I've worked on and off on a tree tutorial. The hard part is that everything is interrelated so discovering/explaining the concepts of the advanced settings (without really understanding the nuts-&-bolts under the hood) means you are sort of like a shamen trying to discover/explain a carborator.... Anyway, someone mentioned the term "L-system" which I assume the plant modeller is based on, and after googling half a dozen college papers I didn't feel any more knowledgable.... Still working on it tho. It's much easier to do a tutorial for a specific tree, than a tutorial on "advanced tree concepts and controls".