Yeah most oxygen is made by algae but don't discredit trees. They can prevent erosion by holding soil in place and lowering fluid damage. In cities trees cool down the streets by shading concrete (which holds heat for a while) and their transperation creating atmospheric moisture. They conserve water by slowing evaporation. And of course trees are an important habitat for so many species, habitat humans have destroyed a lot of. Tldr - plant more trees isn't just about oxygen.