scale pipe according to the distance from ground to the top of the object

Hello everyone

I need help!I experiment to create a tree by L-system (Rabbit plug-in, Morphocode) but the main problem is how having different width of pipe from the trunk of the plant to the smallest branches of the higher foliage part. Does exist a good way to manage this kind of shape by a few parameters? I hope this message can be understandable by each one.