Cornell's Sugar Maple Tree
Improvement Program

The amount of sugar in maple tree sap is variable. Some trees, especially those grown in the open, may have high sap sugar concentrations, ranging from 3 to 4%. There are also differences in sap sugar concentration from year to year. One year, a tree might have 2% sugar and the next year 3%. What factors might influence sap sweetness?

Why are these differences in sap sugar concentration so important? The answer is simple: the higher the sugar concentration, the less water needs to be boiled off to produce syrup (~66-67% sugar), and the less the producer will need to pay for the fuel used in the boiling process.

Knowing that trees grown in the open generally have higher sugar concentrations would help maple producers decide which trees to tap, or even where to plant young trees. But what about all the trees growing in the forest? Is there any way we can increase their sugar content?

For many years, maple scientists and farmers wondered if there might be genes that controlled sugar content in maple trees. If you’ve had biology in school, or simply observed family and friends, you know that tall parents are more likely to have children who grow to be tall. But is a maple tree with high sugar content more likely to produce seeds that will grow into new trees with high sugar content? Or are environmental variables, things like whether a tree is grown in the open or yearly variations in the weather, the only thing that controls sugar content of sap?

Starting in the late 1950’s, maple scientists began to search for an answer to this question: Can high sugar content be inherited from a parent tree? Before you read the next section that describes how the scientists tried to answer this question, you might want to imagine you are a scientist investigating this question. How would you go about finding out if parent trees pass on a trait for high sap sugar to their offspring?

Field Examination and Selection
The first thing the maple scientists did was find trees that had high sugar content. Scientists from the US Forest Service worked with county foresters to test 21,000 trees from throughout the Northeast. They measured the sugar content of sap from each tree using hydrometers and refractometers (click here for details on these instruments). Many of the trees they tapped were identified by maple producers, who knew from years of working their sugar bushes which trees were "sweeter" than others.

You might also want to measure sugar content in sap of maple trees in your area, and perhaps help find sweet trees that could become part of the Sugar Maple Tree Improvement Program. Click here for details.

Screening to Select the Very Best Trees
Of the 21,000 original trees tapped, the scientists identified 53 trees that they felt had promise for the tree improvement program. These trees had higher sap sugar content than their immediate neighbors and were healthy and free of any defects.

Clonal Bank

Do you remember "Dolly," the cloned sheep? Foresters have been cloning trees for years. This allows them to produce trees that are genetically identical to the parent tree. Trees are also produced by seeds. Seeds have a portion of their genetic material from pollen, which may come from a different tree than the tree producing the female flowers and seeds. Thus, trees grown from seed have only half of their genetic material from the mother tree.

In the Sugar Maple Tree Improvement Program, scientists knew that the sugar content of sap was influenced by the environment. They wanted to determine whether the genetics of the tree also influences sugar content of sap. If the scientists had used seeds from the 53 original trees, the pollen would likely have come from a tree with lower sugar content than a tree selected for the Sugar Maple Tree Improvement Program. Thus, trees produced from seeds would have been less likely than trees that were cloned from the original trees to have high sap sugar content, if sap sugar content is genetically controlled. Therefore, the scientists wanted to test trees that were genetically identical (clones) of the original 53 "sweet" trees. But how do you get clones of trees?

The first step in cloning a tree is taking a "cutting" or branch of the parent tree. The cuttings can then be "grafted" or allowed to grow onto roots ("root stock") of an existing tree. If you are familiar with grafting apple trees, you know that any genetic characteristics of the apple tree cutting will be expressed above the point where the cutting is grafted onto the rootstock. For example, if you take a cutting from an apple tree with crisp, green apples, and graft it onto rootstock from a tree with mushy, red apples, the new tree will produce crisp, green apples above the point where it is grafted. This same principle would hold for maple trees. If sap sweetness is genetically controlled, then a grafted tree should have similar sap sweetness to the parent cutting above the point where the tree is grafted.

Alternatively, cuttings can be "rooted." This involves treating the cuttings with hormones under special greenhouse conditions so that the cuttings form their own roots. It can be a tricky business as it involves growing the cuttings under just the right temperature, moisture, light, and nutrient conditions. Scientists from the U.S. Forest Service and Cornell University have pioneered new methods of developing roots on sugar maple cuttings.

Another advantage of using cuttings rather than seeds has to do with how fast the next generation of seeds is produced. Whereas a tree produced from a seed takes more than 20 years to produce its own seeds, a tree produced by grafting or rooting cuttings can produce seeds in just three or four years.

Uihlein greenhouse

One year old sugar maple seedlings

Uihlein Clonal Orchard

In 1968, the cuttings from the trees selected for the Maple Tree Improvement Program were grafted onto root stock in Grand Isle, VT, at a site owned by the US Forest Service. In 1983, a second group of cuttings were grafted onto root stock or "rooted" in the greenhouse and planted at Cornell’s Uihlein Sugar Maple Field Station in Lake Placid, NY. These two plantings of the 53 original trees are called clonal banks. Their purpose is to provide seeds and cuttings to grow more "sweet" trees.

Every year researchers at the Uihlein Station plant seeds of adult clonal bank trees in small pots. These seeds grow into saplings in the greenhouse during the spring and summer, and are stored in the underground storage bunker during the winter (funding for the bunker was provided by the New York State Maple Producers Association). The cycle is repeated until they have completed two full winters; at this point they are available for outplanting. Contact, Uihlein Station Director, for more information.

The clonal banks thus represent years of work involving examining over 20,000 trees and research to determine how to propagate them. They are valuable genetic resources that we need to maintain for the future.

Progeny Tests
When they established the clonal banks, the scientists didn’t actually know whether trees grown from seeds of the cuttings would have high sap sugar content. Can you figure out a reason why they couldn’t be sure?

Maybe the 53 trees selected for the clonal bank all happened to be growing in a slightly different environment than other trees nearby. For example, there could be slightly more light or higher nutrients in the soil. If sugar content were only controlled by factors in the environment, then the trees produced from cuttings, growing in a different environment, would not be expected to have high sugar content.

To determine whether sap sweetness is genetically controlled, the scientists established "progeny tests" using seeds from the clonal bank in Vermont. The progeny tests were established in 1983 at two sites: the Uihlein Sugar Maple Field Station in Lake Placid, NY, and on private land in West Salisbury, PA. A progeny test is an experiment in which seeds are taken from a number of different trees that were originally growing in different environments. The seeds are planted at a site that has a uniform environment. The differences between the trees are then measured. Because the environment in which the plants are grown is constant, any differences between the trees in a progeny test are genetic.

The trees in the sugar maple progeny tests were grown from the seeds of trees in the clonal banks. Because the trees in the clonal banks are isolated from other sugar maples, it was assumed that the pollen that fertilized the seeds came from sweet trees in the clonal bank. Once the progeny test trees were about seven years old, they were large enough to test for sap sweetness. The scientists measured their sap sugar content. They determined that sap sweetness was indeed partly controlled by genetics. (Remember, the scientists already knew that environmental factors, such as weather and light, also influence sap sweetness. Many characteristics of humans as well as trees are controlled both by the environment and genetics.)

First Generation Seed Orchard

Once the scientists knew that it was possible to breed trees with higher sap content, they set out to produce more trees from the original "sweet" trees. Scientists at the Cornell Uihlein Sugar Maple Field Station selected the best trees from the progeny test to plant in the Lake Placid seed orchard. Eventually this orchard will produce seed for growers throughout the Northeast.

Establishing Maple Seedlings
Most maple producers tap trees that reproduced naturally from seeds in the forest. In the future, maple producers may want to plant trees that have been bred for sap sweetness and other characteristics. Because many farmers in the Northeast have recently taken land out of farming, abandoned fields are abundant and could be used for sugar maple plantations. But not much is known about growing maple seedlings in open areas like abandoned fields. Thus, the next step in the Sugar Maple Tree Improvement Program was to test different ways to establish seedlings in abandoned agricultural fields.

Currently, twelve farmers from New York State and Pennsylvania are conducting experiments in cooperation with the Maple Team of the Cornell Sugar Maple Program, to see how well maple seedlings grow in abandoned fields in different regions of these states.

We are hopeful that in the not too distant future, a commercial or state nursery will grow sugar maple seedlings from the seeds of the best trees in the Uihlein seed orchard. Maple producers will be able to buy the seedlings grown from the seed of trees with high sap sugar content, and plant them on their land. When this occurs, research conducted by universities, the federal government, and private landowners working together over the past 40 years will have benefited maple producers throughout the northeastern US.

Growing Maple Seedlings in Fields

Once the maple seedlings are taken from the nursery or greenhouse and planted in the field, they are exposed to many factors that could affect their survival. Therefore, another set of experiments is being conducted to test different means of protecting maple seedlings from deer and other animals that might browse on them. Included in this field experiment are also several treatments to test the effect of different ways to control weeds and of applying fertilizers to seedlings. These experiments are being carried out at the Cornell Uihlein Sugar Maple Field Station in Lake Placid, NY, and the Cornell Arnot Teaching and Research Forest in VanEtten, NY.

Can you think of different ways to control weeds when growing sugar maple or other seedlings? How about ways to prevent deer or rabbits from browsing on seedlings? You might want to design your own experiment on growing maple seedlings.

The Future of the Sugar Maple Tree Improvement Program

Of the 21,000 maple trees originally tested for sap sweetness, 53 of the best trees were selected to be part of the Sugar Maple Tree Improvement Program clonal banks. Some of the original 53 have not performed well and are no longer part of the program.

Eventually, commercial seedlings may be produced from the seed orchard trees and planted by maple producers throughout the Northeast. Can you think of any problems that might occur in the future?

In any species of plants or animals, individuals differ in many genetic traits. For example, some trees may be susceptible to a disease or particular insect, while others are able to resist attack. Beech bark disease is killing most beech trees in forests over much of the Northeast. Less than 1% of beech trees are genetically resistant to attack by the scale insect that allows the disease to develop.

What happens if you have maple trees planted throughout the Northeast from less than 50 seed sources? It is possible that none of the selected trees will be resistant to an insect or disease that will attack maple in the future.

Past research has shown that an ideal genetic breeding program for trees includes about 300 seed sources. Thus, the Sugar Maple Tree Improvement Program is seeking new "sweet trees" from throughout the Northeast to add to the clonal bank. As more clonal bank trees produce seeds, these too will be added to the progeny tests.

Maple producers, families, and youth groups can help to locate the new "sweet trees" through screening trees for sap sugar content using the Comparison Tree Method.

For more information and history on The Sugar Maple Tree Improvement Program