The pasuk itself in Devarim 20:19 commands that a fruit tree may not be felled during a siege and then appends the phrase כִּי הָאָדָם עֵץ הַשָּׂדֶה, whose precise grammatical function — whether it is a rhetorical question, a conditional, or a direct identification — is the crux of the dispute among the Rishonim.
Rashi (Devarim 20:19) reads the word כִּי as meaning 'perhaps' or 'possibly,' rendering the clause as a rhetorical question: is the tree of the field perhaps a man, capable of entering the besieged city and enduring the suffering of famine and thirst like its inhabitants? — and this question itself supplies the reason not to destroy it.
Ibn Ezra (Devarim 20:19) rejects that grammatical reading and argues instead that the phrase means 'for the life of man is the tree of the field,' comparing it to the construction in Devarim where 'a life' is what is pledged — so the tree is identified as the very sustenance of human life, which is why cutting it down is forbidden.
The Ramban (Devarim 20:19) similarly reads it as a positive identification rather than a question, explaining that the tree is the man's source of food and life both during the siege and after the city is conquered, and that Israel should trust God to deliver the city rather than destroy what will sustain them.
Whatever the precise reading of the phrase, the Mishneh Torah, Melachim and Wars 6:8–10 draws out the practical consequence shared by all these interpretations: the prohibition on wanton destruction of fruit trees is not limited to siege conditions but applies everywhere, and anyone who cuts one down with destructive intent is liable to lashes.