The importance of factors contributing to the natural resistance of twelve Malaysian wood species was studied.
Twelve Malaysian hardwoods species were selected from three different durability
classes. The tests involved physical properties, fungal and termite natural durability, and extractives as antitermitic, antifungals and antioxidants. The wood densities were examined in order to know the variation between and within wood species. Statistical analyses indicated that significant differences existed between and within trees for density. Density was largely influenced by wood species while within tree, it was significantly affected by location of the samples.
For the natural durability of the outer heartwood of these twelve wood species, a series of tests against biological agents were done. Tests with two species of subterranean termites ( Coptotermes curvignathus and C. gestroi) and three species of white-rot fungi (Pycnoporus sanguineus, Trametes versicolor and Lentinus sajor-caju) showed that N heimii was the most resistant and H brasiliensis the most susceptible wood against all biological agents. However, the order of resistance and susceptibility of the other wood species varied to a minor extent to the challenge by termites and fungi.
Extractives were quantified from all species. Standard extraction with Toluene:
IMS (2:1) according to standard method ASTM D1105-96 (2001) was followed by hot water extraction. Results showed that high amounts of wood extractives were found in the durable class timbers and lowest from non-durable class. The highest yields were detected in bark and the lowest in heartwood.
Bioassay studies were done to investigate the toxicity of wood extracts to termites and fungi. Extracts from four selected wood species (N. heimii, M utilis, C. lanceolatum and S. curtisii) were impregnated into filter paper discs (termite test) and H brasiliensis blocks (fungus test) in different concentrations. The above termite species (Coptotermes curvignathus and C. gestroi) were used and the brown-rot fungus (Coniophora puteana) was added to fungus test together with T versicolor and L. sajor-caju. The study showed some very promising results. Both subterranean termites (C. curvignathus and C. gestroi) had high mo1iality with N. heimii extracts at the highest dose, 2%, whereas other species gave lower mortality. S. curtisii had the least effect with some 30% survival at the highest
dose. However higher concentrations were needed to protect the wood from fungal decay.
A study was also undertaken to examine the antioxidant activity of the wood
extracts of all twelve wood species. Two tests (total phenolic content and radical
scavenging activity - DPPH) were performed for bark and heartwood extracts. The
differences in heartwood mass losses were best explained by the antioxidant capacity as well as by the concentration of total phenols determined by the Folin-Ciocalteu method. A significantly higher total phenolic content and stronger antioxidant activity were measured in N heimii bark and heartwood extracts compared to the other extracts examined in this study. However, some of the extracts showed higher values than some well-known antioxidant-rich plant extracts and synthetic antioxidants.
Correlation analyses were run to explore the relationships between the properties studied. This study found that wood density had negative correlation with durability against termites but not with fungi. Wood extractive also has a significant influence on the durability rather than wood density. However, it is observed that higher wood consumption was generally obtained in species with lower wood density and lower extractive contents. The results of this study indicated that in some species there is a strong relationship between densities and extractive contents with durability and that this could be used as a tool to determine the durability of Malaysian hardwoods.
Correlation also exists between the total extractives and total phenolic contents in
majority of wood species studied. The total extractive contents had a significant positive correlation with total phenolic content and thus gave a significant correlation between durability and total phenolic content. It was also found that total phenolic content was significantly correlated with antioxidant activity.
In parallel to the termites, fungus, bioassay and antioxidant studies, the extractives of the most durable wood species, N heimii, were characterised by MALDI-TOF. Analysis of methanol extracts afforded two major polyphenols; stilbenes (resveratrols like derivatives as polymers) and flavonoids (kaempferol) which were suspected as defence against both termites and fungi. These two classes of compounds (polyphenols) also could be the reason for higher antioxidant activity in N heimii. A possible way ahead for wood preservation could be to examine in more detail the nature of polymerisation of such compounds inside wood cell walls during heartwood formation.