@Fil Albuquerque,
It would take a long time to make Mars habitable by trees from earth. The "soil" is intensely salty--unlike earth, there is not sufficient free oxygen to bond with metallic rocks. On Earth, cyanobacteria giving off O2 lead to the first great extinction event, about two an a half billion years ago.
It's not just the lack of dirt suitable for organic interaction, either. The atmospheric pressure runs about 10 to 12 millibars at mean surface level. To put that in perspective, the atmospheric pressure on the Earth at mean sea level runs about 1013 millibars. There is a down and dirty way to pump up atmospheric pressure, wich would be to park a carbonaceous chondrite in a polar orbit, and put a primitive robot on it so that it drops carbon dust on the south pole of Mars whenever it passes over. It could be refined by waiting for southern hemisphere summer on Mars to dump the carbon dust. That would melt the southern polar ice cap, which is about 99% dry ice, CO2 ice. Then you could plant all manner of tress on the surface, on patches of soil created by robtos. Most of the trees would die off, but eventually, they prosper. It would also be useful to spread halophyte plants on the surface.
Then, we'd only have to wait about 30,000 years (and a Martian year is bout 22 months, by our calendar) for the trees to scrub enough CO2 out of the atmosphere for terrestrial animals to be able to live there. Easy-peasy . . .