Ramban ¹ : It is giving precedence to cutting down a non-fruit bearing tree to a fruit-bearing one.

Seforno: To teach us that, even a species of fruit-tree, if one knows that, due to old age or damage, it is no longer capable of bearing fruit, one is permitted to cut it down when laying siege.

Rashbam: Refer to 20:20:1.1:1.

Bava Kama, 91b: "Eitz asher Teida" is referring to a fruit-tree, and "Ki Lo Eitz Ma'achal Hu", to a non-fruit-bearing tree - to teach us that one is permitted to cut down either of them if necessary.

Rashbam: In order to permit cutting down species of non fruit-bearing trees even if they grow far from the town that is being besieged ¹ - but not fruit-trees.

Bava Kama, 91b: In order to give precedence to cutting down a non-fruit bearing tree over a fruit-tree wherever possible.

Bava Kama, 71b: It precludes there where the non fruit-bearing tree is more valuable than the fruit-tree, in which case it is permitted to cut down the fruit-tree first. ¹

Rashi: It means until its inhabitants surrender.

Rashbam: It means 'until its walls are brought down'. ¹

Targum Onkelos and Targum Yonasan: It means 'until it is vanquished'.

Moshav Zekenim: It comes to permit [fighting the city] even on Shabbos. ²