A tree does take in carbon dioxide and expel oxygen we need to breath, but the tree uses the carbon dioxide to maintain its life (like we need oxygen to maintain human life), not store it.

Carbon dioxide is being converted into oxygen on Mars thanks to an invention ... of oxygen per hour–about the rate of an average tree on Earth. The findings were published in the peer-reviewed ...

But if it fails and grabs oxygen instead, the tree wastes energy without producing any sugar. This process is called photorespiration and is different from photosynthesis." An increase in ...