As we come to late winter and early spring, a tradition begins in many Appalachian communities, that of collecting maple sap to boil down into maple syrup. As we can see from the psalmist, the Lord created the trees that are full of sap. “The trees of the Lord are full of sap; the cedars of Lebanon, which he hath planted” (Psalms 104:16). Explaining this process has not been as easy as scientists might have liked, so let’s take a look at the mystery of the rising sap of the maple tree.
Plants were created with a pipe-like vascular tissue that provides them with the crucial tubes through which water and nutrients flow. The vascular system is composed of two types of specialized tissue called xylem (which transports water and dissolved minerals) and phloem (which conducts food from the leaves to all other parts of the plant). The xylem is located more closely to the center of the plant, and wood is the best known of the xylem tissues. It is mostly composed of dead cells with a tube-like structure. The phloem, however, consists mostly of living cells. Located closer to the outside of the plant and just under the bark in trees, the phloem moves organic nutrients (known as photosynthate). The dependence of the plant upon phloem is why trees can be killed simply be removing a circular layer of bark around the trunk. There are many other types of supporting cells that work together to allow these tubes to function and allow movement of water and food in trees more than 350 feet high like the Giant Sequoia.
The processes that move fluids around the plants are complex. Water is moved from the roots to the rest of the plant due to two factors: Root pressure (in which osmosis moves water from the soil into the roots) and transpirational pull (where water is lost by transpiration in the leaves and the resulting surface tension pulls water up the xylem). Sugars are concentrated during the summer and makes some water flow through osmosis, and the fancy name for the movement in the phloem is called the Pressure-flow Hypothesis. Yet, the flow of maple sap this time of year is not by any of these processes and is a totally unique physiological event that God has created.
Maple syrup makers have long known that the key to sap flow is cold nights followed by warm days. Only when the day and night temperatures fluctuate above and below freezing will the trees release their sweet elixir. At night, when it is normally cold, there is little sap flow. In the daytime, once the temperature warms above freezing, sap flow begins. The amount of sap flow is related to how cold it got during the night and is most directly connected with the temperature of small branches in the canopy of the tree.
It has been discovered that as night falls and the temperature drops, air bubbles in the sap contract and dissolve, decreasing the pressure within the cells. This initiates a suction pressure that pulls water from nearby cells. In turn, these cells are refilled by water absorbed from other cells and ultimately from the roots. As the temperature continues to drop, water freezes inside in the xylem and in the intercellular spaces. The next day, when the temperature warms, the ice bubbles melt and the compressed gases expand, producing the pressure that pushes the sap up the stem to produce maple sap flow. So, the cycle of freezing and thawing from night to day is what causes the sap to move up the stem where the taps intercept the flow and collect it for boiling into maple syrup.
The collection of sap is not harmful for the trees, and the amount of sap produced depends on many factors, such as the health of the tree, the amount of sugar produced last summer and variation of the day and night temperatures. Not all trees have been created with the ability to produce sap that contains enough sugar to boil down to obtain a sweet tasting treat. Besides maple trees, other trees, producing smaller amounts of sap, are butternut, walnut, sycamore, birch, boxelder and ironwood — all of which can be tapped. God has provided for all of the needs of mankind, including the “…trees of the Lord…” being “…full of sap…” Let us always praise the Lord for the blessings He has provided.
