Ashaya, Soul of the Wild

A land that's a Forest has the intrinsic mana ability "{T}: Add {G}."

As long as Ashaya is on the battlefield, it's affected by its second ability and thus its first ability counts itself.

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to Ashaya may become lethal if lands you control leave the battlefield during that turn.

The ability that defines Ashaya's power and toughness applies in all zones, not just the battlefield.

You can't play creature cards as lands; you'll still have to cast them as spells, and they'll enter the battlefield as lands (in addition to their other types).