Spring.wmf (18300 bytes) Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

Biology of Maple Sap Flow

There is in some parts of New England a kind of tree...whose juice that weeps out of its incision, if it be permitted slowly to exhale away the excess moisture, doth congeal into a sweet and saccharine substance. 

Robert Boyle
Philosophical Works (1663)

Objectives: Upon completion of this laboratory you should be able to:

  1. describe methods used to tap sugar maple trees, collect sap and prepare maple syrup from the sap.
  2. explain the physiology of maple sap flow and describe the chemical composition of maple sap
  3. use and describe the principle of operation of a sugar refractometer

PreLab:   Read the following materials: 

An event associated with spring is the production of maple syrup from the sap of the sugar maple tree (Acer saccharum Marsh.).  This process, which was originally discovered by Native Americans, is one of the relatively few uniquely North American crops and it is one of the oldest American crops (Willits, 1958).  The monks of St. Johnís have made syrup since 1942 when, perhaps spurred by sugar shortages during the war, the monks tapped 150 trees, collected 1440 gallons of sap, and boiled it down in the candle shop to make their first 45 gallons of pure maple syrup.  This springtime tradition has continued until the present.  During todayís lab we will visit the St. Johnís maple sugar operation and learn more about this monastic ritual. 

Sap Chemistry
When a sugar maple tree is tapped, sap flows out of the hole.  A single tap will produce about 10 gallons of sap per per and yield approximately one quart of finished syrup.   The sap contains sucrose and trace amount of oligosaccharides including raffinose (Willits, 1958).  The concentration of sucrose in the sap is typically 2-3%, though it can range from 0.5 - 10% (Kozlowski & Pallardy, 1997).  Environmental conditions can affect the yield of sugar.  Trees grown with adequate moisture and fertilizer produce higher yields than trees in infertile soil and dry conditions. Sap yield is also lower if leaves are defoliated in the previous season. Trees with an exposed crown produce greater amounts of sap than trees grown under crowded conditions - presumably because of the advantage due to photosynthesis (Kozlowski & Pallardy, 1997).

    Other organic compounds in the sap include organic acids, amino acids, amides, ammonia, and peptides. The organic acids in the sap include malic (0.21%), citric (0.002%), and traces of succinic, fumaric and several others. The total ash (mineral) content of the sap is 0.66 %. Common minerals include potassium (0.26%), calcium (0.07%), silicon oxide (0.02%) and lesser amounts of manganese, sodium, and magnesium (Willets, 1958).

To make maple syrup, the sap is concentrated by boiling to yield a solution which must legally contain 66.7% sugar or have a specific density of 66.5 degrees Brix or 36 degrees Baum.  In practice a syrup-maker uses a hydrometer to measure the density of the syrup. An alternate method is to monitor temperature - the syrup is finished when it boils at 7 F above the boiling point of water.

    It takes approximately 40 gallons of sap to produce one gallon of syrup.  Thus, there is a 40 to one ratio of sap-to-syrup.  The lower this ratio the better because it means that less boiling is required to produce a gallon of syrup.  At St. Johnís our average sap/syrup ratio is almost exactly 40.  However, early in the season our sap/syrup ratio is usually slightly higher (50 or 60 to one) but it declines toward the end of the season.  The reason this occurs is because the concentration of sugar in the sap varies with the season.

    To estimate the number of gallons of sap needed to produce a gallon of syrup (sap-to-syrup ratio), a syrup-maker can use the Rule of 86 (Willits, 1958) which is mathematically expressed as follows:

Equation 1:  Gallons of sap per gallon of syrup = 86/[sap sugar concentration]

This equation is derived from the fact that finished syrup has 86.3% solids.  Thus, to calculate how much sap is required to produce one gallon of syrup, divide 86 by the sugar concentration of the sap.  For example, the sugar concentration in trees at St. John's is typically 2.0%. Thus, according to the Rule of 86, it would take approximately 43 gallons of sap (86/2 = 43) to make one gallon of syrup. Or, in other words, you must boil off 42 gallons of water to leave one gallon of maple syrup (Willits, 1958).

    We can rearrange Equation 1 to use it to estimate the concentration of sugar in sap as follows:

          Equation 2:   [Sap sugar concentration] =  86/(gal sap/gal syrup)

    For example, if we found that we cooked down exactly 40 gallons of sap to produce one gallon of syrup we would know that the sugar concentration in the sap was approximately 2.15% ([sap sugar] = 86/40).

    Maple sap has little maple flavor.  The distinctive flavor of the syrup is caused by the heating which changes certain nitrogenous chemicals in the sap (Kramer, 1983). Part of the flavor of maple syrup is due to vanillin and furanones; the darker the syrup the more the furanones and the stronger the taste.

    During the boiling process, minerals and other insoluble materials form a sediment, called the sugar "sand," which must be filtered and removed from the final syrup. The main constituent of the sugar sand is calcium malate (Willits, 1958).

Sap Flow
Sap flow requires cool nights (below freezing) followed by warm days. In central Minnesota, sap typically flows best from mid-March to mid-April although it can flow anytime the trees are dormant from October to late April (Kramer and Kozlowski, 1960). Sap flow stops when the buds expand and the leaves develop (Marvin, 1958). Flow will also stop if the temperature is continuously above or below freezing or if the night temperatures are no longer below freezing (Kramer and Kozlowski, 1960). At night there is little sap flow. As the day warms, sap flow begins. By noon, approximately 60% of the flow has occurred and the flow begins to decline (Kramer, 1983). The temperature of the previous night appears to be one of the most important factors for flow (Marvin, 1958).  

Physiological Explanation for Sap Flow
First, let's address a common misconception about sap flow. Since grade school we've learned that the xylem 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 lead to the erroneous idea that sucrose-rich maple sap is being removed from the phloem - which is wrong. Maple sap that drips out of a spile in the 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.

    The cause of maple sap flow is complex and our understanding of the process is relatively recent. Sap flow is not related to the normal process (Cohesion-Tension Theory) by which water is transported in stems during the growing season (Kozlowski & Pallardy, 1997). According to the cohesion-tension model, water is essentially "pulled" up through a plant as it evaporates (transpiration) from leaf surfaces. Clearly this can't be important to 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 as is measured in maple stems during sap flow.

    Sap flow is not related to root pressure. Plants can generate sizable root pressures that can play a role in water movement. In some species, like birch (Betula sp.) and grape (Vitis sp.), the sap that flows from cuts or wounds in the stem in the spring is a consequence of root pressure. The root pressure increases the stem pressure which results in sap flow. However, root pressure is not responsible for maple sap flow (Marvin, 1958; Kramer, 1983; Kozlowski & Pallardy, 1997). Root pressure is absent in maple trees, even when there is stem pressure (Kozlowski & Pallardy, 1997).

    So, if root pressure and normal water transport mechanisms are not involved, what causes sap flow? The crucial factor is apparently related to the age-old observation that sap flow requires warm days and cool nights. Stems must experience a freeze-thaw cycle for sap flow. When pieces of maple stems are given a source of water and then placed in a freeze-thaw cycle, they exhibit sap flow. During the cold period the stem pressure decreases and the stem absorbs water (Kozlowski & Pallardy, 1997).

    As the temperature cools, gases in the xylem dissolve and the pressure decreases. This draws water from adjacent cells which, in turn, are refilled by water absorbed from adjacent cells and ultimately from the root. As the temperature continues to drop, water freezes along the inside walls of hollow xylem cells 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. As the temperature warms, the ice melts and the ice-compressed gases expand forcing the sap out of the stem (Tyree, 2001).

    This hypothesis explains why freezing and thawing temperatures are required and why sap flow is always followed by re-absorption of water (Marvin, 1958). However, it doesn't explain why sap flow requires: (1) sucrose in the sap, and (2) living cells. It is possible that both are necessary for cellular respiration that yields carbon dioxide. This gas may be the main component of the gases that undergo thermal expansion and contraction during the freeze-thaw cycle (Marvin, 1958; Kramer, 1983).

    The sugars in the sap are derived from carbohydrates that accumulated in the stem during the previous season (Kramer and Kozlowski, 1960). These are converted to starch when the weather becomes cool in the autumn. The starch in living ray cells is hydrolyzed to sucrose as the temperature warms in the spring. The sugary sap is then pushed into the xylem (Milburn, 1979).

Why maple?
Spring sap production is a relatively rare phenomenon, and occurs in the maples (genus Acer) and just a few others. So, what it is about maple? According to Dr. Mel Tyree (2001) the distribution of sap and gas in maple stems is the critical factor. Species like sugar maple and butternut (Juglans cinerea) that have air-filled fiber cells and water-filled vessels will exude sap. 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.

Syrup/Sap From Other Species
As mentioned above, when grapes or birches are pruned in the late spring they will exude sap. This process is not temperature dependent as is the production of sap from maple trees and is due to root pressure. Because of the amount of bleeding that can occur you should avoid pruning grape vines in the late spring. Syrup can be made from birches, and is a commercially important product in some areas.

    Hickory syrup is a sugary syrup flavored by an extract of the bark of Shagbark hickory (Carya ovata). The bark is gathered, extracted, strained and aged.

Climate Change
Evidence strongly suggests that global climate is changing. Climate models suggest that winters in the Great Lakes region will likely be warmer and wetter. One advantage of this warming trend is that it could extend the region of maple syrup production further west in the state. Currently Minnesota is on the western edge of the range for sugar maple trees.

Lab Activity
    During today's lab we will visit and take a tour of the St. John's Maple Syrup operation.  While there, you will learn how to identify maple trees in winter condition, tap trees, measure sugar concentration in maple sap, and learn how to prepare maple syrup.  If time, as a group we will try to answer one selected research question.  Possible questions include:
  • How does sap flow relate to temperature?
  • What sorts of microbial contaminants grow in the sap?
  • What sorts of insects are attracted to the sap?
  • Do different trees vary in sap flow?
  • Do trees vary in the duration of sap flow?
  • Do trees vary in the temperatures at which flow occurs?
  • Does the sap contain cells?
  • Which species produces sap with the highest sugar concentration? 
  • Does sap flow rate vary with species?
  • Is the sugar concentration of the sap higher in large or small trees?
  • Is the sugar concentration greater in the trunk or twigs?
  • Does tree height affect sugar concentration or sap flow?
  • Is the sugar concentration in the sap or rate of sap flow uniform along the height of the tree?

Post Lab

  1. Print and complete the online quiz.
  2. Complete the maple exercise
  3. Prepare an abstract for the mini-experiment that we conduct in the field.

Literature Cited and References

  • Barr, Jesse.  Maple Syrup Production at St. John's University.  History and Sustainability   (Environmental Studies Practicum; pdf file)
  • Cleveland, Mark (1987) Maple syruping in your back yard. Minnesota Volunteer. March-April, pp. 9 - 14.
  • Coons, CF (1975) Sugar Bush Management for Maple Syrup Producers.  Ministry of Natural Resources, Ontario.
  • Davis, MB, K Walker, S Sugita (2001) Climate change and sugar maple.  Minn Maple News, December, pp 6-7.
  • Harborne, J. B. (1973) Phytochemical Methods. Chapman and Hall, NY.
  • Holan, F. (1986) Sugaring Made Simple. Country Journal 13: 38- 45.
  • Houston DR, Allen DC, Lachance D (1990) Sugarbush Management:  A guide to maintaining tree health.  Northeast Forest Experiment Station, General Technical Report NE - 129.
  • Kozlowski, TT and Pallardy, SG. (1997) Physiology of Woody Plants. Academic Press, NY.
  • Walter Kieffer, OSB - How We Make Maple Syrup At St. John's
  • Kramer, P. and T. Kozlowski (1960) Physiology of Trees. McGraw-Hill, NY.
  • Kramer, P. (1983) Water Relations of Plants. Academic Press, NY.
  • Martin, J.W. (1958) The physiology of maple sap flow. In The Physiology of Forest Trees. K.V. Thimann, ed. Ronald Press, NY.
  • Milburn, J.A. (1979) Water Flow in Plants. Longman Group Ltd., London.
  • Nearing, H. and S. Nearing (1970) The Maple Sugar Book. Schocken Books, NY.
  • Richardson, M. (1968) Translocation in Plants. Edward Arnold, London.
  • Ross, C.  (1974) Plant Physiology Laboratory Manual. Wadsworth Publishing, California.
  • Saupe, S.G. (2005)  "The Maple Syrup Crystal Ball"  Sagatagan Seasons 8: 4 (Spring)
  • Saupe, S.G. (2005)  Photograph (maple syrup collecting) published; Abbey Banner Vol 5 (2): 19.
  • Saupe, S.G. (2006) Bibliography of articles about the St. John's Maple Syrup Operation.  On-line at: http://www.employees.csbsju.edu/ssaupe/essays/maple_bibliography.htm
  • Saupe, S.G. (2006) Making maple syrup at St. John's:  Records show shifts in the 'Sticky Business.'"   Headwaters. A CSB/SJU Faculty Journal 23: 25 - 38. (pdf)
  • Saupe, S.G.  (2006)  Does sap flow better during Holy Week?  Maple Syrup Digest, 18A (3): 29 - 33.  (pdf)
  • Saupe, S.G. (2006) Maple syrup:  St. John's sweetest springtime tradition.  submitted to Understanding Home. (pdf)
  • Tyree, M (2001) Chapter 3. Water Flow in Plants.  Unpublished manuscript.
  • Vanilla, beef-broth compound flavor syrup.  Science News 143: 158.
  • Vogt, C.  Homemade Maple Syrup.  University of Minnesota Extension Service.
  • Vogt, C.  Identifying Maple Trees for Syrup Production.  Univ. of Minnesota Extension Service
  • Willits, CO (1958) Maple-Sirup Producers Manual.  USDA.  Agriculture Handbook 134.

Web Sites

A sap-run is the sweet goodbye of winter.  It is the fruit of the equal marriage of sun and frost

John Burroughs
Signs & Season (1886)


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Last updated:  01/07/2009     © Copyright  by SG Saupe