Sap runs occur in March and April. Through photosynthesis, the tree fixes carbon dioxide, which is immediately assimilated and used for the synthesis of starch (a storage metabolite), for respiration, or for the synthesis of sucrose and other carbon compounds. The starch accumulated over the summer is stored in the tree’s roots. In spring, when the required enzymes become active again, the starch is converted into sucrose. Maple sap contains 2% to 3% sugar on average (a small amount of glucose is present in addition to the sucrose). This “sweet water” contains all of the ingredients the tree needs to resume vegetative growth in the spring: inorganic salts, peptides, amino acids, amylase, and certain growth regulators.
The sap run phenomenon is highly dependent on air temperature. Freezing conditions at night alternating with higher temperatures during the day are essential. There are two phases in the sap run: an absorption phase during cold nights (-5o C or below) and an exsudation phase that occurs when the temperature rises. It is believed that the sap is drawn up to the crown of the tree when the branches most exposed to the cold freeze.When the temperature rises above the freezing point (to approximately 5o C), the sap thaws and flows down towards the base of the tree under the effect of gravity.