When using a surface, I would suggest you to always double check it's local coordinates (your surface coordinates should correspond with intended Forest object orientation). For example a plane created in Front view will work differently with Forest objects as another one created in Perspective / Top view.
Sometimes it's handy to apply the Reset XForm utility on it (as in this case) to "reset" it's local coordinates. Also, the Forest object rotation is actually set to 90º - to get the correct distribution it can be easily fixed by setting 0º. Attaching the modified scene.