Trees have trouble in urban areas with a lot of traffic

Because they pick trees that have gender. They plant male trees, see dogwoods, the flower but not bare fruit. So you get allergies because of tree cum. And as far as I know plankton cum doesn't come with side effects.

Trees and algae have different utilities. Trees are beautiful, reduce temperature, offer shade, and produce a modest amount of oxygen. Algae tanks produce vastly more oxygen by volume and cause you to question whether it’s really enough to continue meandering through life in this stone and metal postmodern hellscape and maybe it’s okay to finally indulge in a vacation near open fields and untamed wilderness. The local camping spots might be available in a few weeks if my sanity can hold out. One doesn’t completely substitute the other.

Because Trees are for rich people only.

Poor people don't get natural shade. They just get to see "luxury" apartment towers with tenant only "public parks from a distance.
/s

They grow upside down

the shade is also important