When middle prut valley is the land formation what is the highest of which currently forests, km² ?
CREATE TABLE "natural_habitat" (
  "land_formation" text,
  "area_km" real,
  "of_which_currently_forests_km" real,
  "pct_forests" text,
  "habitat_type" text
);
SELECT MAX("of_which_currently_forests_km") FROM "natural_habitat" WHERE "land_formation"='Middle Prut Valley';