It is still quite unreal to imagine there will be a time all trees will get chopped down on this planet though. People is more aware than ever on the importance of trees and the ecosystem in general for us to continue to leave on this planet.
Needless to say, trees generate oxygen we need and the many of the crops we consume, people won't chop down apple trees, orange trees and so own, so there will be always at least fruit trees, for the sake of consumption.
Also, I am aware there are alternatives for human beings to generate oxygen, through chemical reactions, for example, however such a thing will never be a full replacement to the natural way we get our breathable air, since the beginning.

Even in space stations there are on going experiments to study whether it would be possible to sow trees in man-made structures in space. It tells a lot on the importance we give to them, don't you think?