Juvenile 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. 

Fast grown softwood from plantation sources is generally characterized by a high proportion of juvenile wood and often poorly developed heartwood. A fast rate of growth results in wide growth rings, producing low-density timber that exhibits inferior mechanical... 

It is possible that there will be differences in the reactivity of heartwood and sapwood, or between juvenile and mature wood in acetylation reactions. As a consequence, larger-dimension wood specimens containing both heartwood and sapwood, or mature and juvenile wood, could be liable to distortion when modified. Considering the importance of this to any commercial acetylation operation, it is surprising to see that there is, in fact, very little literature dealing with this issue. 

In a study of acetylation of loblolly pine oriented strand board (OSB) flakes, it was found that reactivity of the wood increased in proportion to the juvenile wood content, when moving from the base to the top of the tree (Hon and Bangi, 1996). However, no difference in reactivity was found in moving from the core to the outer shell (i.e. radially). Rowell and Plackett (1988) found no difference in the reactivity of the heartwood and... 

The value of kML is likely to be in the vicinity of 1.7 for mature normal softwoods (Saka et al. 1982). However, compression (Westermark 1985) and juvenile woods (Donaldson and Ryan 1987) are reported to have a different correction factor. This may be due to the fact that such abnormal softwoods contain different types of lignins in both the secondary wall and middle lamella regions. 

Figure 35. The general location cf juvenile wood and knotwood in tree stems. (A) Cross-section qf a SiUsa spruce dwwing portions of two branch traces (knots). (Reproduced from Rff. 39. CopyrWtt 1982, American Chemical Society.) (B) Schematic indicating the tupUm location qf juvenile wood or corewood in softwood tre. ...

At the ultrastructural level, juvenile wood fibers have a much greater S2 microfibril angle than normal mature wood fibers. The net... 

There is limited information regarding potential differences in the nature and amount of juvenile wood and mature wood hemicel-luloses (2). What data are available indicate that there can be limited changes in the relative amounts of some simple sugars, but there is apparently little or no significant difference for either softwoods or hardwoods. 

The amount and nature of extractives in normal juvenile wood are generally similar to those of mature sapwood in the same tree, except that extractives near the tree center can be altered with time in many trees by the development of heartwood. Specifically, the amount and toxicity of juvenile wood extractives may both increase from the pith to the heartwood/sapwood boundary (2). However, resin extractives are a somewhat special case. In trees with or without heartwood, resin content is reported to be highest near the pith and at the tree base. It then decreases outward and upward in the stem (2). 

Traditionally, around the world the terms juvenile wood and mature wood have been taken to relate to cambial age, i.e. juvenile wood is the wood surrounding the pith that is formed by the young (juvenile) eambium. Confusingly, in some Southern Hemisphere countries the terms corewood and outerwood refer to the same radial gradient in wood quality. To avoid - or add to ( ) - any potential confusion, this text follows the new convention proposed by Burdon ei al. (2004) that has yet to achieve broad consensus. 

While the juvenile zone only extends upwards for a few metres, this zone is encompassed by the butt log that, by tradition, is valued because of its size. As with the corewood-outerwood boundary, there are tree-to-tree and species-to-species variations in the vertical extent of juvenile wood and in the rate of change in wood quality toward the tree top. Cuttings from older branches lack juvenile responses to silvicultural practices and the benefits of physiologically aged cuttings are improved form and stiffness, i.e. minimizing the juvenile core but at the cost of some loss of vigour. 

Indeed in some parts of the world plantation-grown trees harvested for commercial uses are composed entirely of juvenile wood, examples being 10-15 yr-old Paulowania and Populus sp. in parts of China (Bao et al., 2001). Fortunately the differences between corewood and outerwood, and between the juvenile wood and mature are not nearly as obvious in hardwoods (or maybe they are less well categorized). 

As just noted, for many plantation species the corewood is of lower density than is the outerwood. Further Figure 5.5 indicates that there is little differenee in basic density between the corewood-mature wood zone in the topmost part of the stem and the corewood-juvenile wood in the butt log that had formed years earlier when the green crown of the younger tree was much lower. 

Bao FC, Jiang ZH, Jiang XM, Lu XQ and Zhang SY (2001) Differences in wood properties between juvenile wood and mature wood in 10 species grown in China. Wood Science and Technology, 55(4) 363-75... 

Bhat KM (1999) Is fast grown teak inferior in wood quality An appraisal from plantations of low input management. Wood News, 5(4), 27-31 Bhat KM, Priya P and Rugmini P (2001) Characterisation of juvenile wood in teak. Wood Science and Technology, 34 517-32... 

Clark A and Saucier JR (1989) Influence of initial planting density, geographic location, and species on juvenile wood formation in southern pine. Forest Products Journal, 59(7/8) 42-8... 

Zohel BJ (1975) Using the juvenile wood concept in the southern pines. South Pulp and Paper Manufacturers, 38(9) 14-6... 

TF Yeh, JL Braun, B Goldfarb, HM Chang, JF Kadla. Morphological and chemical variations between juvenile wood, mature wood, and compression wood of loblolly pine (Pinus taeda L.). Holrforschung 60 1-8, 2006. 

A major potential between-stand difference for loblolly pines is caused by the density and moisture content differences between juvenile and mature wood. The southern pines produce juvenile wood during an approximate 10-year period beginning with germination. The juvenile wood type has larger cell lumens, resulting in increased moisture content because more water is contained in the cells. Mean southern pine juvenile wood moisture content is reported to be... 

Zobel, B. J., Sprague, J. R. (1998). Juvenile wood in forest trees. Berlin Springer-Verlag. 

Despite the success of many adhesives in a variety of applications, other challenges still exist with wood adhesives. How can we get equal performance out of a lower cost adhesive As the wood supply changes from old growth wood to more juvenile wood, how do we develop the proper adhesives, and how do we address performance standards when the wood itself is weaker ... 

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