Mature 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. 

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. 

All cells in the cambial zone are living. However, as xylem derivatives (i.e., developing wood cells) begin a sequence of transformations that will convert them into mature wood elements, they embark on a path of cell specialization or differentiation that will lead eventually (for fibers, vessel elements, and certain other cells) to cell death. 

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. 

When wood is laid down by the cambium of a living tree, two major types of wood cells are formed—thick-walled fiber cells that make wood strong and thin-walled parenchyma cells in which reserve foods are stored. Wood fiber cells die a few days or weeks after they are formed and lose their cytoplasmic contents as soon as they become functional in water transport. Thus, mature wood fiber cells consist almost entirely of cell wall polymers—cellulose, hemicellu-loses, and lignin. For this reason, wood fiber cells can be degraded only by organisms that have the ability to decompose these structurally complex high-polymeric materials. 

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. 

Wood quality varies within trees both in the radial and axial direetions. Burdon et al. (2004) propose a two-dimensional framework with the radial variations deseribed in terms of corewood and outerwood and the axial variations deseribed in terms of juvenile and mature wood. Arbitrarily, corewood has been described as a eylindrieal zone enclosing the first few growth rings around the pith. Typically, for fast grown pines this zone around the pith is considered to be of poor quality, having a number of undesirable features (Zobel, 1975) ... 

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. 

As one might expect, the within-tree stiflhess distribution reflects the corewood-outerwood and juvenile-mature wood patterns observed for many characteristics and properties furthermore between-tree variations are very noticeable. 

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... 

Yamamoto H (1998) Generation mechanism of growth stresses in wood cell walls roles of lignin and cellulose microfibril during cell wall maturation. Wood Science and Technology, 32 171-82... 

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... 

The moisture content of various forest biomass varies widely with species, geographic locations, genetic differences, tree components used, and tree age. Published data indicate that moisture content of mature wood may range from about 30 percent to more than 200 percent ( ). Also, moisture content of the stem sapwood portion is usually higher than that of the associated heartwood. For young hardwood sprouts (6 to 15 years old), an average... 

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. 

Chemical composition, as discussed in the next section, is closely related to the caloric values of biomass, and also affects the efficiency of conversion, particularly when using a biological approach. For example, the presence of phenolics, particularly lignin, presents a major roadblock for enzymatic conversion of polysaccharides to alcohol. The conversion of juvenile biomass has been shown to have a higher moisture content and lower specific gravity than mature wood (47), and may respond more favorably to such a treatment process. The energy conversion of juvenile biomass materials by a thermal or biological methods needs to be explored. 

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