Hi, i'm afraid it's not possible to use only a spline.
ForestIvy requires a mesh to grow, because the alignment of leaves and other components depends of the surface's normals.
But for next versions we want to include options to export/import the Baseline from a spline.
Using that you would create a dummy surface from the curve, and the import the Baseline from the same curve. That makes the process more simple, without having to "paint" the Baseline.