“The sap is rising” is an often heard description of early spring. If you cut into the stem or branch of certain trees – sugar maple, birch, hickory, walnut, or sycamore – on a cool spring day, you may see sap dripping from the cut end, or an icicle of sap forming. The sap is slightly sweet and is the source of maple syrup. Yet if you cut a tree a few weeks earlier or later, nothing happens. What is going on?
During the winter, when trees are dormant, the stems store large amounts of starch, a polymer made of long chains of glucose, a simple sugar. The starch is stored in living cells in the wood, called parenchyma. When nights are cold and days a bit warm, enzymes in the stem break down the long polymers into the simple sugar sucrose. Suddenly, the number of molecules in the parenchyma cells goes from a small number of starch polymers to a huge number of small sucrose molecules. The sucrose creates an osmotic potential that causes water to flow into the parenchyma cells. This is the simple and familiar principle of osmosis. As water moves into the cells, the pressure inside the cells rises. Some of the sucrose is pumped out of the parenchyma cells into the dead xylem cells. Water continues to flow from the soil, raising the pressure in the stem. When a branch breaks or is cut, the pressure causes the sap to flow out.
In maple syrup production, a metal spile is driven into the stem and connected to plastic tubing, allowing the sap to flow from the tree to the sugar shack. Maple syrup production is only commercially practical in places with the right weather – cold soil, cool nights and warm days. The cool nights promote the conversion of starch to sugar, while warm, sunny days allow water to flow from the soil into the stem.
In Kentucky, where I live, commercial maple production is not practical. The spring tends to warm up rapidly and there is usually little snow to insulate the soil, making the sap flow season too short to make any money. A year like 2015, though, could be a good sap year. I have already seen bleeding sap in a number of maple trees.
The positive pressure inside the stem lasts for only a few days to weeks. Trees do not push water up the stem, they pull it from the top. For the entire growing season, the stem is under strong negative pressure (<0). Negative pressure is not something familiar in everyday life: it is not possible to create a pressure less than zero (a vacuum) in a gas. But it is possible in tightly constrained narrow columns of water in the xylem of a tree.
In the best years for sap production, spring weather produces cold soil, cold clear nights and warm days. Sudden spring warming, as happens more frequently as climate change take hold, reduces sap yields.