Stephen G. Saupe, Ph.D., College of St. Benedict/St. John’s University
Biology Department, Collegeville, MN 56321, (320)363–2782; firstname.lastname@example.org
An annual springtime event from Maine to Minnesota is the production of maple syrup. Each spring, syrup-makers head to the woods to collect sap from the sugar maple tree (Acer saccharum Marsh.) and then cook it into one of nature’s greatest gifts. Although this ritual has been practiced for many years since its discovery by Native Americans, surprisingly the actual mechanism responsible for sap flow is still something of a botanical mystery. So, exactly what do scientists know about why sap drips out of a sugar maple tree in the spring? To answer this question we must first understand the conditions that affect the flow of maple sap, since our final explanation must account for these observations.
Photo 1: A young visitor to the St. John's Abbey & University maple syrup operation enjoying sap straight from the tap.
Conditions for Sap Flow
Maple syrup-makers have long known that the key to sap flow is cold nights (below freezing) followed by warm days (above freezing). Only when the day and night temperatures fluctuate above and below freezing, will the trees release their sweet elixir. Maple syrup season in central Minnesota typically runs from mid-March to mid-April when the weather provides the optimal temperatures for sap flow. Although the best sap flows occur in spring, maple sap is reported to flow anytime the trees are dormant from October to late April. Sap flow stops when the buds expand and the leaves develop. Sap flow also ends if the temperature is continuously above or continuously below freezing. As a recent grade-school visitor to our maple syrup operation wisely noted, there is no sap flow when the weather is cold-cold or warm-warm, but only when it is warm-cold.
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 sap flow is initially rapid and then declines during the day. The amount of sap flow is related to how cold it got during the previous night and is most directly correlated with the temperature of small branches in the canopy of the tree.
Photo 2: Brother Walter Kieffer using a modified chainsaw to tap trees in the St. John's Abbey & University sugarbush.
Maple sap comes from the xylem
Before discussing the hypothesized mechanism for sap flow, let’s address a common misconception. Since grade school we've learned that the xylem, or wood, transports water from the roots to the aerial parts of the plant, while the phloem transports sugars and other organic materials. Though true, this has resulted in the notion that sucrose-rich maple sap is extracted from the phloem – which is wrong. The sap that drips out of a tap, or spile, in a sugar maple tree comes from the xylem. In fact, this is the only time during the year when the fluid in the xylem is rich in sucrose, and is an exception to the wisdom we garnered in grade school.
Maple sap is under pressure
The fact that maple sap drips out of a tree through a spile or wound in the bark tells us that the pressure inside the tree must be greater than the pressure outside. When a pressure-measuring device such as a manometer is attached to a spile or the cut end of a branch, sugar maple stems can generate a pressure of about 100 kilopascals, or roughly 15 pounds per square inch, when the sap is flowing. Further, careful studies have shown that the flow rate of sap from a sugar maple is proportional to its internal pressure.
Photo 3: Late spring scene of tapped trees with bags in the St. John's Abbey & University sugarbush.
Maple sap flow occurs by a different mechanism than water transport
The flow of maple sap is not related to the normal process by which water is transported in the xylem during the growing season. According to the transpiration-pull model for water transport, a column of water is essentially "pulled" up through the xylem as individual water molecules evaporate (transpiration) from leaf surfaces. This works because of the cohesive property of water molecules that causes them to “stick together.” Thus, transpiration literally sucks water out of the stem creating a negative pressure, which is also called a tension, and which gives this hypothesis its alternate name of “cohesion-tension model.” Clearly this can’t be responsible for maple sap flow since: (1) maple trees lack leaves during the time period when sap flows; and (2) the xylem in trees that are transpiring and transporting water is under a negative pressure (or tension), not a positive pressure, like in maple stems during sap flow.
Root pressure does not cause sap flow
Gardeners know that grape vines pruned in the spring will bleed. Sap exudes from the cut stems due to root pressure. Root pressure occurs when water osmotically enters the core of a root and builds up pressures that can reach 40 pounds per square inch and higher. However, these pressures are not responsible for maple sap flow. A simple experiment shows why. When branches are cut from a sugar maple tree they will produce sap from the cut end. Since the branch is no longer connected to the roots, the roots can not be involved. Further, the stump of a dormant sugar maple tree cut off at the base produce little sap. Hence, root pressure is usually absent in maple trees when sap flow occurs. Further, root pressure is not temperature-dependent and doesn’t rely on alternating cold/warm periods as does sap flow. Although sap flow in maple is not caused by root pressure, it is responsible for birch sap flow. When a birch tree is tapped in the springtime it produces sap by root pressure. The sap can also be made into syrup and is a commercially-important product in some areas.
Photo 4: Sap sack ready to be collected.
Maple sap flow is caused by temperature-induced changes in stem pressure
So, if root pressure and normal water transport mechanisms are not involved in maple sap flow, what is the cause? The crucial factor is apparently related to the age-old observation that sap flow requires warm days and cold nights. Clever experiments have shown why the freeze-thaw cycle is necessary. When a cut maple branch is given a source of water and allowed to freeze, the stem absorbs water. This occurs because the internal stem pressure decreases. When the branch is warmed up, the stem exudes water as the pressure increases.
Now, let’s summarize our various observations and explain how sap flow occurs. At night, as the temperature gets colder, air bubbles in the sap contract and dissolve decreasing the pressure. This initiates a suction (tension) that draws water from adjacent cells. In turn, these cells are refilled by water absorbed from other cells and ultimately from the root. As the temperature continues to drop, water freezes inside hollow fiber cells in the xylem and in the intercellular spaces. Additional ice forms as water vaporizes from surrounding cells, much like the formation of frost on a misty winter morning. When ice formation is complete, the remaining gases in the stem are compressed and locked in ice bubbles. The next day, when the temperature warms, the ice bubbles melt and the compressed gases expand producing the pressure that pushes the sap out of the stem. Sap flow eventually slows by late afternoon when the stem pressure declines due to the evaporation of water from branches, internal leaks, and perhaps other causes.
Photo 5: The smoke and steam from the St. John's Abbey & University sugar shack.
This hypothesis explains why freezing and thawing temperatures are required and why sap flow is always followed by re-absorption of water. However, questions about the process remain. For example, experiments have shown that maple sap will only flow if the sap contains sucrose. Since the current model for sap flow is based primarily on temperature-induced pressure changes in the stem, it’s not understood why sucrose should be required for sap flow. Our current model suggests that the composition of the sap, whether it contains sucrose, glucose or other solute, shouldn’t matter. But it does. Continuing experiments will hopefully provide the answer to this mystery.
Photo 6: Autumn in the St. John's Abbey & University sugarbush.
Spring sap production is a relatively rare phenomenon. It occurs in the maples, genus Acer, and just a few other species. So, what it is about maple? The critical factor appears to be related to the distribution of liquid and gas in the xylem. Xylem is comprised of several types of cells including fibers and vessel elements. Vessel elements are the main water-transport cells while fibers are longer and thinner and serve primarily to support and strengthen the wood. In species that produce sap, like sugar maple and butternut (Juglans cinerea), the fiber cells are air-filled and the vessels are water-filled. In contrast, species that do not exude sap, such as willow (Salix), aspen (Populus), elm (Ulmus), ash (Fraxinus) and oak (Quercus), have gas-filled vessels and water-filled fibers.
The mysterious process of maple sap flow likely evolved as a way that maple could provide water to the leafless canopy during a freeze-thaw cycle. Fortunately for us, there’s no mystery about how to take advantage of the resultant stem pressures to make a sweet, springtime delicacy.