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.