Seasonal Sap Flow and Moisture Content in Trees

Debunking a few misconceptions about winter and summer tree sap behavior. December 31, 2012

The other day I started thinking about the idea that the sap "drops" in trees in the colder winter time. So where does it go? Is it really that the water intake decreases when the weather gets cold? I can speculate as well as the next person, so I really would like a scientific answer.

Forum Responses
(Sawing and Drying Forum)
From contributor D:
The sap is put under pressure during the growing season. During the dormant season the pressure drops.

From Professor Gene Wengert, forum technical advisor:
The amount of sap in a tree, or the moisture content, is essentially the same throughout the year. This has been measured time and time again, especially by pulp companies that buy wood by weight and are sensitive to the amount of water in the tree when pulping.

The sap does not go into the roots in the winter. In fact, with some species we see a 1% MC increase in the wood above ground in the wintertime. The difference between summer and winter is the flowing of the sap.

However, anyone who has tapped maple trees knows that the best flow occurs before the buds have even begun to swell, like in February, with freezing nights. In fact, with this warm winter, maple flow has begun and may actually be poor.

When sawing, because the logs dry some in warmer weather, at the mill, the MC is lower in the summer.

Also, a similar issue is that the sap does not freeze in the wintertime in the tree. It has natural anti-freeze. We may get some freezing under -40 F and then trees will get so-called frost cracks due to expansion of the ice.

From the original questioner:
Thank you; that makes more sense than the sap drops. It is funny how we continue to use the wrong, misguided terms without thinking of the logic. I just wish I had questioned this a long time ago as I hate to perpetuate misinformation.

From contributor J:
I think the phrase "drop" may come from something known as Turger Pressure in trees, which measures the natural flow of sap up to the canopy of the tree during the normal growing season due to evaporation of moisture through the leaves. Obviously once the tree goes dormant the turgor pressure drops.

From Professor Gene Wengert, forum technical advisor:
It is my understanding that turgor pressure refers to the minute action within a cell, but does not account for sap flow itself in a tall tree, especially in a species like sugar maple in the early spring before leaf growth starts. In fact, we need to recognize that virtually all cells in the stem of a tree are dead. Further, the cells of a tree are filled with about half water and half gas... If they were all water, trees or logs would not float, as wood itself is 1.5 times heavier than water. Unfortunately, the literature is full of theories on sap flow that are incorrect when applied to tall trees, but do work for short plants.

In a tree, does water flow up in the sapwood and down in the bark? Or is it the other way? In other words, how do the sugars of photosynthesis get down the tree? Another comment is that wood cells, when first formed in the cambium layers (which was once thought to be a single layer of cells) are full size and do not grow longer or fatter. In fact, soon after creation, most of the cells die... Maybe some ray parenchyma cells remain active.

Joyce Kilmer may have said it best:
I think that I shall never see
A poem lovely as a tree.
A tree whose hungry mouth is prest
Against the earth's sweet flowing breast;
A tree that looks at God all day,
And lifts her leafy arms to pray;
A tree that may in summer wear
A nest of robins in her hair;
Upon whose bosom snow has lain;
Who intimately lives with rain.
Poems are made by fools like me,
But only God can make a tree.

From contributor D:
It was explained to me some 30+ years ago while taking Tree Physiology that turgor pressure is a capillary action of liquid. This capillary action theory held true until you got to the tall trees: redwoods and sequoias. It was also explained as Gene stated - most of the wood cells in the tree are dead. They can be functional but cannot reproduce - one of the defining terms for a living organism. The cambium only a few cells thick is layers of longitudinal pipes, the only living tissue in a stem of a tree. Xylem conducts water up the stem and phloem carries the sugar down the stem. I recall a question being asked in class, maybe by me: If you could bore a tree stem out from the bottom of the tree without harming the cambium, could the tree survive? The answer was yes, assuming the tree had support and proper environmental conditions, water nutrients, etc.

New discoveries and theories have been brought forth in the tree world in the past 30 years, Iím sure, but this is how I recall things.