Sugaring has come a long way since people slashed a chunk from a tree’s bark to let the sap run out. And it’s come a long way since sugar makers used their arms to measure whether or not a tree was big enough to warrant a second tap. Guidelines written by state regulators or trade associations haven’t always kept pace with changes in industry practices or understanding of the science of sap flow, and researchers are working to update tapping guidelines.

For example, existing guidelines were written when most producers were using 7/16-inch taps and relying on gravity to draw the sap from the trees. Now most sugar makers use 5/16-inch taps, and many are using vacuum systems to pull more sap out of each taphole. The smaller taps create less of a wound, which is certainly healthier for the trees, but vacuum systems are taking as much as 50 percent more sap from trees than gravity systems.

What’s more, there’s not a lot of continuity among the tapping guidelines that have been written in various states, says Dr. Tim Perkins, director of the Proctor Maple Research Center at the University of Vermont. Some suggest that tapping 8-inch-diameter trees is acceptable, while others recommend not tapping anything smaller than 12 inches. Where one set of rules says the threshold for adding a second tap to a tree is a 14-inch diameter, a more conservative set suggests it should be 21 inches. Few, if any, of the guidelines were written based on scientific studies, Perkins notes; instead, they’re based on common practice or anecdotal evidence.

There’s no way to remove sap from a maple tree without creating some sort of wound, which naturally pressures the tree by creating a dead zone in the wood where sap can no longer flow. The size of that dead zone is based on the diameter and depth of the hole, extending upward and outward beyond it. That’s why sugar makers know not to tap near old holes – there won’t be any sap. However, it also means that the tree isn’t able to move sap that it needs for its own growth.

Perkins says that guidelines may have taken into account the effect of the wound on the tree and how quickly they heal, closing up the hole and growing new sapwood on top of the dead zone, but not the effect of taking some of the tree’s carbohydrate supply. The tree isn’t making sap for the sugar makers; it’s a vital energy source for the tree’s growth. So each time sugar makers take sap, they’re removing some of what the tree needs to sustain itself.

A single tap on a gravity system in a 6-inch-diameter tree can take as much as 5 percent of that tree’s sap, says Perkins, while one tap in an 18-inch-diameter tree only removes about 0.5 percent of that tree’s energy reserves. Adding a second tap to a tree on a vacuum system increases that tree’s production far less than a second tap on a gravity system does, since the efficiency of vacuum systems is based in part on the fact that they are drawing sap from a bigger part of the tree already. These and other factors have not been taken into account in tapping guidelines to date.

A recent study by Proctor Center researchers that was published in the December 2013 Maple Syrup Digest ( examined the impact of the most conservative guidelines on growth rates of tapped trees, along with the long-term sustainability of those guidelines. The results yielded a model that allowed the researchers to create a spreadsheet tool (downloadable at where sugar makers can enter information about the size of a tree and their tapping practices, such as depth and size of hole and length of dropline, and the tool will calculate the sustainability of the practice over 100 years of tapping based on the probability of finding sap-conductive wood in each successive year.

Meanwhile, Perkins is starting a 10-year project to investigate the effects of tapping on the long-term health of trees. Variables such as the use of vacuum and the number of tapholes and their depth will be examined, and observations will be made about the trees’ growth rates and health. In addition, a study is being conducted at the Proctor Center to examine how faster-growing red maples heal and recover from tapholes. These studies will add to the growing body of knowledge in the maple industry that allows producers to maximize efficiency and steward their resources wisely.