Final answer:
Maple sap's sweetness in spring is a result of the conversion of starch to sucrose in the xylem rays, independent of root activity. The sap flow is facilitated by freeze/thaw cycles and pressure changes in the xylem. The sap is collected and concentrated to produce the much sweeter maple syrup.
Step-by-step explanation:
Maple sap is sweet in spring due to a unique process within the maple trees. The sap's sweetness derives from the sucrose that is found in the xylem sap, originating from the starch stored in the xylem rays being converted into sucrose during late winter. This conversion happens independently from root activity and is possible even in stems removed from the root system. The flow of maple sap, which is collected by tapping into the xylem, is facilitated by the pressure changes in the xylem related to freeze/thaw cycles.
Unlike the phloem, which contains much higher sugar concentrations and is capable of rapidly plugging holes to prevent sugar loss, the xylem tissue allows for the bleeding of sap over a prolonged period. For maple sap flow to continue, it needs to be exposed to the proper freeze/thaw conditions without the pull from growth or evaporation from leaves. This typically occurs when leaves are absent, and no growth is occurring.
Maple sap's sweetness is only slightly perceptible until the sap is concentrated. Through a process of collection and reduction, the sap, with higher viscosity than water, is turned into the much sweeter and thicker maple syrup that is a well-known delicacy. The wood anatomy of maples and a few other species enables this pressurization and sap flow, which is not found in most other tree species.