Yes, it could have been - and in that case, quite like with the sentence that was mentioned at the beginning of this discussion, “in the park” would be an adverbial of place and “a tree” the subject. 
[quote]You can call this the subject or not, I’ve seen different interpretations, but it’s irrelevant to the question at hand…
The question at hand is the verb form which can appear after this element, whatever you care to call it, which is filling the place of the subject in this sentence.[/quote]
Well, it appears that not only has the initial question been answered but people have been talking about the (grammatical) subject of the sentence and ended up disagreeing on what/where that would be… 
And i still hope that it will become clear to all involved that whoever suggested the subject is “the oak tree” had gotten it right… 