Well, 'Red Planet' was an entertaining movie, but a number of factoids left out to overcome.
The hardest would be the creation of a suitable magnetosphere to keep solar radiation from turning any vegetation to a crisp. (May be why none there) Protective biomes could solve that.
A viable notion why Martian atmosphere is so thin is solar flares and wind blasting it into space.
Plant trees, make an atmosphere, only to loose it in the solar winds. Trees soon become ashes.
Lets not forget the super tornadoes also, even on earth those uproot trees and toss them many miles.
And what of the soil,,will plant even root there??
Red planet suggested algae and chloroplast. That would singe in daylight.