Wood quality Juvenile wood and its implications Dave Cown and Leslie Dowling Abstract Juvenile wood has been known as a concept since the 1960s, but its full impact has not been felt until it comprises a major proportion of the wood harvested. A conservative estimate would put it currently at around 50 per cent by volume of wood in a standing tree. Its characteristics vary according to a number of internal and external factors but it consistently represents the lowest density, highest microfibril angle, and highest spiral grain within each individual stem. For most high value wood products it is considered undesirable. Unless there are changes in the overall approach to management of wood quality in radiata pine, it will have an increasing financial impact on plantation forestry due to the proportion of the harvest that will not meet users’ requirements. What is juvenile wood? Much of the research that has been done on wood quality in New Zealand has been concerned with aspects of juvenile wood. The term ‘juvenile wood’ is a convenient method of describing the wood formed around the pith in radiata pine and it has loosely been defined as the inner 10 to 15 rings (Burdon et al., 2004). This has significant ramifications for tree growers and wood processors when silviculture and breeding approaches are adopted to enhance growth. This is because of the resulting wide annual rings, low wood density (DENS), high microfibril angle (MFA), high spiral grain (SGA) and knot frequency (Cown, 1992), which collectively result in high variation in stiffness and strength (Watt et al., 2013). Despite its economic importance, there is no universally accepted definition of juvenile wood and hence it has been difficult to quantify its real importance in forest management. We know that the zone occupies a very significant proportion of radiata pine stems at rotation age, and effectively it means that upper logs in particular comprise predominantly juvenile wood. Wood property gradients – strength and stiffness, stability – are greatest within this zone and tend to stabilise afterwards in the mature wood. What determines the extent of juvenile wood? Given that juvenile wood occupies the inner part of the tree, to what extent can its impact be modified? Evidence to date suggests that the extent is strongly biologically controlled rather than by external influences. This means that the internal patterns of wood properties will remain constant, although the actual values will vary according to site, silviculture and genotype. Management practices that encourage early growth rates – fertilisation, initial spacing, weed control, thinning – will increase the physical volume of juvenile wood produced as well as the proportion at harvest. In the worst case scenario rapid early growth followed by stagnation, e.g. highly stocked un-thinned stands on poor sites, will result in stems comprised of mostly juvenile wood. Standing tree acoustic tools may indicate ‘high stiffness’ in such stands, but this only applies to the outer shell of mature wood which will likely be lost during solid wood processing. Recent analyses of spacing trials have indicated that early growth rates are not highly influenced by spacing except at the extremes (Carson et al., 2014). The greatest impact on juvenile wood content is therefore thinning and the rotation age – thinned and older stands will have a greater proportion of mature material with better stem average wood properties. Wide juvenile growth rings. Source: Scion Digital Print Library 10 NZ Journal of Forestry, February 2015, Vol. 59, No. 4