Plants & Human Affairs - Introduction
Cherries.wmf (7140 bytes) Plants & Human Affairs (BIOL106)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;


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)

An event associated with spring is the production of maple syrup from the sap of the sugar maple tree (Acer saccharum Marsh.). When a sugar maple tree is tapped, the sap flows out of the hole (Kramer, 1983). Most of the sap comes from the stem above the hole (Kramer and Kozlowski, 1960). A maple tree will produce approximately 4 liters of sap per day or 35-70 liters per year (Kramer, 1983). The sap contains sucrose and a variety of inorganic salts and organic compounds (including organic acids, amino acids, amides, ammonia and peptides).

    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 Baume (Swain, 1981). It takes approximately 40 gallons of sap to produce one gallon of syrup (Swain, 1981; Holan, 1986). The distinctive flavor of the sap is caused by the heating which changes certain nitrogenous chemicals in the sap (Kramer, 1983). 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.

    Sap flow requires cool nights (below freezing) followed by warm days. In Minnesota, sap typically flows best in mid-March 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 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).

    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 lowered if leaves are defoliated in the previous season. Trees with an exposed crown also produce greater amounts of sap than trees grown under crowded conditions - presumably because of the advantage due to photosynthesis (Kozlowski & Pallardy, 1997).

    The cause of maple sap flow is complex and not completely understood. 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 water evaporates (transpiration) from leaf surfaces. Clearly this can't be important to maple flow since: (1) maple trees lack leaves during sap flow; and (2) the xylem in trees that are transpiring and transporting water is under a negative pressure (suction), not a positive pressure as is measured in maple stems during sap flow.

    Plants can often 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 is a consequence of root pressure. The root pressure then 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 that is pushed out during the thaw cycle as the stem pressures increase (Kozlowski & Pallardy, 1997).

    As the temperature cools, gases in the xylem dissolve and the pressure decreases. This draws water from adjacent cells which  are refilled by water absorbed from adjacent cells and ultimately from the root. As the temperature continues to drop, water freezes along the inside wall 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 (Kozlowski & Pallardy, 1997).

    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 is then released enzymatically from ray cells in the xylem as the temperature increases in the spring. The sugary sap is pushed into the xylem (Milburn, 1979).


Tapping a Sugar Maple Tree

Background Information:
    Under the appropriate conditions, sap will readily flow from a wound in the xylem of sugar maple trees. To collect the sap, Native Americans used to simply chop into the trunk with an axe (Swain, 1981; Cleveland, 1987). This technique was ultimately replaced by the less destructive method of boring a hole into the xylem and inserting a spile to collect the sap. In this lab, we will use a brace and bit to drill a hole. However, large sugar operations use a drill bit mounted on a chainsaw. Spiles can be made from hollowed sumac stems (Swain, 1981) or from a variety of other materials such as copper tubing. A sack, can or other container is hung on the spile to collect sap. For sources of maple sugaring supplies, see Holan (1986). We will use the following procedure for tapping:


  1. Find a sugar maple tree. In the winter, this is not quite as easy as it may sound. Sugar maple trees have dark, sharp-tipped buds, that are arranged oppositely (two per node). Look for opposite branching and the characteristic bark. I will show you some sugar maple trees in the field. You may want to experiment with other species including birch (Betula sp.), silver maple (Acer saccharinum L.), or box elder (Acer negundo L.). Basswood (Tilia americana L.) looks like sugar maple but does not produce sap when tapped. Basswood can be recognized by its alternate leaf arrangement (one bud per node), bark and by the numerous sucker shoots that are typically found at the base of the tree.

  2. To determine if the sap is running, break off the tip of a small branch. If it "bleeds" the trees are ready to be tapped.

  3. Using a brace and 7/16th inch bit, bore a hole approximately 2.5 � 3.0 inches deep into the tree at a slight upward angle (about 10 degrees). The hole should be on the sunny side of the tree (south side) approximately chest high (1 meter). Avoid scars, burls and previous tap holes. Drill one tap per tree, although larger trees can have more taps (Swain, 1981; Holan, 1986). As a general rule, <31 inches circumference = no taps; 31-44 inches = 1; 45 � 60 = 2; and >60 = 3 taps.

  4. Gently tap the spile into the hole with a hammer. Hitting the spile too hard could damage the wood surrounding the spile and retard the healing process. Some commercial producers used to put a paraformaldehyde tablet in the drill hole to prolong flow by inhibiting the trees ability to seal the xylem vessels. The benefit of this practice is debatable (Swain, 1981) and is now illegal in Canada and the U.S. (Dr. T. Perkins, personal communication).  To reduce microbial contamination a dilute Clorox solution (10%) can be squirted in the hole followed by a rinse with sterile water (Swain, 1981).

  5. Hang the collection sack on the spile and record the date/time.

  6. Check the sacks periodically until sufficient sap (at least 50 ml) has been collected. Record the time and then return the collection bag and spile and return to the lab. Many commercial operations attach plastic hoses to the spiles under a vacuum to help withdraw sap.

  7. Measure the volume of sap and then transfer the sap to a labeled bottle for storage.

  8. Rinse the spile, collecting sack and other materials in warm water and allow to dry.

Questions: Does the sap have a taste? If so, describe it. How much sap did you collect? What was the rate of sap flow? How does sap flow relate to temperature? What color is the sap? Is the sap contaminated with foreign materials? If so, what are they and where did they come from? Squirrels like sugar maple sap; did you see any evidence of squirrels feeding on maple sap? Do the taps attract any insects? 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?


Determining the Sucrose Concentration of Sugar Maple Sap by the Refractive Index Method

    The concentration of sucrose in a sample can be determined by measuring the refractive index of the sample. This measurement is based on the principle that the index of refraction of a solution containing sugar is proportional to its concentration. In this experiment, we will use an Atago Hand Sugar Refractometer to measure sugar concentration. (Note: other types of refractometers will work equally well, but in order to obtain a concentration value, the user needs to refer to standard tables or run a standard curve with sucrose solutions of known concentration).

  1. Open the prism cover of the refractometer. Rinse with a few drops of water from a squirt bottle and then gently blot dry. Be careful so that you don't scratch the lens/prism!

  2. Place a few drops of sample on the face of the prism and then close the cover.

  3. Turn the instrument until the window hole is oriented to a light source.

  4. Adjust the eyepiece to give a sharp focus on the scale and the border line between the dark and light field is read directly as percentage sugar.

  5. Measure the temperature of the sap sample.

  6. If the sap temperature is not 20 C, correct the reading (by referring to the table provided with the instrument).

Questions: What is the concentration of sugar in the sugar maple sap? How does this value compare with the one cited above? Is the sugar concentration of the sap higher in large or small trees? Is the sugar concentration greater from the trunk or twigs?

Literature Cited and References

  • Cleveland, Mark (1987) Maple Syruping in your back yard. Minnesota Volunteer. March-April, pp. 9 - 14.
  • Harborne, J. B. 1973. Phytochemical Methods. Chapman and Hall, NY.
  • Holan, F. 1986. Sugaring Made Simple. Country Journal 13: 38- 45.
  • Kozlowski, TT and Pallardy, SG. (1997) Physiology of Woody Plants. Academic Press, NY.
  • 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.
  • University of Vermont - Proctor Maple Research Center web site 
  • Vermont Agricultural Extension - Maple Pages web site

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Last updated:  08/25/2003 / � Copyright  by SG Saupe / URL:
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