Sugar or Gas? – What Drives the Springtime Flow of Sap in Sugar Maple Trees?

179193754 4e933af4f6Pancakes With A Side of Math

What better time to talk about where maple syrup comes from than “sap season”? This is time of the year – early March – when the sap typically is flowing in the sugar maple trees that grow in the northeastern part of North America.

What drew my attention to this timely subject was a recent paper (see Ref. 1 below) about what exactly causes the sugar maple sap to flow in such abundant quantities in the early spring.

This paper about maple trees was published in a most unlikely place – a mathematics journal. The reason for this is because the authors provide mathematical calculations to support a current theory regarding what drives the springtime sap flow in sugar maple trees.

However, before we get bogged down in this sticky subject, first a let’s sop up little background information.

Go With The Flow

A general explanation about why sap flows in some deciduous trees during the transition from winter into spring goes something like this:

The story really begins in the previous fall. Before their leaves drop in the autumn, the trees try to retrieve and store as many photosynthetically-produced carbon compounds from the old leaves as they can. A lot of these carbon compounds are converted into glucose, which is then stored over the winter as starch in the stems and in the roots of the trees.

At the end of the cold winter when temperatures start to warm up a bit, living cells in the trees begin convert the starch back into sugars in order to support the new growth in the spring.

Also, with the melting of the snow, the soils may become saturated, or nearly so, with water.

4415049940 e80a64fdbeThe combination of the abundant amount of sugars in the tree along with the plentiful source of water in the soil allows the plant to osmotically take up large amounts of this water.

In some trees, such as birch, this so-called “root pressure” may indeed drive water flow (sap flow) from the roots up the trunk and into, at least, the lower branches of the tree.

Alternatively, “stem pressure” may drive sap flow in some trees in the spring, in particular, maple trees.

Unlike root pressure in birch, stem pressure in maple is highly dependent on temperature; several freeze-thaw cycles are needed to initiate significant positive pressures.” (from: Ref. 2 below)

What Causes This Stem Pressure?

Because maple syrup is economically so important in the northeastern parts of Canada and the United States, this question has attracted a lot of attention, along with ample research funding, I might add.

Basically, two theories have emerged.

The first, and older, of the two has to do with expanding gases in the xylem.

Briefly, this “physical” model attributes sap flow entirely to pressure and volume changes resulting from passive, physical effects in the tree’s vascular tissue, that is, the expansion and contraction of gas, as well as freezing and thawing of sap, which occur in response to temperature changes. (The calculations presented in Ref. 1 below support this theory.)

The second theory states that positive pressures are developed in sugar maple stems when temperatures fluctuate around freezing mainly due to osmotic processes. This “osmotic” theory relies on the involvement of living cells that convert starch into sucrose, which is then actively transported into the vessels. This increased sucrose concentration in the vessels osmotically attracts water, which results in increased stem pressure. (The experimental results of Ref. 2 below support this theory.)

Bottom Line: Looks like “the jury is still out” regarding the driving force chiefly responsible for sugar maple sap flow in the spring.


1. Ceseri, M. and J. M. Stockie (2013) “A mathematical model of sap exudation in maple trees governed by ice melting, gas dissolution, and osmosis.” SIAM Journal on Applied Mathematics, Vol. 73, pp. 649–676. (Abstract)

2. Cirelli, D., R. Jagels and M. T. Tyree (2008) “Toward an improved model of maple sap exudation: the location and role of osmotic barriers in sugar maple, butternut and white birch.” Tree Physiology, Vol. 28, pp. 1145–1155. (PDF)

For More Information: The life of a sugar maple tree from Cornell University.

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