By Jim Gale

I thought transpiration caused the Sugar Maple sap to rise; but no.

The cohesion tension model is the rise of water through the xylem from the roots to the leaves.

I just recently, realized that the veins in leaves with their vascular bundles bring the water to the leaves, and of course, return of sugars to the roots through the tubular cells of phloem. As the leaf photosynthesizes the stomata open to grab CO2 and release oxygen and release water which is called transpiration.

The release of water, pulls the continuous stream of water through the xylem. Therefore, the cohesion tension model describes the typical movement for water in trees.

But Sugar Maples have no leaves in late winter. The cohesion tension model has nothing to do with the rise of sap in Acer saccharum. There are no leaves and the root pressure is negative. So what’s happening to get the Sugar Maple’s sap flowing? The first days’ temperatures in early spring that reach 40-50 F range with freezing night temperatures triggers the rise of sap in sugar maples!

The dead cells in the xylem responsible for the rise of sap are the vessel cells.

Fiber cells are like the threads of tough strings you chew through while eating celery. The vessel cells are barrel-shaped and stacked on top of each other with holes in the sides of the barrel called pits. Therefore the barrel cells can receive water nutrients from surrounding cells. I remember that vessels are like barrels and barrels hold water. This is key for Sugar Maples.

The fiber cells have air and the vessels have water.

Now what is the role of temperature? At night, the gases dissolve and water freezes in the vessels. This reduces the pressure and the vessels pull in water from the surrounding cells and the roots. This creates a positive pressure that pushes the sap up to the adjacent vessels.

Now where does the water in the xylem gets it sweetener?

The vessels are in xylem, and in the fall starch from the leaves is deposited in xylem. Hydrolysis of the starch in the spring turns to sugar and tada! we have 2% sugar in the sap.

Now why don’t oaks, aspens, or willows have sugar in xylem rising? Simply their vessels are filled with air (instead of water like Acer) and their fibers have water. The fibers will be acting in the cohesion tension model to bring the water to the trees pushed up by positive root pressure.

So to recap, vessels are shaped like barrels and barrels hold water and with the freeze every night the water freezes that creates negative pressure. So as water thaws it’s drawn into through the pits and pushes the sap up the tree.