The main reason for its rare use is set-recovery, which reduces the degree of densification over time so that the mechanical properties deteriorate. For instance, hunters in north Eurasia and the American Arctic were known to harvest compression wood for bow staves, because the increased density and compression strength of this wood enabled them to make functional weapons out of tree species that would otherwise be unsuitable for this purpose, due to their low strength and low density.Thermo-hydro-mechanical (THM)-densified timber is rarely used in construction, although its mechanical properties are in many cases excellent. Traditionally, compression wood does have niche applications. Reaction wood also responds to moisture differently from normal wood. For instance, ripping a piece of reaction wood on a table saw without a splitter or riving knife installed can lead to kick back of the stock. This movement can occur during the milling process, making it occasionally dangerous to perform certain operations without appropriate safety controls in place. Reaction wood can twist, cup or warp dramatically during machining. The term ‘adaptive growth' therefore includes, but is not synonymous with, the formation of reaction wood.Īs a rule, reaction wood is undesirable in any structural application, primarily as its mechanical properties are different from normal wood: it alters the uniform structural properties of timber. The formation of reaction wood may act in conjunction with other corrective or adaptive mechanisms in woody plants, such as thigomorphism (adaptive response to flexure) and gravitropism (the correction of, rather than the support of, lean) and the auxin-controlled balance of growth rates and growth direction between stems and branches. In a leaning stem, the normal flow of auxin down the tree is displaced by gravity and it accumulates on the lower side. The controlling factor behind reaction wood appears to be the hormone auxin, although the exact mechanism is not clear. Its lignin content can be as high as 40%. Compression wood has only about 30% cellulose compared to 42% in normal softwood. Compression wood has a higher proportion of lignin than normal wood. Compression wood forms on the side of the plant that is under compression, thereby lengthening/straightening the bend.
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