Maple sap can be collected with or without the producer following any published "tapping guidelines," which are recommendations on how to best preserve maple trees while exploiting them for sap. Tapping guidelines exist in several, sometimes contradictory, forms, and they are not always based on research. This article describes some new efforts to formulate more comprehensive guidelines.
To collect sap for making syrup, maple producers annually drill holes in living trees and drain away part of the tree's carbohydrate reserves. This might not sound like a sustainable process to a non-sugar maker, but maple sap collection is a relatively benign process compared to the way many other forest exudates are gathered around the world.
A few years ago, I was contacted by a researcher who was interested in borrowing maple sap collecting techniques in hopes of promoting a new method of tapping resin-producing trees in Southeast Asia. Indigenous people had a tradition of collecting resin by a method known as "boxing and firing," which consisted of chopping a hole in the trunk the size of a soccer ball, and then lighting a fire in the hole to induce resin flow. Several such holes were often made before these large and valuable trees were abandoned; this kind of harvesting was clearly not sustainable in the long term.
The scientific literature is filled with reports of trees that are the sources of various gums and resins used in the manufacture of petroleum and gas, textiles, paper and pulp, pharmaceuticals, etc., being damaged by harsh tapping methods.
There is ample evidence that taphole wounds alone are not likely to kill or even seriously harm a sugar maple.
Recently, the University of Vermont Proctor Maple Research Center (PMRC) obtained a section from a large sugar maple that lived from approximately the late 1700s until 2006 and provides a fine example of the resilience of this species. The tree was first tapped around 1860, when it was 10 inches in diameter, and kept in production until the 1970s. Dozens of stain columns from old tapping wounds are visible, some indicating spout diameters as large as .75 inch, and many appear to be overlapping, as this was a bucket tree and spouts would have been placed at about the same height each year.
Despite the 110 years of tapping, the trunk showed no indications of decay and grew an additional 3 inches in diameter between 1970 and 2006, after tapping ceased. As growth rates vary from site to site, it is worth noting that not every tree will respond to tapping in this manner.
Merely concluding that tapholes do not cause serious injury does not provide the guidance needed to meet the goals of most producers in their woods, which can be summarized as protecting the health of the trees, collecting the highest-quality sap in order to make the best syrup and maximizing the production of sap.
Meeting these goals is the objective of maple tapping guidelines. Properly researched guidelines should answer many of the questions that producers are faced with. Among these are:
- What is the minimum trunk diameter for placing one or more than one tap?
- How deep should the taphole be?
- How far from an older taphole should the new hole be?
- Should tapping intensity be modified if the tree has suffered defoliation due to insects or frost?
Guidelines have existed for many years in many forms and may offer a variety of recommendations. However, attempting to follow them can lead a sugar maker on a somewhat crooked path. For example, the new second edition of the North American Maple Syrup Producers Manual offers both "conservative" and "traditional" guidelines for the number of taps per tree. Several organic certifying organizations offer additional, different guidelines. Virtually all of these suggest a different minimum diameter for a single tap or for two taps in a tree. Suggestions for placing new holes in relation to older ones vary according to the author of the guidelines, as do suggestions for how deep to tap, or how to proceed when there are questions about how some adverse weather or insects might have affected carbohydrate storage.
Researcher Mark Isselhardt taps a maple at the UVM Proctor Maple Research Center. Note the use of a drill stop to control the depth of the hole.
Photo by Gary Graham.
It is questionable if these guidelines are based on research or just "best guesses." Further complicating the issue is the fact that many guidelines were written during the long era of bucket collection. Research has shown that management techniques such as having taphole vacuum in the range of 24 to 27 inches of mercury (Hg), a modern two-pipe conductor system and good sanitary practices typically double the per-tap sap yield compared to bucket collection, and today, the vast majority of syrup is made from sap collected using these high-yield techniques. As we collect more and more sap, are we putting the trees in jeopardy?
A variety of research studies at the PMRC are ongoing, or recently completed, with the goal of fulfilling the diverse needs for a modern, revised set of tapping guidelines. Dr. Abby van den Berg, working with researcher Mark Isselhardt, also of the PMRC, recently examined hundreds of cores from sugar bushes around Vermont in order to estimate the growth rate of maples that have been subjected to the high-yield collection techniques described above. Her current studies will help determine what tapping guidelines are appropriate and sustainable for use with high-yield practices.
As part of this work, she and her cooperators have developed a model that estimates the proportion of functional wood within the tapping zone of a tree over time based on parameters including tree diameter and growth rate, and management practices such as drop line length, tapping depth and spout size. This will result in a method that producers can use to estimate the chances of tapping into new, functional wood when they are placing their tapholes. For example, producers will be able to observe how lengthening the drop line can dramatically expand the tapping zone.
As most producers know, tapping into clean wood not only ensures the maximum potential sap flow, but also helps prevent the collection of poor-quality sap that can come from tapping into brown wood. A second study van den Berg hopes to begin soon will compare the growth rates of trees that have been tapped using high-yield methods with the growth rates of nearby trees that have not been tapped, in order to determine if the extraction of tree carbohydrates has affected the rate of tree growth.
A study I completed in 2011 provides data that can help improve the guidelines. I found that a second tap in 16-inch-diameter trees yielded an average of 52 percent more sap than a single tap, while a second tap in 19-inch-diameter trees yielded an average of 66 percent more than a single tap. Both averages were based on two seasons of sap collection under high vacuum. These numbers give a partial answer (by indicating the benefits) to the question of when, or if, to place a second taphole, while van den Berg's data will help illustrate the cost of multiple tapholes in the tree.
Differences in yield according to taphole depth have been examined in recent years, and new data may revise the way we drill our tapholes. Stéphane Guay, of Dominion & Grimm, Inc. and formerly of Centre ACER, reports that holes 1.25 inches deep typically yield much less sap than holes as deep as 2.75 inches. I began examining this issue in 2012. My preliminary data from a single season showed that holes 2.25 inches deep yielded 70 percent more sap than holes 1.25 inches deep, all under 24 inches of vacuum. This is an ongoing study and additional hole depths will be examined.
Sugar makers should weigh the benefit of more sap against the cost of a deeper hole, including the increased chances of striking nonfunctional wood from an old hole and the greater amount of carbohydrate harvested from the tree.
The term "nonfunctional wood" refers to the stained volume of wood that is associated with a taphole. This is a part of the stem that the tree has walled off, and it is no longer used for sap transport. There is still some question whether the amount of wood withdrawn from future sap flow is limited to stained wood or if the walled-off area goes beyond the stain. This could impact all our models of tappable wood, for it could change the distance that a new taphole needs to be from an old one. This subject will require future research.
Guidelines specify the minimum tree size for tapping. Recently we have observed some producers tapping every maple in their woods down to diameters of 6 inches or even less. Many of the trees in this size range have their tops below the canopy, and in this suppressed position grow slower and gain less carbohydrate annually than larger trees. A study by Mark Isselhardt of the PMRC determined that with trees in the 6 to 7-inch-diameter range, vacuum may remove as much as 10 percent of the stored carbohydrates, and thus tapping these trees could seriously impact their future vigor. For some producers, tapping small trees is considered a method of "thinning," although this method may not result in fewer trees but instead many weaker, suppressed trees.
It may be several years before new and comprehensive tapping guidelines are completed, including guidelines for tapping red maples, which are increasingly used for syrup production in much of the Northeast. All of the research described above may not lead to dramatically new guidelines, but hopefully will enable producers to more clearly see the costs and benefits of various practices. When in doubt, conservative practices are best. Close observation of the trees, including assessing foliage health during the growing season and checking for the ability to consistently find new white wood when drilling tapholes, is always a good idea.
The author is a maple specialist with University of Vermont Extension and Proctor Maple Research Center in Underhill Center, Vt.