Syrup Update

Last week we made 33 quarts of syrup and 6 pounds of sugar. Pretty good. The expectation is to get about 1 quart of syrup per tap, so we are expecting about 130 quarts for the season. But this week the sap won't run too well at all. It's too warm. Sap runs really only when it's below freezing at night and above freezing during the day. The North American Maple Syrup Producer's Manual has a very interesting section on the science behind sap flow. Here is a little bit of it:


Fluctuations in wood temperature above and below freezing during the dormant season are responsible for sap flow in maples. These temperature changes create periods of alternating negative and positive stem pressures. At this time of the year, negative pressures can develop when the air temperature falls below freezing and postivie pressures may develop when the temperature rises above the freezing point. ... These changes in pressure are the result of certian properties of maple wood, which, althought not unique in the plant world, are extremely uncommon. In hardwood trees, a great many fiber cells in the xylem (the wood just beneath the bark) surround the sap conducting vessles. In maple trees these fiber cells are air filled, whereas in most other tree species they are sap filled. When freezing conditions are present in late winter and early spring, small ice crystals begin to form inside each of these air filled fiber cells, much like forst forming on a windowpane. As the ice crystals form, the humidity within the fiber cells falls rapidly, causing moist air to be drawn in from adjacent vessels. As this moisture is pulled into the fibers, the layer of ice crystals within each fiber thickens and the air bubble becomes increasingly compressed. The movement of sap into fiber cells pulls water from the vessels, and this pull is transmitted throught the branches and the trunk down to the roots. Strong negative pressure (suction) results throughout the tree. At the same time, other weaker forces caused by the contraction and the dissolving of air bubbles in the cooling sap also add to the suction. The suction results in water being taken up by the roots (if the soil is not frozen). This process continues until all the sap in the tree is frozen, effectively blocking the pathway for water uptake. When cooling takes place slowly over many hours the resulting accumulation of sap in the fiber cells and throughout the tree is greater than if rapit freezing of the twigs occurs.

When the temperature next rises above freezing and the frozen branches thaw, the pressure in the tree at the taphole transitions rapidly from negative to positive. Thawing of the ice in the fiber cells allows compressed bubbles to expand and push the sap back into the vessels. Coupled with the pressure from bubble expansion are two additional forces: gravity and osmosis. Since the sap accumulated as ice in branch fiber cells that are located high abvoe the taphole, this addtional sap, once returned to the vessels exerts a doward force similar to a standing column of water, with the greatest pressure at the base of the tree. In addition, it is believed that osmosis contributes to sap pressure because the movement of water into the fiber cells during the cooling period excludes sucrose (the sugar molecules cannot pass through the fiber cell walls). Once thwaed sucrose in the sap draws water out of the fibers by normal osmotic behavour. This helps explain the observation that a tree with higher sap sugar concentration than its neighbor typically has higher sap pressure and yeilds more sap.


Cool huh? The book goes on to say that eventually things happen that counter act this positive pressure thing - evaporation from the branches in the sun for one and the sap flow stops until the temperature cools down enough to start it again. And of course, eventually the tree starts to heal the hole you made in it, and that stops the sap flow as well. Once the trees start to bud, the composition of the sap changes and makes it unpalatable for people and that is usually what ends sugaring season.

So, maple producers everywhere hope that it will get colder again to prolong the sap run. It does sometimes take a break and then start again. Some big farms even tap in February for this reason if it's supposed to be warmish - to get some early sap. But, if it gets so warm for so long that the trees start to leaf out, well, that's all she wrote and sugaring is over.