Birch sap or Birch water is plant juice tapped from Birch trees in Spring. You just drill a hole into the tree and start tapping. The same method seems to work with other trees. Very traditionally maple trees in canada where the sap is concentrated to produce maple syrup.
Birch water tapping was and is an old and ancient northern Europe thing which took place in Russia, Latvia, Estonia, Finland, Lithuania, Poland, Balearus and parts of northern China as well as other countries of the northern hemisphere. The Sap of the Birch tree can also be fermented like Pulque or Palmwine but it never reaches high alcohol content because the sugar content is very low, around 10g/l which only means 0,5%Vol. at it’s best.
So the water is often boiled to increase the brix. But the main sugar in birch sap is fructose which is more temperature sensitive than glucose which is the main sugar in maple syrup. Birch syrup is more expensive and harder to produce than maple syrup because of the usage of reverse osmosis to concentrate it today properly. Birch Water is often mixed into drinks and used in the kitchen.
Here is an very refreshing example of Birch Wine Making from another blog. Enjoy!