Sugar maple bleeding

© Jardin botanique de Montréal (Normand Cornellier)

I don't understand the phenomenon of bleeding in sugar maples, which allows us to harvest maple syrup.
Anonymous, Montreal (Quebec)

Hello!

The phenomenon of spring bleeding in sugar maples depends mainly on temperature: an alternation of cold nights (below 0°C) and warm days (above 0°C) is imperative. Bleeding takes place in two stages: an absorption phase during cold nights and an exudation phase during the warming of the weather.

At night, the sap freezes in the thin branches at the ends of the tree. The ice core attracts the sap upwards and dries up the notches. The trunk, under vacuum, absorbs water like a sponge. For an abundant bleeding, the air temperature must lower gradually so that the sap freezes slowly. When the gel is fast, sap hardens in the vessels of xylem (Wood conducting network.) without reaching the wood fibers.

During the day, when the temperature rises above freezing, frozen sap becomes liquid again and falls by gravity towards the base of the tree and towards the notches.

This phenomenon is due to the special properties of maple wood, whose fiber cells are filled with gas rather than water as in the case for most trees. The presence of gas in the wood fibers attracts water by capillary forces (Phenomenon allowing elevation of fluids into the vessels of the tissues of a plant.) and leaves space for the ice. When the ice melts, air bubbles push sap by pressure and accelerate the phenomenon of bleeding.

This mainly physical explanation of bleeding doesn't account for some biological phenomena. For example, the presence of sucrose in the xylem is necessary for there to be spring bleeding. Starch accumulated during the summer is transformed into sucrose in early winter, when the enzymes involved in this process are activated by cold temperatures.

Physical theory (pressure, etc.) explains, however, why the trees with the highest yield of sap are those with the largest vascular rays. There is more space for the rise of sap during the absorption phase.

Have a good day!

Céline Arseneault
Botanist and librarian

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