I guess it would take a lot of time to accommodate Mars for trees. More than for algae ;)

Back when I was a kid, trees still lived under water.

The problem with trees is they are used as lumber. The national parks has always been protected. But Trump has unprotected parts of the national parks to be cut down for lumber.

even your trees will be slop. nice.

Can't they just put the algae in the ocean?

What happens when one of these breaks and drains into the sewer system? Algae blooms cause noxious odors and would proliferate quickly in the nitrogen-rich environment of human waste water, potentially building up as clogs in the sewer lines. And if the system drains into a natural body of water, the algae can have devastating toxic effects on the natural wildlife. If it doesn't drain and instead gets recycled, then the water treatment process becomes much more difficult and expensive.

Alternative in what sense?

Keep in mind that tree roots can brake through anything