Scientists are using the pine-forested slopes of a Mexican volcano as a test bed to see if trees could grow on a heated-up Mars, part of a vision of making the chilly and barren red planet habitable for humans one day. Planetary scientists at NASA and Mexican universities believe if they can warm Mars using heat-trapping gases, raise the air pressure and start photosynthesis, they could create an atmosphere that would support oxygen-breathing life forms. Getting trees growing would be a crucial step.