of the great mysteries of maple syrup is what causes the sap to flow
out of the trees
is an explanation of how temperature fluctuations and pressure and
suction in the tree cause sap to flow.
Early in the
spring, when the maple trees are still dormant, temperatures rise
above freezing during the day but drop back below freezing at night.
This fluctuation in air temperature is vital to the flow of sap in
sugar maple trees.
What causes the
sap of maple trees to flow in the spring? During warm periods when
temperatures rise above freezing, pressure (also called positive pressure)
develops in the tree. This pressure causes the sap to flow out of
the tree through a wound or tap hole. During cooler periods when temperatures
fall below freezing, suction (also called negative pressure) develops,
drawing water into the tree through the roots. This replenishes the
sap in the tree, allowing it to flow again during the next warm period.
Although sap generally flows during the day when temperatures are
warm, it has been known to flow at night if temperatures remain above
Thus, pressure and suction are essential to sap flow. But how do the
pressure and suction develop?
Sap flows through
a portion of the outer tree trunk called sapwood. Sapwood consists
of actively growing cells that conduct water and nutrients (sap) from
the roots to the branches of the tree. During the day, activity in
the cells of sapwood produces carbon dioxide. This carbon dioxide
is released to the intercellular spaces in the sapwood. In addition,
carbon dioxide in sap is released into the spaces between the cells.
Both of these sources of carbon dioxide cause pressure to build up
in the cells. A third source of pressure is called osmotic pressure,
which is caused by the presence of sugar and other substances dissolved
in the sap. When the tree is wounded, as when it is tapped by a maple
producer, the pressure forces the sap out of the tree.
At night or during
other times when temperatures go below freezing, the carbon dioxide
cools and therefore contracts. Some of the carbon dioxide also becomes
dissolved in the cooled sap. Finally, some of the sap freezes. All
three of these factors create suction in the tree. This causes water
from the soil to be drawn up into the roots and travel up through
the sapwood. When temperatures rise above freezing the next day, sap
flow begins again.
Thus, the cycle
of warm and cool periods is essential for sap flow. Temperatures too
warm or too cool during the short, six-week "sap season"
will reduce the amount of sap flow. This will result in lower maple
syrup production or a "bad year" for maple producers in
The sap in sugar
maple contains a high concentration of sugar compared to the sap of
other trees. The sugar in maple sap is the product of photosynthesis
that occurred during the previous summer. Carbohydrates produced by
photosynthesis are stored in the tree in the form of starch. Starch
is converted to sucrose (sugar) and dissolves in sap. Amino acids
in the sap give maple syrup its distinctive flavor, which differs
from pure sugar.
Many people wonder if tapping the tree and taking away so much of the
trees sap might harm the tree. In fact, when producers follow
tapping guidelines, and tap only healthy trees, no damage to the tree
results. It has been estimated that tapping removes only 10% or less
of the trees sugar, an amount too small to hurt a healthy tree
under normal environmental conditions.