Sugar Maple Tree
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.
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 youve 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
Starting in the
late 1950s, 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?
Examination and Selection
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
to Select the Very Best Trees
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.
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
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.
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
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.
year old sugar maple seedlings
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 Cornells 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.
established the clonal banks, the scientists didnt actually know
whether trees grown from seeds of the cuttings would have high sap sugar
content. Can you figure out a reason why they couldnt 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.
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
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.)
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
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.
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.
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
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.
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
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.