Softwood compression wood

Other distinct classes of wood in a tree include the portion formed in the first 10—12 years of a tree s growth, ie, juvenile wood, and the reaction wood formed when a tree s growth is distorted by external forces. Juvenile fibers from softwoods are slightly shorter and the cell walls thinner than mature wood fibers. Reaction wood is of two types because the two classes of trees react differentiy to externally applied stresses. Tension wood forms in hardwoods and compression wood forms in softwoods. Compression wood forms on the side of the tree subjected to compression, eg, the underside of a leaning tmnk or branch. Tension wood forms on the upper or tension side. Whereas in compression wood, the tracheid cell wall is thickened until the lumen essentially disappears, in tension wood, tme fiber lumens are filled with a gel layer of hemiceUulose. 

Nimz H, Robert D, Faix O, Nemr M (1981) C-13 NMR spectra of lignins 8 Structural differences between lignins of hardwoods, softwoods and compression wood Holzforschung 35 16-26... 

Thioacidolysis of extractive-free softwood meal or softwood MWL essentially leads to the guaiacyl structures listed in Table 6 4 2 and referred to previously in the section on thioacidolysis of hardwood lignins When subjected to thioacidolysis, compression wood additionally gives rise to significant amounts of p-hydroxyphenyl QC-, monomeric compounds (Lapierre et al 1987)... 

Reaction wood in softwoods is normally concentrated on the underside or lower side of the affected tree or branch. Because the wood in such regions appears to be subjected to compressional forces, this wood is given the nontechnical designation of compression wood (Figure 30). In hardwoods the location and concentration of reaction wood tissue is normally opposite that in softwoods, i.e., on the upper side of branches and leaning stems, and the tissue is called tension wood (Figure 31). Reaction wood in softwoods and hardwoods is also located in the vicinity of points on any tree bole where branches originate. 

Compression Wood. The formation, structure, and chemistry of compression wood have been given considerable attention (see Reference 30 and references cited therein). Compression wood tissue exhibits both major and subtle differences from normal softwood xylem. These differences (Table II) are either directly or indirectly responsible for any distinguishable changes in the behavior and treat-ability of compression wood versus the behavior generally noted for normal wood. 

As with softwoods, the stems and branches in hardwoods that are subject to asymmetric loading produce wood that is characterized by different anatomical, chemical and physical properties. Thus tension wood is quite common in vertical stems, especially below the point of attachment of lateral branches. However, more usually it is associated with inclined stems and branches, the upper side shows a marked eccentricity with wider growth rings. This is the opposite situation from softwoods where the eccentricity and modified wood lies to the lower side of the inclined stem or branch. The modified wood in the eccentric growth in hardwood is referred to as tension wood on account of its position, and like the compression wood of softwoods, it develops in response to stress and acts to straighten the leaning stem. Tension wood is usually harder and denser than normal wood and is sometimes darker in colour. In sawn timber it shows up as having a woolly appearance. 

Clearly shrinkage anisotropy is a eomplex issue. A number of faetors ean contribute and the relative importance of each will vary between timbers. In some cases a large microfibril angle might be significant, as in corewood and in compression wood. Ray tissue will be important in species such as beech and oak. Contrasting earlywood and latewood densities is a likely cause in Douglas fir, but would be irrelevant for a tropical hardwood. The effects of elastic anisotropy would be more apparent in low density softwoods. 

If the stem of a young sapling is bent into a loop in the vertical plane, then at the top of the loop the uppermost part of the looped stem is stressed in tension and its underside is in compression whereas at the bottom of the loop the stresses are reversed. In such a looped stem softwoods form compression wood tissue on the underside at the top of the loop (where the bending stress is compressive) and on the underside at the bottom of the loop (where the bending stress is tensile), whereas hardwoods form tension wood on the upper part of the stem in both positions, i.e. compression wood can form in that part of the stem that is in tension as well as that part of the stem that is in compression, and similarly for tension wood. 

Compression wood forms on the underside of all softwood branches and tension wood on the upper side of hardwood branches. Thus reaction wood is responding to and resisting the additional incremental mass of branch wood that is weighing it down and which would otherwise result in the branch drooping from its preferred inclination. Most curious, if a branch is forced upwards compression wood forms on the upper side with softwoods, while tension wood forms on the lower side with hardwoods. Reaction wood is seeking to maintain some pre-determined, natural branch angle. 

In summary and confusingly, under certain circumstances compression wood (in softwoods) and tension wood (in hardwoods) can each be found on the underside or upper surface of a branch/stem, and they can also be found where the local bending stress is tensile or compressive. 

Reaction wood also forms in the stem immediate below branches. It is a continuation of the reaction wood tissue in the branch downwards into the stem. In softwoods the volume of this associated compression wood can be from one to several times the knot volume (Von Wedel et al., 1968). 

Softwoods compression wood Curious Found in aU genera... 

Burdon (1975) in a study of 12 yr-old Pinus radiata growing on four sites estimated that 30-45% of the stems eontained mild to severe compression wood although Timell (1986) suggested that 15% would be a representative figure for virgin spruce forests and for plantations of southern pines and Pinus radiata. Lindstrdm et al. (2004) reeorded eompression wood oeeupying from 6-30% of the stem eross-section in 3 yr-old elones of radiata. Whiehever number is taken, there is a lot of eompression wood in young softwoods. 

Nimz, H. H., Robert, D., Faix, O., and Nemr, M. (1981) Carbon-13 NMR spectra of lignins, 8. Structural differences between lignins of hardwoods, softwoods, grasses and compression wood. Holrforschung 35(1), 16-26. 

There are definite changes in the chemical composition of reaction wood. Compression wood has a significant increase in lignin and a corresponding decrease in polysaccharides as compared to normal softwood. Tension wood has just the opposite relationship. Since juvenile wood tends to contain a high level of reaction wood, its chemical composition should differ from that of mature wood. 

As plants cannot move, they have developed several features to adapt to their environment such as the effects of wind and site slope. Reaction wood is part of this capacity for adaptation it allows the plants to maintain or modify the orientation of different axes in space. In softwood trees, the reaction wood produces growth stresses smaller than in normal wood the tensile level is smaller and can even become negative in this compression wood. In hardwood trees, reaction wood forms wood with higher growth-stress level than in normal wood and is known as tension wood. For example, branches are horizontal elements that must work against gravity. In the case of hardwoods, tension wood is present in the upper part of the cross sections of branches whereas, in the case of softwoods, compression wood is found in the lower part. However, the compression wood can also be found in some primitive groups of hardwoods (Carlquist, 2001). 

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