When it comes to parallelism, you want to make sure that the elements being compared - nouns, verbs, or modifiers - are parallel, but that doesn't mean that you have to force whole structures to look exactly alike!

Here, we want to say that the yield of a tree depends on two things: A) size, and B) climate and altitude. These are both nouns, so it makes sense to compare them. We want our structure to say "depending on (A) and (B)."

Your question, as I understand it, is - why is it ok to have ITS in one element, but not in the other? Well, we don't have to make the parallel elements look exactly alike, as long as the meaning is clear. We're talking about the size of a particular tree, so we need the possessive "its" to specify that it's the size that belongs to this particular tree. If we just said "depending on size," that would be too vague/general. We don't use the possessive "its" for climate and altitude, however, because those are not particular to the tree - they're the climate and altitude of the whole region, so they should be general.

The nouns here are parallel - it's fine to compare "size" to "climate and altitude." The possessive is there for meaning clarification, so it doesn't have to match. If it matched, it would change the meaning!

The same is often true for articles. See more here: https://www.beatthegmat.com/gmat-prep-qu ... tml#554171