As it grows, the maple sugar transforms starch into sugar. The sugar combines with water absorbed by the maple’s roots, slightly sweetening the maple water. In the spring, when temperatures begin to rise again, the water found in the trunk and roots expands, building up pressure inside the tree. The alternation of cold nights, when the temperature is sub-zero (frost) at night, and when the temperature rises above zero during the day, encourage the flow of maple water, which is then transported by tubes to the sugar house. There, an evaporator turns the maple water into maple syrup. It takes on average 40 litres of maple water to produce one litre of syrup.
Source : Federation of Quebec Maple Syrup Producers