Because trees are living they breathe, but unlike humans and animals which take in oxygen and give off carbon dioxide, trees take in carbon dioxide and give off oxygen.
(trees; are living; ) (trees; give off; oxygen) (they; breathe; ) (humans and animals; take; L:in oxygen) (humans and animals; give off; carbon dioxide) (trees; take in; carbon dioxide)
trees trees they humans and animals humans and animals trees
are living give off breathe take give off take in
oxygen
L:in oxygen
carbon dioxide
carbon dioxide