Hi,
The
Spline Area without having any Surface assigned (under the
Surface tab) is using the highest point of that spline to determine at which altitude all items should be placed.
Without having such surface, you might find handy the
Custom Edit mode, which includes a tool for this purpose:
Tree Editor -
Creation Tools -
Along a Path. It works in a slightly different way, because it creates single items along the path, instead using the area surrounding the spline.
Hope that helps.
Best regards,