While the wood was part of the standing tree, sap flowed trough their vessels. Thus, how can there be water-soluble extractives in a dried wood sample if many liters of sap have passed by it over the years carrying these extractives solubilized in the sap?
Perhaps I misunderstand the question, but trees are about 50% water weight when harvested. When you cut them, the water/sap stops moving. When you dry them the soluable components don't volitolize with the water.
Extractives are formed when water stops flowing, which takes place during heartwood formation. The still living cells switch from aerobic to anaerobic conditions and form chemical substances, which we call "extractives". Some will be water soluble. Example: Larix has up to 30% d.w. water-soluble extractives (arabinogalactans) located in the lumina and the apparent free space of the cell walls. But thats all in the heartwood, the sapwood has none.
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Dear M Eisenbies,
Thank you for the response. That was exactly what I thought. These soluble water extractives represent a small proportion of chemical components that remained in the wood after the evaporation of water and not volatilized when the wood dried.
Dear prof. R Wimmer,
Thank you very much for your explanation and suggested papers. You raised interesting questions. I had never thought about changing the aerobic and anaerobic condition during sapwood to heartwood transition. However, some questions remain:
i) The content of extractives soluble in water in a living tree is different from the content of these extractives on a dried wood?
ii) What does it mean that a (dry) wood presents extractives content of 5%? What is the relationship of this information with the actual content of extractives on the wood of a standing tree?
When a heartwood, which has a moisture content at or under fibre satuation has lets say 28% MC, is then dried down to lets say 5% MC, why should the wood loose extractives? At and under fibre satuation water is located in the cells walls only and does not fill up the lumens. The high-volatiles ones might get lost in part. 5% means, if you have 100 kg of wood (at zero moisture), 5 kg of it refers to an extractable content (whatever is extracted). As this extractive content is the same as in the heartwood of a standing trees, the question about the "relationship" is not important. If you include some water content, then the precentage will change, since we are calculating relative contents here. Or am I not getting something?
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