Welcome.
You did paste the centre of the population in front of the render camera I suppose? What I usually do is hover above the scene in the preview, with the camera in view and locate the population some distance in front of the camera, so that its outer border just covers the camera. If you need a bigger population that the default 1x1km, move the location further away from the camera of course. I also rotate the population with the same rotation of the render camera, for maximum efficiency of the instances.
And the population needs to be anchored to the planet and compute terrain. When it isn't, all trees may be underground (or hover).