OK, so I'm pretty sure that "orange tree" is considered one subject. If I'm diagramming a sentence it would go on the left as "orange tree."
However, what about "Anjou pears" or "green pears" or French toast?
Would I diagram the following on the left of the vertical line?
"Anjou pears" as one subject instead of Anjou being a proper adjective?
"French toast" as one subject instead of French being a proper adjective?
Re: Compound Nouns
I am not a great diagrammer,but I see them all as adjectives, no matter what their form.
The orange tree is heavy.
The Anjou pears are heavy.
The green pears are heavy.
The French toast is heavy.
I'd make tree, pears, toast the subjects to the left of the vertical, and orange, Anjou, green, French on slanted lines down from the subjects-- they tell us what kind of tree, pear or toast.