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During photosynthesis, energy from sunlight combines with water (from the soil, through the trees roots) and carbon dioxide (from the air, through stomata in the leaves). These chemical reactions produce oxygen and a type of sugar called glucose. Most of the oxygen is released back into the atmosphere, but the tree holds onto the glucose, which is a rich source of energy for the tree. Some of the glucose is used within the growing season, but much is sent to the roots and converted to starch for storage through the dormancy period of late fall and winter. Which brings us back to that snowshoe trip on a sunny late-winter afternoon.
In late winter and early spring, as temperatures start to fluctuate from above-freezing during the day to below-freezing at night and back again, pressure builds up in the tree. At the same time, encouraged by rising temperatures, enzymes and microbes within the tree get busy converting the stored starch back into sweet glucose, which is needed at the top of the tree for the springtime process of bud formation. This creates a perfect storm that humans have been taking advantage of since time immemorial: the sap run.
Under this pressurized system, the glucose, which is dissolved in water along with minerals, is pushed with force through the tree’s vascular system and easily pours out from any natural or human-made holes in the bark of the tree. This sap, which is thin and slightly sweet, is boiled down into our familiar maple syrup—which is a whole story in itself.
Though all deciduous trees produce sap, we use maple trees most commonly for syrup production because of their higher sugar content and because maple trees tend to produce more sap, as explained in this resource from the Wisconsin Maple Syrup Producers Association.
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