H Wrote:Hi Ron,
Does "depending on its size and on climate and altitude" modify "is"? or "varies"? or the entire subordinate clause - "a single tree is able to produce..."?
it's an adverbial modifier that modifies the action of the clause with which it's associated. technically, that action is encapsulated by "is (able)", although it's probably easier to think of it as "modifying the entire clause".
with modifiers that serve as adjectives (such as "..., which ..."), it's crucial that you isolate the single noun modified by the modifier.
on the other hand, with adverbial modifiers, it's unimportant to isolate the single word being modified, because the placement of the modifier isn't restricted anywhere near as severely as with adjective-type modifiers.