There are over 20 million hectares of eucalypt plantations in the world (Iglesias-Trabado and Wilstermann 2009). While most eucalypt plantations are grown for pulpwood, 1.2 million hectares are managed for sawlog production (FAO 2005). Log-end splitting is one of the many factors affecting the recovery of timber and veneer from plantation-grown eucalypts (Priest et al. 1982; Bariska 1990; Yang et al. 2005; Chauhan et al. 2006). It manifests immediately after felling and tends to increase further with time in storage.
Log-end splitting occurs as a consequence of growth stress (Kubler 1987). Growth stress is generated during cell maturation in the cambium zone (Yang et al. 2005). These forces produce tension in the cells near the cambium, while the cells near the pith are in compression. This contrast generates a residual stress distribution (Kubler 1987; Okuyama 1997; Chauhan et al. 2006). When a tree is felled or a log is cross-cut, the elastic energy of the residual stress is released through formation of end splits at the cross-cut surface (Okuyama 1997). In the absence of drying with shrinkage, cross-cutting creates a free surface in a pre-stressed volume (growth stress field) which leads to a redistribution of stresses. On a log end, this is equivalent to axially pulling its centre and applying a pressure on its periphery (Archer 1987). These forces induce heart splitting when the radial stress generated by the redistribution reaches the ultimate strength of green wood in the tangential direction. From a solid mechanics point of view, the initiation and progression of the cracks depend on the geometry of the log, the growth stresses, the elastic and non-elastic mechanical properties and the transverse strength or toughness of green wood (Jullien et al. 2003). The mechanical properties, tangentially to the log axis, can be significantly affected by grain angle and the end-split pattern
Growth stress and consequently log-end splitting varies within and among trees, as well as among sites, and are potentially influenced by genetic, environmental and silvicultural factors (Archer 1987; Kubler 1988). Genetic effects in Eucalyptus, for example, have been reported at the clonal (Malan and Verryn 1996), family (Barros et al. 2002), provenance and species levels (Nixon 1991). In addition, Nixon (1991) found that E. nitens showed the lowest level of splitting when it was compared among 13 different eucalypt species in South Africa. Environmental factors which can increase growth stress include those which cause stem re-orientation, such as slope, aspect and wind exposure (Archer 1987; Kubler 1987; Malan and Verryn 1996). These factors affect the manner in which the stem and crown are oriented, inducing a redistribution of internal growth stress in the stem (Kubler 1988; Mattheck and Kubler 1997). Wind pressure, for example, has been observed to promote splits at right angles to the main wind direction (Mattheck and Kubler 1997).
Silvicultural practices can also influence the magnitude of growth stress (Malan 1995). Thinning is the most important silvicultural intervention in terms of its effect on growth stress and log-end splitting because it affects competition among trees. Thinned stands, which experience lower competition, produce trees with high taper and large crown, which tend to exhibit low growth stress (Mattheck and Kubler 1997). In contrast, trees under strong competition have slender stems and narrow crowns and exhibit high levels of growth stress (Becker and Beimgraben 2002). However, the effect of competition on growth stresses may change with development stage of the stand (Biechele et al. 2009). When thinning is frequent and light, trees do not need to re-orientate their crowns to catch light, so growth stresses are likely to be lower. On the other hand, infrequent and heavy thinning results in drastic changes in the amount and direction of light, which may lead to high growth stress (Kubler 1988).
In addition to these genetic, environmental and silvicultural influences on growth stress, there are industrial factors that directly affect the magnitude of log-end splitting during processing. In veneer processing, the most relevant factors are storage time and log heating. Logs are commonly stockpiled to avoid production stoppages or to buffer the production system against market price fluctuations (Shmulsky 2002). In eucalypt logs, two phases of split development during storage have been described (Priest et al. 1982; Bariska 1990 ). The first phase starts immediately after felling, with splitting increasing rapidly until 6 to 20 days after felling. This is followed by a second phase when splitting is significantly slower. In terms of the wood supply chain, the first splitting phase usually occurs between felling and the log-yard, whereas the second phase is usually evident after storage in the log-yard prior to processing.
Heating hardwood logs using hot water or steam is common practice prior to peeling for veneer (Shmulsky 2002). This is done to improve veneer yield, smoothness and thickness uniformity and to reduce energy consumption (Becker and Beimgraben 2002 ; Dupleix et al. 2012). In species prone to developing high growth stresses, log heating has been reported to relieve growth stresses and improve timber quality (Severo et al. 2010). However, heating logs at high temperatures can also exacerbate log-end splits (Marchal et al. 1993). This is believed to be due to the expansion and contraction of wood associated with heating and cooling—a phenomenon called hygrothermal recovery (Kubler 1987). Tangential dimensional change in the wood is the most important driver of log-end splitting as it is far greater than radial and longitudinal dimensional changes (Kubler 1987; Marchal et al. 1993). The magnitude of split propagation within a log depends not only on the inherent growth stresses but also on the temperature and duration of heating (Marchal et al. 1993; Gril and Thibaut 1994; Becker and Beimgraben 2002; Dupleix et al. 2012).
The present study aimed to evaluate the relative importance of the above factors on the magnitude of log-end splitting using Tasmanian plantations of Eucalyptus nitens (Deane and Maiden) Maiden. Plantation-grown trees of this species can produce log-end splits when felled and cross-cut into logs (Blackburn et al. 2011; Valencia et al. 2011). E. nitens is widely planted in temperate regions of the world (Hamilton et al. 2011) and is the second most prevalent hardwood plantation species in Australia (Gavran 2014). Within Tasmania, there are approximately 208,400 ha of E. nitens plantations (Hamilton et al. 2008; Gavran 2014) which are mainly managed for pulpwood. Fifteen percent of these plantations, however, have been pruned and thinned to produce high-quality logs for sawing and/or veneer production (Forestry Tasmania 2011). It is anticipated that these pruned and thinned plantations will be used by Tasmania’s forest processing industries along with traditional sources of hardwood logs from the island’s native forests (Forestry Tasmania 2011).
A few studies have investigated factors affecting log-end splitting or growth stresses in E. nitens. With respect to genetic factors, Blackburn et al. (2011) showed that trees grown in a Tasmanian trial of families from different geographic races exhibited significant genetic variation in log-end splitting within the races. However, no significant genetic difference has been reported between races of this species (Blackburn et al. 2011). In terms of silviculture, Valencia et al. (2011) showed that thinning to different spacings did not affect log-end splitting after accounting for the positive effect of stem diameter. Several recent studies of Tasmanian E. nitens have shown log-end splitting increases with height up the tree (Blackburn, Hamilton et al. 2011; Valencia et al. 2011). We are not aware of any previous studies investigating the effect of post-felling treatments, such as storage and steaming on log-end splitting for E. nitens.
The specific objective of this study is to quantify the development of log-end splitting in plantation-grown E. nitens and determine the influence of:
Site, log position, diameter and inter-tree competition
Log storage time
A pre-peeling steam treatment
Improved understanding of the factors that determine log-end splitting will contribute to improved use of the E. nitens plantation resource and ultimately to more effective forest management.