Scientists have been trying to explain sap flow for decades. Maples, along with a few other species like walnuts and butternut, develop positive pressure in their stems rather in the root system. Several freeze-thaw cycles initiate positive stem pressure. The magnitude of pressure is correlated to sucrose (sugar) concentrations in the sap. Higher concentrations of sugar mean higher positive stem pressure.
Two theories were posed to explain how high positive pressure developed in maple stems when temperatures fluctuate around freezing. The Milburn-O’Malley theory, named after the scientists who first described the idea, suggested that pressure development was simply physics. While on the other hand, the osmotic theory suggested that living cells and sucrose generated osmotic pressure differences between fibers and vessels in the sapwood.
Way back in 1860 it was thought that expanding gasses caused sap flow. The Milburn-O’Malley theory further suggested that gas-filled fibers contract when cooled and pull water from liquid-filled vessels through the fiber walls. The water would freeze and compress gases. As it thawed the next day, the gas would expand and push the water out of the fibers and back into the vessels. More recent studies have shown that there is a flaw in this theory. Gas bubbles under pressure can actually dissolve which reduces the pressure to the same as the atmosphere within just a few hours, yet stem pressure continues for days.
The osmotic theory explained it via osmotic processes that caused long-term pressurization of maple sap. Sugar concentration in the vessels becomes higher and higher as the sap run progresses suggesting that living cells are involved in the process rather than simple physics.
Recently, scientists from Maine and Vermont found anatomical evidence to support the osmotic theory. They then used a fluorescent dye that was molecularly similar to sucrose to trace the pathway of the sugar and found they were indeed excluded from the fibers. Without the sugar, compressed air bubbles in the sap lost pressure in just a few hours.
In a nutshell, at night as the sap cools it creates suction in the tree and water is pulled up from the roots. During the day the sap heats and positive pressure is created in the stem. When the stem has a wound, such as a tap, the pressure forces the sap out of the wound and in this case, into our metal bucket.