Heartwood durable

Table 9.1. Natural heartwood durability of certain timbers in groimd contact, based on 50 x 50 mm stakes indicative figures only. Hardwoods (HMSO, 1969) softwoods (Hughes, 1982).

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. 

The heartwood of various timber species shows a wide range of resistance to biological attack. The natural durability of heartwood) is assessed by ground contact tests and according to survival, classified into five ctegories - very durable (>25 years), durable (15-25 years) moderately durable (10-15 years) non-durable (5-10 years) and perishable, (<5 years). 

The polyphenols, i.e. stilbenes such as pinosylvin, in Pinus sp. are considered to be responsible for the better durability of pine heartwood compared to sapwood. The fiavonoid dihydroquercetin is considered to be responsible for the durability of Douglas fir heartwood while tectoquinone, 2-methyl anthraquinone, is responsible for the durability of teak and cedrol contributes to the durability of Chinese fir,... 

Historieally, heartwood of certain timbers would be used where decay was foreseen, a practice that declined with the rise in wood preservation. Today environmentally friendly solutions to decay relying on naturally durable wood are increasingly sought. 

Wood-boring molluscs and crustaceans are the most destructive agents of timber submerged in seawater. Indeed the marine environment is recognised as the most hazardous situation for exposed timber. The rate at which these organisms attack is dependent primarily on water temperature and the natural durability of the wood species, notably the heartwood which has greater natural resistance than sapwood. 

A recent review by Scheffer and Cowling (60) summarizes extensive evidence that the natural durability of many wood species is because of the toxicity of certain phenolic substances that are deposited in the process of heartwood formation. These substances act as poisons to the cellulolytic microorganism rather than by direct action on the enzymatic process of deterioration. 

Heartwood contains very few or no primary extractives, but usually significantly more secondary extractives than sapwood. The secondary substances include almost all the other extractive classes, hence they have enormous structural variety including most different substance classes like alkaloids, quinones, fats, oils, saponins, terpenes, tropolones, waxes and a number of aromatic or phenolic compounds (stilbenes, lignans, flavonoids, tannins, etc.). The secondary substances play an essential role in the use of wood, for example the heartwood of most of the wood species is more durable than the sapwood. 

VisorWood is furfurylated Scots pine (sapwood and heartwood) and is used for outdoor applications like cladding and decking roof boards, where durability is required. 

Heartwood of Scots pine is more durable than sapwood because it generally contains more extractives and absorbs less water. Heartwood is nearly always much less permeable to water than sapwood due to pit aspiration, obstruction by extractives, and vessel tyloses (outgrowth from an adjacent ray or axial parenchyma cell through a pit cavity in a vessel wall, partially or completely blocking the vessel lumen) [9]. 

Ebony is the name for wood species belonging to the genus Diospyros. Most prized are those with a deep black heartwood, extreme hardness and durability. 

Secondary cell wall formation is finalized by lignification and followed by programmed cell death, during which the plasma membrane collapses and the cell dies. Lignification proceeds from the middle lamella through the primary cell wall and inwards over the secondary cell wall. In the final stages of xylogenesis, the deposition of chemical components known as extractives, result in the formation of heartwood, which is the least permeable and most durable form of wood (Plomion et al. 2001). 

Although normally present in wood in relatively small quantities, flavonoids may be of importance in vivo in protecting the heartwood from fungal decay, along with other phenolics that may be present. Dihydroquercetin, for example, is one of the most potent fungicidal materials of Douglas-fir, a fairly decay-resistant species. In vitro, too, flavonoids may contribute to the long-term durability... 

Toxic extractable substances deposited during the formation of heartwood are the principal source of decay resistance of heartwood (45). This conclusion is based on the following observations 1) Extracts from durable heartwood are much more toxic to decay fungi than those from the sapwood of the same tree 2) extracts from nondurable heartwood have little inhibitory effect on decay fungi 3) extraction of the antimicrobial compounds with various solvents decreases the decay resistance of the wood from which the compounds were obtained 4) the toxicity of extracts obtained from heartwood of various species corresponds broadly with the decay resistance of those species and 5) generally, but not always, nondurable wood impregnated with extractives from durable heartwood increases in decay resistance (24, 45). 

The various compounds responsible for the decay resistance of heartwood, their structures, their biosynthesis, and their chemotaxonomy are discussed in Chaps. 7 and 8. Most of the important antimicrobial compounds are phenolics, although the terpenoids may also be involved. Often the major portion of the phenolic extractives are polymeric materials (MW > 500) that cannot be identified. These materials may be very important in increasing the durability of heartwood but little work has been reported. 

Durability of heartwood is dependent on the kinds and concentration of extractives present, their chemical stability, and their resistance to microbial inactivation. Many tree species (e.g., Madurapomifera or Robiniapseudoacacia) contain only one or two toxic compounds, often in very small quantities, yet the wood is extremely durable (45, 54). In other species a number of similar toxic compounds may occur together (Libocedrus decurrens (5)), or toxic compounds of a very different chemical nature may be present in the heartwood of a single species (stilbenes and ellagitannins in Eucalyptus sideroxylon (23). In either case, the combination of compounds acts additively or perhaps even synergistically to produce an inhibitory environment. 

Rudman P 1959 The causes of natural durability in timber. III. Some conspicuous phenolic components in the heartwood of eucalypts and their relationship to decay resistance. Holzforschung 13 112-115... 

Extractives Extractives are organic chemicals that can be extracted from wood with solvents. They are generally low molecular weight compounds with the content accounting for 2-5 % of the wood. Comparatively, softwoods have a higher extractives content than hardwoods and most of them are located in the heartwood. The component of extractives is responsible for some wood properties such as the color, smell, density, hygroscopicity, flammability, and durability. 

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