Birch bark is one of the materials in the game The Long Dark. It can be found in Sandbox mode. It is used as Tinder to start a fire. Like all tinder, it provides no extra bonuses to either fire length or temperature, and is not needed once the player reaches level 3 in the Fire Starting Skill Level.
It can be found in plentiful supply in the Ravine. More can be found to the east of the Lonely Cabins in Pleasant Valley, and around in Timberwolf Mountain.