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Note: Although many of you may still be receiving snow, spring IS coming. Spring rolls across different parts of the provinces at different times and so we may mention some happenings a little early for the benefit of our southern-most readers. Officially spring arrives in the Northern Hemisphere on March 19 or 20 (Vernal Equinox). It's a great reason to have a celebration!
Featured Event: The Sap is Rising!
Nothing seems to unify Canadians around spring in the outdoors more than maple syrup season, which by mid-month or sooner should be full swing in many parts of Ontario and Eastern Canada.
Maple syrup was first collected by Native Americans and has been appreciated for a long time:
"There is in some parts of New England a kind of tree...whose juice that weeps out of its incision, if it be permitted slowly to exhale away the excess moisture, doth congeal into a sweet and saccharine substance." Robert Boyle Philosophical Works (1663)
But how does it work? Why does sap “rise” at this particular time of year in this particular kind of tree? To answer this question, we first must review the parts of a tree, in particular the xylem and the phloem.The phloem is a thin layer of living cells just under the bark that carry sugars down from the leaves to the rest of the tree. The xylem is a wider band of sapwood just inside the cambium that carries water, nitrogen and minerals up the tree, pulled along by the gentle tug of evaporation as water leaves the leaf pores.
But two things are very different in the spring: it’s the xylem that contains the sugary sap, not the phloem, and there is no way, with the leaves missing, to ‘pull’ it up the trunk. In addition, root pressure is not involved – pieces of living Sugar Maple stem by themselves, standing in water, can be made to create sap flow.
What does happen is linked directly to this specific time of year, when nights fall below freezing and days warm above it. Take away either of those conditions, and sap does not rise. What happens is this. First, in the fall, sugars are transported down into the stems and converted to starch. In the spring, during the warm days, living cells convert that starch into sugar. They also generate carbon dioxide gas. This gas diffuses into the xylem. As the temperature cools, the gas dissolves, lowering the pressure and pulling the sugary water from the living cells into xylem. This water is replaced from adjacent cells, which form a conveyor belt for water down to the roots. As night comes and the temperature drops further, water freezes along the inside walls of the xylem and in between its cells. The remaining gas is compressed and locked in this ice. Suction is created, which draws water from the soil into the roots. With morning, things warm, the gases expand and force the now liquid sap out of the trunk or stem and into the tap. As the day cools in the afternoon, the process repeats itself. For more details, go here or here. The process stops when the temperature remains above freezing and the buds begin to open.
Only a few trees besides Sugar Maples (distribution) contain the correct cell structure to produce this kind of temperature-related pressure, and include Butternut (distribution). If you’re from much of the rest of Canada and feel like you’ve missed out on a childhood tradition, pout not -- syrup can also be made from birch trees (distribution), but the sap flow comes later in Spring and is dependent on root pressure. Make your own!
Red Squirrels discovered this process long before we did. They will nip at a twig to start the flow, then go away for a day or so to allow some water to evaporate and the sugars condense before coming back to consume the thicker sap. It may have been an observation of this behaviour by indigenous peoples that led to their development of the syrup-making process.
Related lesson plans and resources can be found here.