Date Published: December 17, 2012
Author(s): Daniela Tudor, Sara C Robinson, Paul A Cooper.
Eight fungal species known to produce wood pigmentation were tested for reaction to various moisture contents in two hardwood species. Fungal pigmentation by Trametes versicolor and Xylaria polymorpha was stimulated at low water concentrations in both Acer saccharum (sugar maple) and Fagus grandifolia (American beech), while Inonotus hispidus and Polyporus squamosus were stimulated above 22-28% and 34-38% moisture content in beech and in sugar maple respectively. Fomes fomentarius and Polyporus brumalis produced maximum pigmentation in beech at 26 – 41% and in sugar maple at 59 – 96% moisture content. The pink staining Scytalidium cuboideum pigmented both wood species at above 35% moisture content. This research indicates that controlling the moisture content values of wood substrates can stimulate the intensity of pigmentation of specific fungi when spalting wood for decorative and commercial purpose.
Spalted wood is considered a value added product and can be produced by selected fungal inoculation of the wood substrate to create unique patterns (Robinson et al. 2007), and its considerable artistic and economic value is already established (Nicholls 2002; Donovan and Nicholls 2003a, b). The main characteristics of spalting are the spatial barrier demarcation of the fungal colonies by black pigment deposition, and the distinct colored zonation (Figure 1). The black pigment is a melanin type pigment (Butler and Day 1998) and it has a protective role against unfavorable environmental conditions (Campbell 1934; Pearce 1991; Henson et al. 1999). The stained wood of various colors is an effect of secondary metabolites in the form of pigmented fungicides, produced in the wood substrate by specific fungi (Margalith 1992; Duran et al. 2002). Wood moisture content (MC) is one of the most important conditions that influence fungal behavior and wood colonization patterns (Boddy 1983b). It is established that optimal fungal growth is achieved at 35–50 % MC on a dry weight basis, with a minimum required of 20–30 % necessary for fungal development; the values vary for different fungal species and inhabited wood substrates (Cartwright and Findlay 1958; Rayner and Todd 1979). Cartwright and Findlay (1958) also mention the ability of some fungal mycelium to survive below the fiber saturation point moisture content (26- 27% of the dry weight for most wood species), while spores, as well as mycelium of several fungal species, can survive for many years in dry condition (Schmidt 2006). Very high wood moisture content also inhibits fungal activity in wood substrate by limiting the quantity of the oxygen available in wood, preventing degradation (Cartwright and Findlay 1958; Boddy 1983a, b).
For commercial and artistic purposes, spalting should result in a high intensity of wood pigmentation, while minimizing the loss in strength and integrity of the wood after exposure to fungal activity. The natural wood resistance to decay is influenced by many other factors such as wood extractives and ambient temperature. In our tests, wood integrity was reflected by mass loss measurements at the end of period of incubation. As expected, based on the specific gravity of the wood species tested, sugar maple samples proved to have higher degradability than beech, with few exceptions. Polyporus brumalis and S. cuboideum degraded both wood species at the same rate, and F. fomentarius was more effective for beech degradation, within the same treatments of water availability. However, sugar maple is more prone to spalting, as the occurrence of fungal pigmentation and zone line is much higher than in beech.
MC: Moisture content; WHC: Water-holding capacity.
The authors declare that they have no competing interests.