How large is the Boreal Shield in km2?
CREATE TABLE "table1_15555661_2" (
  "ecozone" text,
  "area_km" real,
  "percentage_of_total_area" text,
  "percentage_of_land_area" text,
  "percentage_protected" text
);
SELECT COUNT("area_km") FROM "table1_15555661_2" WHERE "ecozone"='Boreal Shield';