|
You
probably know that several species of maple grow in
Michigan, but the two that turns our late-winter woods
into a bucket kingdom are the sugar maple and the black
maple (though people collect the sap of any & all
maples). All trees have sap, but only maple sap has a
high enough sugar content to make processing it into
syrup a viable economical venture. And sap flow (sap
exiting the tree at a wound—either natural or
human-caused) only occurs in maples.
Sap flow is caused by a combination of late-winter
weather conditions and the unique physiology of maple
trees. The stage is set in the fall: carbohydrates,
produced by photosynthesis during the summer, are moved
down out of leaves and twigs to be stored over winter as
starches in the trees’ trunks and roots. During the
winter, some of these starches are converted to sucrose,
much of which is released into the sap and dissolved.
(The amount of sucrose in the sap depends on the
genetics of each individual tree, tree health, leaf
mass, soil composition, and the environmental conditions
of the previous growing season.) With the spring thaw,
the sap begins moving back up toward the leaves and
twigs.
But the calendar doesn't determine the start of sugaring
season; sap flow determines the season. It may come as
early as mid-February; it may not come until mid- to
late March. It may start, stop, and start again. Sap
flow begins after a freeze–thaw cycle of days with above
freezing temperatures followed by nights below
freezing. This could happen at any time during the
winter dormant period, but a strong flow is produced
only after several consecutive days of temperature
fluctuation. Once the sap begins to flow, it will
continue at a steadily declining rate for approximately
8 to 15 hours, providing it doesn’t re-freeze. When it
does freeze and thaw again, the sap flow resumes at the
peak rate. These intermittent flow periods will
continue for as long as temperatures fluctuate between
above and below freezing. When spring advances and
nighttime temperatures no longer drop below freezing,
sap flow and sugaring season end. That is one reason
why, although maple trees grow all over the world,
including Siberia, maple syrup is made only in Canada
and the northeastern and north central United States: in
other regions, temperatures are either too consistently
warm or cold.
The mechanics of the sap flow process are complex and
not completely understood, largely because the maple
species has evolved a method of storing and moving sap
unlike other trees. The trunks of hardwood trees are
composed of living sapwood filled with four types of
longitudinal cells—“pipes” which transport
sap—surrounded and supported by deadwood fibers called
xylem. In most hardwood trees, the xylem is filled with
water. In maples, the xylem is filled with CO2. It is
the distribution of sap and gas in maple trees that
seems to be the critical factor: the gas-filled xylem
provides the conditions necessary for maples to exude
sap under the correct weather conditions.
When temperatures fall below freezing, negative pressure
is created within the sapwood of maple trees due to:
-
sap
freezing,
-
gas
contraction in the xylem (caused by cooling)
-
carbon dioxide dissolving in the cooling sap—CO2 is
twice as soluble at 0°C as at 20°C. (Any
undissolved CO2 is compressed and locked in ice when
the sap freezes.)
The
negative pressure “sucks” sap and water up from the
roots and soil into the tree, increasing the sap
volume. Suction continues as long as the temperature
remains below freezing and sap from the roots or water
from the soil is available.
When the temperatures rise above freezing during the
day, the previously ice-bound, compressed CO2 expands,
exerting as much as 40+ pounds/square inch of positive
pressure relative to atmospheric pressure. Osmotic
pressure further increases the internal trunk pressure.
This pressure, normally exerted by liquid flowing
through a semi-permeable membrane separating two
solutions with different concentrations of solute, is
amplified by the presence of sugars and other substances
dissolved in the sap. Positive pressure develops first
in the twigs, moves downward into the trunk, and finally
in the roots of the maple tree.
When temperatures fall again at night, the process is
reversed and pressures are reduced. The cycle repeats:
200 to 250 gallons of sap rise through a mature maple
tree each day in late winter/early spring.
I hope you agree that even a rudimentary understanding
of the physiology of maple trees makes sugaring season
even more wondrous. What better way to shake the winter
blahs that this: walk in the woods; check out the maple
trees and ponder their unique complexity. Find a
sapsicle; peek under the lids of the collection buckets;
breathe in the aroma in the sugarhouse where the sap is
boiled; get a taste of pure maple syrup. Late-winter
will never (completely) feel depressingly like mud
season again.
back to Maple Syrup
Season
|
