Seasonal Moisture Content of Living Trees

      Moisture content of living tree wood is constant throughout the year. May 31, 2010

Question
I hear a lot of anecdotal evidence that the moisture content of trees varies through the seasons. Is there any truth to it? It certainly seems that ash trees are pretty dry in the winter.

Forum Responses
(Sawing and Drying Forum)
From contributor T:
I would love to learn anything about this - it's a very touchy topic between me and some much older friends! How about the moisture going down with the phases of the moon?! The moon can move the ocean with its gravitational pull, so why not the water in trees? My argument is that if the water going into the roots is low tide, then the equivalent of high tide is the water going into the branches. This kind of throws my friend's argument off, as he does not want to believe the water concentrates into the branches. Who knows, maybe this is actually how a tree lives and gets moisture to its branches? I don't know what to believe! Scientific evidence please!



From contributor U:
Read what the Wood Doc has written - for example, where does the water that travels down the tree into the roots go? Wouldn't it make more sense to think of the tree as a system of capillaries that draws water up from the soil and expires it through the leaves?


From contributor A:
Trees are always the same wetness inside if they are alive. The little tubes (xylem) carry water up to the leaves (capillary action). When the leaves fall in the fall (deciduous trees) the water has to remain in the tubes. If they dry out or get an air bubble then the capillary action will be lost and the leaves will not come back in the spring. The taller the tree the more important it is to keep the system going. That is why most very tall trees are evergreens. It is also why no trees are over 420 ft tall.

The sugar content of the tree changes with the season of the year and I think the phrase "the sap is down" comes from this, when the leaves are gone and not making sugar (deciduous trees). But the tree is just as wet inside. White oaks plug their tubes as they grow and red oaks do not.



From contributor U:
A tree with a bacterial infection can have a much higher MC than a non-infected one - red oak is a prime example around here. They can reach 200% MC and be ruined for most uses.


From Professor Gene Wengert, forum technical advisor:
It is correct that the MC of a living tree does not change more than 2% from season to season. However, as noted, the moisture does move more in some seasons than others.

I am not aware that the sugar content changes (like in Feb or Mar when the maple trees are tapped) as I cannot imagine where the sugar would be stored or how it could be generated if the leaves are not on the tree. If it were stored in the roots, then what replaces that sugar when it leaves the roots?

It is easy to calculate the maximum MC that wood can have when it is soaking wet without any air bubbles. The density of solid wood is about 1.5 times the density of water. Depending on species, it can be 150% MC maximum for US species, but I do not think that it can reach 200% MC, unless the wood is decayed. The bacteria do not decay the wood.

You can calculate the maximum MC using a formula 3-1 in the US Dept of Agriculture "Wood Handbook." For red oak (SG = 0.56), it is 114% MC. If the basic wood density, expressed as specific gravity (SG), of a species is 0.40, it is 185% MC. Of course, these values are for 100% elimination of all air, which is probably impossible, so the actual MC that we will see will be lower.



From contributor D:
I can't claim much more than having stayed at a Holiday Inn Express, but I believe sugar can be converted to cellulose and also to starch to be alternately stored by the rays and then turned back to sugar for a burst of energy in spring.


From Professor Gene Wengert, forum technical advisor:
I am not much of a chemist, but I do believe that cellulose is built using sucrose sugar molecules. Any conversion from cellulose to sugar at very cold temperatures in the late winter would be very slow. Further, that would mean that the density or specific gravity of the wood substance would change seasonally. On the other hand, the ray cells do store starches and sugars, so it makes sense to me that it is from the content of these cells that the sugar in the sap comes. Or it could be that the sugar content in the sap is the same, but it is only that it starts to flow in the later wintertime. It is very interesting that the tree itself seems to get a signal to enhance sap flow in the early spring or late winter, even though the leaves are not yet developing. I have wondered how this message is conveyed throughout the tree, especially at cold temperatures when chemical reactions are very slow. It is certainly an amazing piece of engineering.


From contributor M:
I've cut cherry bark red oak in the dead of winter and after a good rain. I've seen water drip and puddle up as it was bucked up. Roots suck up water as it is available year round.


From contributor D:
It's no secret that it is highly stressful to keep watering freshly transplanted trees throughout the winter. Along those lines... one winter I saw two large apple trees that had blown over about a month before. They produced large root balls and were still alive. Many of their exposed roots were translucent and their texture was somewhat akin to a carrot. These translucent roots were ~1" in diameter and were an off-white color. You could flex them slightly and stick your thumbnail into them and they would snap in two. Their texture seemed more vegetable than wood. They seemed to be swollen with food and moisture.

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