When 312 is  of which currently forests, km² what is the land formation? 
CREATE TABLE "natural_habitat" (
  "land_formation" text,
  "area_km" real,
  "of_which_currently_forests_km" real,
  "pct_forests" text,
  "habitat_type" text
);
SELECT "land_formation" FROM "natural_habitat" WHERE "of_which_currently_forests_km"=312;