Hello. I want to distribute some trees over several surfaces, limited only by one shape (multiple splines):

Every green surface is a separated part. The yellow spline is the limit. At first, I select the spline as base for the forest generation and all works right, but as soon as I select the first surface all the trees are confined into a part of the spline. The surface selected at first doesn't affect the result. Any ideas? I've included the reduced scene showing the problem.
sceneRegards