Wood structure

Wood is composed of cells, and is an anisotropic material. Commercial timbers are broadly classified into two categories, namely softwoods and hardwoods.

Softwoods are relatively simple in structure: more than 90% of the volume is composed of tracheids (Walker et al., 1993), axially elongate cells of 2 to 5 mm in length. Density will commonly be in the range of 350 to 700 kg/m³, basic density at 12% moisture content (Desch and Dinwoodie, 1996). Conversion and drying procedures for softwoods are better established than for hardwoods. A prime reason is that softwoods comprise the bulk of the wood used, and commercial plantings are mostly softwood. Research institutes in Europe and North America, also in New Zealand and Australia have devoted time to softwoods. The plantation area for softwoods is about 10,000 square kilometres in Australia according to the Australian Bureau of Agricultural and Resource Economics (ABARE, 2000) and about 17,000 square kilometres in New Zealand (source: New Zealand Forestry, 2002). Much of this is radiata pine (Pinus radiata)

The processing of hardwoods is more complex because of diversity. There are up to a hundred times as many species of hardwood trees as there are softwoods, and hardwood trees grow in a much wider range of ecological niches than softwoods. Density of commercial hardwoods commonly is in the range of 450 to 1250 kg/m3 basic density at 12% moisture content (Desch and Dinwoodie, 1996). Hardwood may have low lateral permeability, compared with softwoods. For example, the transverse permeability of green wood from Eucalyptus delegatensis is in the order of 4.6x10e-18 m2, whereas the permeability of green wood of Pinus radiata is 263 to 410x10e-18 m² (Langrish and Walker, 1993). Blackbutt (Eucalyptus pilularis) is a difficult to dry hardwood species (Bootle, 1994). Also the structure of hardwood shows a much greater range of variation than for softwood.