The Physics of Sap

by Barbara Walvoord

from Lathrop Lamp Post, March 30, 2017

A bruised branch we see in the woods these days is likely to be oozing sap.  As we drive through our countryside, smoke rising from a shack along the road tells us the inhabitants are “sugaring.” Those of us who don’t do our own “sugaring” trek to the Hadley Sugar Shack for pancakes drowned in real maple syrup.

Behind this common New England scene lie some amazing physics.  Sap flows because of carbon dioxide–yes, that gas we have too much of, causing climate change.  But inside a tree, carbon dioxide is essential. A tree has a problem–it has to get nutrients and water to its branches and roots, especially in spring when it’s trying to nourish new shoots and buds.  It can’t burn coal for electricity, as we do to move heat and water through our homes.  Instead, a tree uses the properties of carbon dioxide, and the spring changes in temperature, to push around the life-giving sap.

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, the sapwood cells produce carbon dioxide, and the sap itself releases carbon dioxide.  This build-up of carbon dioxide causes pressure to increase in the cells, like a tummy full to bursting. A third source of pressure is called osmotic pressure, which is caused by the presence of sugar and other substances dissolved in the sap.  The pressure from all three sources causes the sap to flow throughout the tree, reaching even those high-up new buds. When the tree is wounded, as when it is tapped by a maple producer, the sap, pushed by the high pressure, can literally stream 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 to produce more sap. When temperatures rise above freezing the next day, pressure builds and the sap flow begins again. (http://maple.dnr.cornell.edu/produc/sapflow.htm)

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