Plant Physiology

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

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 simply chopped 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 tube called 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 portable electric drill or 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. Many commercial operations attach plastic hoses to the spiles under a vacuum to help withdraw sap. We will use the following procedure for tapping:

Protocol:

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. (Click here for a brief primer on maple identification) You may want to experiment with other species including birch (Betula sp.), silver maple (Acer saccharinum L.), box elder (Acer negundo L.), red maple (Acer rubrum) or Norway maple (Acer platanoides). The latter is rarely used because it produces milky sap. At first glance, 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. One way to determine if the sap is running is to break off the tip of a small branch. If it "bleeds" the trees are ready to be tapped.
Using a brace and 7/16th inch bit, bore a hole into the tree. Modern producers are using smaller tap holes (19/64th, 5/16th) and spiles, particularly in operations that use tubing and vacuum to collect sap. The smaller holes provide more rapid sealing of the hole and are easier to drill (a battery-operated drill can be used) and install.
The hole should be at a slight upward angle (about 10 degrees). This angle is to allow the sap to drain more easily from the hole and prevent ice from accumulating. In addition, my minimizing sap accumulation in the spile it will help to reduce bacterial contamination.
Traditionally, the holes were approximately 2.5 – 3.0 inches deep or until dark-colored (heartwood) shavings appeared. However, new guidelines suggest that a hole one inch deep is more than adequate and that the hole should be in sapwood (white shavings).
The hole should be approximately waist high (1 meter). Avoid scars, burls and previous tap holes (about 6 inch horizontally and 2 inches vertically). Drill one tap per tree, although larger trees can have more taps (Swain, 1981; Holan, 1986). New guidelines suggest that trees smaller than 12 inches in diameter shouldn't be tapped and that a maximum of two taps be placed in a tree. Traditionally, the number of taps was related to tree diameter: 10-14 inches = 1 tap; 15 - 19 inches = 2 taps, 20 - 29 = 3 taps; 25+ = 4 taps. If the taps from previous seasons haven't sealed, the tree should not be tapped.
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. 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).
Hang the collection container on the spile.

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? Does sap flow rate vary with species?