Variability sapwood

Permeability. Although wood is a porous material (60—70% void volume), its permeability (ie, flow of liquids under pressure) is extremely variable. This is due to the highly anisotropic shape and arrangement of the component cells and to the variable condition of the microscopic channels between cells. In the longitudinal direction, the permeability is 50 to 100 times greater than in the transverse direction (13). Sapwood is considerably more permeable than heartwood. In many instances, the permeability of the heartwood is practically zero. A rough comparison, however, may be made on the basis of heartwood permeability, as shown in Table 3. 

Flavonoid compounds are common in other softwoods. Both Douglas fir and western larch contain the dihydroquercetin (Gardner and Barton, 1960), the distribution of this constituent being rather variable within the tree, but is highest at the heartwood-sapwood boundary. In studies on New Zealand grown Larix decidua and Larix leptolepis (Uprichard, 1963) it was shown that, in the 45 yr-old trees examined, the flavonoid polyphenols (dihydroquercetin and dihydrokaempferol) inereased steadily from the centre of the tree to the heartwood-sapwood boundary after whieh polyphenols dropped to neglible amounts. 

While the durability of many species has been evaluated with post or stake tests (Figure 9.1), evidence for durability of other species is largely anecdotal. A comprehensive review by Scheffer and Morrell (1998) has helped to collate the literature related to durability for a wide range of wood species. Further, usage is also limited by variability in durability. For some species there are wide differences in heartwood durability between adjacent trees and even between boards cut from the same tree. Also boards can contain both sapwood and heartwood as it is often not economic or practical to cut timber so as to exclude all sapwood. Thus only broad estimates of durability can be developed (Table 9.1). As a result of these sources of variability the use of naturally durable species is often restricted to aboveground applications where the biodeterioration hazard is lower and the consequences of an early failure are less severe. 

For 1981, average male dry weight production was Inversely related to the concentration of limonene and carene, and to trees with large diameter at breast height (DBH) and large sapwood/ basal area ratios (Table IV). Male biomass production was positively related to terpene evenness and high bornyl acetate and camphene concentrations. Five of the seven variables (71%) in the model were secondary metabolites. Nitrogen did not enter into the model as an Important determinant of male biomass production. 

To summarize, the analytical information of archaeological or ancient wood shows variability in chemical changes and losses that may result from the burial environment, the wood species, sapwood or heartwood, outer or inner wood, anomalies in growth, and certainly the methods of analysis. The stability of wood biopolymers has been found to be (in decreasing order) lignin, pectin, cellulose, and hemicellulose. As a rule, increase in moisture content indicates increase in degradation. 

Understanding adhesive-wood cell interactions is more difficult because of the tremendous variability in wood cell types. With tracheid, parenchyma, and fiber cells, vessels, resin canals, and ray cells that vary in composition and structure in the earlywood, latewood, sapwood, and heartwood domains, there is a tremendous variety of bonding surfaces, each of which may interact differently with the adhesives. The most dramatic difference is often between wood species because of the large difference in cellular architecture. Bonding of different species often requires changes in adhesive formulation to control penetration into the wood. Although some work has been done on determining penetration into cell lumens and walls [6], this information is usually not related to the performance of the bonded assembly. 

The transverse permeability and the anisotropy ratios are very variable. The heartwood part of logs is usually much less permeable than the sapwood part (Comstock, 1967). This is due to tyloses development and extractives deposition (tannins, gums, etc.) in hardwoods and due to the aspiration of bordered pits in softwoods. 

Wood density is regarded as the easiest and most reliable measure of wood quality. Data from the drying of 106 North American hardwoods show that the volumetric shrinkage of wood increases with its basic density (Stamm, 1964). There is also a correlation between the permeability and the basic density for the sapwood of a softwood (Nijdam and Keey, 2000). Presorting a sapwood load into high-density and low-density groups can reduce the moisture variability of the final recombined kiln-dried boards. 

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