By Richard DeVries
It takes a cold frosty night followed by a nice warm spring day for a
Maple tree to build up pressure inside its trunk. A Maple tree can do
this because some of the cells in the sapwood are filled with gasses
and others with sap. In other trees all the cells are filled with sap
and these trees can not create the same kind of pressure.
When the Maple warms up during the day, gasses in the cells expand and more gasses from the sap-filled cells are released into the gas-filled cells. These cells build up the pressure that the tree needs to push the sap up to the top of the tree. The sap will also leak out of any kind of wound in the trunk when the pressure inside the tree is higher than the barometric pressure. We like the sap to drip out of our tap-hole and into the bucket but it will also flow out of frost cracks and broken branches.
At night when the temperature drops below freezing, the gasses in the tree contract and some of the gasses dissolve in the sap-filled cells. All this re-organizing by the gasses causes negative pressure or ‘suction’. The extra space in the trunk is filled with water that the tree pulls up through the roots. Unfortunately the negative pressure can also suck the sap out of our tubing and back into the tree. After the tree fills the void it is ready to get pressurized when it warms up the next day. The tree will repeat the process every time it warms up after a freezing night.
A Maple tree can build up to forty pounds of pressure per square inch (PSI), this is more than the pressure in a car tire. To show this we outfitted one tree on the south-hill and one tree on the north-hill
with a pressure gauge. Last Tuesday (3/6) we got our first sap on the south hill, the pressure gauge was reading nine PSI and I counted 45 drips per minute on a nearby tree that was tapped with a bag. Thursday the pressure gauge was pointing at twelve PSI and I counted seventy drips per minute. On the north-side the gauge is still at zero and we have yet to receive any sap. We need some more freezing nights followed by warm days but the forecast does not look very promising.