Comparing sap-sugar concentrations:
The Comparison Tree Method

The Comparison Tree Method (also referred to as the Five-Tree Tester Method) is used to identify trees with higher than average sap content. Follow the steps below to use the Comparison Method to identify high sap sugar trees. The Cornell Sugar Maple Research Program is actively looking for trees with high sap sugar concentrations.

1) Select as many trees as possible to test for sugar content.
The larger the number of trees that can be tested per location, the higher the potential for identifying candidate trees. We have found that out of 100 trees, we might be able to identify two candidate trees.

2) Keep as many factors constant as possible.
Factors such as slope, aspect, contour, and microsite are possible sources of variation and should be kept constant.

3) Keep tapholes uniform.
Drill tapholes at the same relative location (height and compass orientation) on all trees. If you are going to test immediately after drilling, drill all tapholes within a one or two hour time period. Drill tapholes as close to the beginning of the spring sap flow as possible. If trees are tapped too late, early sap flows may be missed; if they are tapped too early, the taphole may "dry out" before the season ends. If the tree is tapped when it is frozen, it may be seriously damaged because the bark is more likely to split when the spout, or spile, is installed.

4) Sample all trees within the same time period.
Sampling and testing of all trees should occur within a relatively short time period (one hour if possible). This way you control for changes in sap content that may be caused by weather or different times during the sap season.

5) Sample during the spring sap season.
Early to mid-season is when sap sugar concentration is likely to be at its maximal level. Sampling during this period offers the best opportunity for detecting differences in sap sugar concentration between trees. Because maple sap may flow in late fall when weather conditions are right, some producers have attempted fall sap collection. However, tapholes drilled in the fall produce only about half as much sap as in the spring. Also, sugar concentrations from fall tapping are about two-thirds below that expected from a spring tapping. Fall tapping for sugar production is therefore not recommended.

6) Test healthy trees.
To be selected for the Sugar Maple Tree Improvement Program, a tree must be of good form and free of defects, evidence of harmful insects and disease, and other factors that could influence sap sugar concentration.

7) Test trees that are capable of flower and seed production.
Trees selected for the Sugar Maple Tree Improvement Program should be capable of flower and seed production. This way twigs taken from the tree and cloned will produce individuals capable of immediate seed production. Sugar maples acquire flowering and seed production potential at about 30 years of age, which generally coincides with the minimum commercial tapping diameter. Although sap sugar concentration can be measured on small diameter, immature trees, only trees that meet minimum commercial tapping diameter (10 inches diameter at breast height) should be tested as potential candidates for the Sugar Maple Tree Improvement Program.

8) Tap trees with crowns close to the ground.
Candidate trees should have well-developed crowns within 30 feet of the ground. This allows the use of pole pruners for cutting twigs from the tree crown. These twigs are used to clone the trees by rooting of cuttings or grafting.

9) Determine the sap content of the tree using a hydrometer or refractometer.
Follow the instructions on the hydrometer and refractometer sections of the web page.

10) If you identify a tree with higher than normal sugar content, test neighboring trees.
When a tree is suspected of having sap with above average sugar content, test the sap content of the five nearest trees. Record the results on the Sugar Maple Testing Form. Make sure to mark the tree with higher sap content and the neighboring trees with plastic flagging or in some other way. If possible, measure the selected tree and candidate tree several times during the sap season. The sap sugar content of the candidate tree must be at least 30% sweeter than the average of five surrounding trees (standards) and must exceed the sweetest standard tree by a minimum of 0.5 percent.

11) If you have a tree you think qualifies for the Sugar Maple Tree Improvement Program, contact your Cooperative Extension agent or the Cornell Uihlein Maple Program.
Extension agents in many counties are familiar with the Sugar Maple Tree Improvement Program and should be able to help you to determine if you have a candidate tree. If your county Extension agent is unfamiliar with the Sugar Maple Tree Improvement Program, have him or her contact the Cornell Uihlein Maple Program in the Cornell Department of Natural Resources.