During a long siege, what did Deuteronomy forbid Israel to cut down?

Answer: Fruit trees

Israel was not to destroy a besieged city's trees with an axe, "because you can eat their fruit" (Deuteronomy 20:19).

“When you lay siege to a city for an extended time while fighting against it to capture it, you must not destroy its trees by putting an axe to them, because you can eat their fruit. You must not cut them down. Are the trees of the field human, that you should besiege them?”— Deuteronomy 20:19 (BSB)