Trees exhibit a variety of fascinating behaviors and phenomena in the winter. See the Arb Note from two weeks ago to learn more about exploding trees. We are nearing the late winter and early spring, when freeze-thaw cycles drive sap flow, and sap for syrup production is extracted.
As the temperatures begin to increase toward the end of winter, days are frequently above freezing, and nights are below freezing. Cold nights cause liquid sap to freeze in xylem vessels (water-conducting tubes in a tree). However, as ice forms, the sugars are not incorporated into the crystal structure and become concentrated in the remaining liquid. This reduces the freezing temperature of the liquid – at temperatures just below freezing, not all of the sap freezes. The sap with a higher concentration of sugar has a lower water potential than the surrounding fiber cells. Water moves from high potentials to low potentials, causing the pressure of surrounding fibers to drop as they dehydrate. Gas bubbles in the fibers contract in the cold, further reducing the internal pressure.
Warm days cause the gas bubbles to expand and ice to melt. The expansion increases pressure in the fibers, driving water into the xylem vessels and causing positive stem pressure. This pressure can be enough to push sap several meters vertically within the trunk, and is utilized for extraction. One simply drills a hole into a tree at a slight upward angle and inserts a spile, which is a small spout or tap. If there is enough pressure within the tree, sap will be forced out of the spile and begin to flow. Once sap is collected from a maple (Acer) tree, it must be boiled to concentrate the sugars. See Plant Physiology, Vol 73, #2 for more information on freeze-thaw cycles and sap dynamics.
As we become hopeful about nearing the spring, it is worth considering these remarkable pressure gradients inside trees that allow us to enjoy a sweet, syrupy breakfast.
- Dexter Pakula ‘26, for the Cole Student Naturalists.
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