"For from it you shall eat": a positive commandment;
"but it you shall not cut down": a negative commandment.
Variantly: "for from it shall you eat": a tree providing life for a man. R. Yishmael reasoned hence: If the L-rd is so solicitous of the fruits of a tree, how much more so of the tree itself! And if Scripture exhorts against (cutting down) a tree, which only produces fruit, how much more so (does it exhort against destroying) fruit itself!
(Devarim 20:20) "Only a tree that you know": This is a fruitful tree. (Ibid. 19) "to come before you into the siege": Cut it down.