Genetic and environmental controls of traits affecting profitability of pulp and solid-wood production from plantation-grown <i>Eucalyptus nitens</i>
Creators: name: Rocha-Sepv¿lveda, MF
Publisher: name: University Of Tasmania
Year: 2023

description: This thesis investigates the phenotypic and genetic variation of key wood traits of Eucalyptus nitens affecting pulp-wood and solid-wood production systems. E. nitens is grown in cool-temperate regions mainly for pulp production, but increasingly for solid-wood products such as timber and veneer. Tree breeders and forest managers thus need to understand how genetics and environment impact both product streams. Four studies were undertaken. In the first, an automated system for quantifying drying defects that degrades timber (internal wood checking, shrinkage and collapse) was developed. The second study applied this automated system to explore factors affecting within and between-tree variation in internal checking across three thinning trials. The third study applied machine learning to model the plastic response of growth, wood density and pulp yield to variation in climate, elevation and geography based on data from 86 pulpwood plantations of common genetic origin. The final study collated and analysed data on pulp-wood and solid wood selection traits from two progeny trials comprising 420 open-pollinated families collected from three native races. It focused on the estimation of genetic parameters for the microfibril angle - a critical wood anatomical feature closely linked to stiffness, which has been rarely investigated in genetic studies - and examined its genetic correlations with both pulp-wood and solid-wood selection traits. A fully automated system developed for the quantitative assessment of drying defects using digital images of disc-derived wood wedges provided a time-efficient and accurate method to measure internal wood checking and shrinkage-related traits. Its application to the study of within-tree and between-tree variation in internal checking in E. nitens showed that pruned bottom logs are more susceptible to checking than upper logs. Radially, checking was concentrated in the transition between inner and outerwood. At the tree level, checking was highly variable depending on site, thinning treatment and tree dominance status. Dominant trees in thinned plots had higher checking than the dominants in unthinned plots, but the opposite trend occurred in suppressed trees. While high checking in the valuable bottom log can adversely affect higher-grade product recovery, there are opportunities for log and board segregation to minimise the adverse effects of checking. Models to predict internal wood checking were developed using predicted wood chemistry from global NIR (Near-infrared) spectroscopy models, drilling resistance and shrinkage-related traits as explanatory predictors but these had low predictive power and were not operationally useful for the non-destructive prediction of checking. Characterisation of the E. nitens pulpwood plantation estate in northwest Tasmania showed growth, wood density and pulp yield were influenced by site climate. Growth was mainly influenced by temperature while wood density was mainly affected by rainfall-related variables. Plasticity in wood density was independent of plantation growth rate and density decreased with an increase in annual rainfall and elevation. The model for pulp yield had the lowest predictive accuracy and its variation was influenced by a mix of climatic and geographic variables but independent of growth and wood density. Using these models, the plastic response of these key pulpwood traits to climate was mapped across the E. nitens plantation estate in northwest Tasmania, to help predict current plantation attributes and future site choice. The quantitative genetic study of microfibril angle revealed this trait showed significant additive genetic control (h<sup>2</sup> = 0.27±0.01) and no significant genotype by environment interaction. Microfibril angle was genetically independent of wood density, but strongly genetically correlated with board stiffness. Microfibril angle was favourably genetically related to pulp yield, meaning that breeding for solid-wood and pulpwood may be compatible and suggests that current selection for pulp yield may have indirectly improved wood stiffness. Thus, for a fully integrated company with diversified management and breeding objectives, the improvement of E. nitens for pulpwood and solid-wood appear compatible. This thesis demonstrated the complexities in predicting wood traits and showed how key pulpwood and solid-wood selection traits of E. nitens may be influenced by genetics, environment and silviculture.