AC Foliage v1.1 - https://ko-fi.com/s/b5e655f89b
Documentation: Balkan Moto