Thinning and fertiliser effects on Eucalyptus nitens wood properties

Eucalyptus nitens is a hardwood plantation species widely planted in cool-temperate regions of the world. Most of the plantations have been established to produce fibre, however, they also have potential to produce more valuable solid-wood products. Silvicultural practices for fibre production are well established, but silviculture for solid-wood production for this species is still being explored. For solid wood production, stiffness and basic density are key wood properties as they determine the suitability of the logs for particular products and ultimately the value of production. Managing eucalypt plantations for the production of solid wood on short rotations requires thinning which is used to produce fewer but larger stems, resulting in a greater yield of sawlogs suitable for processing. To inform processing options available for targeting high-value wood products there is a need to understand how wood properties vary within a tree and how thinning impacts wood quality. Our understanding of how thinning influences the pattern of the within-tree variation of basic density and stiffness is still very limited. Similarly, fertiliser is predominantly used to enhance early growth, but the effect of later applications on tree productivity and fertiliser effect, in addition to thinning, on wood quality traits, are also unknown. The aim of this thesis was to study the influence of different approaches to silvicultural management on the wood properties of E. nitens plantations grown under regimes developed specifically for the production of solid-wood products. To optimise utilisation of wood as a raw material it is necessary to evaluate wood property variation within the tree as that impacts the value of logs and the cost of final products. Therefore, understanding how silvicultural management influences longitudinal and radial variation in basic density and stiffness is crucial. It is also important to find the most economic, fast and reliable methods of testing wood properties. The data used in this thesis to assess silvicultural impacts was derived from three 19-22 year old E. nitens thinning and fertiliser trials which were pruned and grown in different environmental sites in Tasmania, Australia. These experimental trials were established with the aim of improving existing growth models to better understand the silvicultural and economic implications of commercially relevant pruning, thinning and fertilising operations. The first experimental chapter of the thesis (Chapter 2) developed methods to improve the prediction of basic density using drilling resistance. Drilling resistance offers a rapid and non-destructive method of estimating basic density, however, to obtain accurate values a correction must be applied. It was not known if the commonly used linear correction provided accurate estimates of basic density or whether an alternative correction was required. To test this correction, 40 discs were drilled radially, then cut into segments which were measured for basic density. In this chapter, the non-linear correction method was developed which allowed a more accurate prediction of basic density across tree stem radius, which is useful for radial basic density assessment. The improved drilling resistance method allows reliable basic density assessments to be conducted on standing trees with minimal costs and labour intensity. The second experimental chapter of the thesis (Chapter 3) examined the effect of non-commercial and commercial thinning regimes on basic density and stiffness using the non-destructive assessment techniques of drilling resistance and acoustic wave velocity on standing trees across three different sites. Commercial thinning to 300 trees ha\\(^{-1}\\) led to trees having lower stiffness on all three different sites. However, thinning did not affect basic density. Non-commercial thinning resulted in larger trees and there was no difference in wood properties between non-commercially and commercially thinned trees. Commercial thinning to 500 trees ha\\(^{-1}\\) had no effect on wood quality. Trees grown at higher elevation on a wetter and colder site had lower basic density and stiffness. The results have confirmed that wood properties are influenced by both silviculture and site environmental differences. The third experimental study (Chapter 4) examined the effect of commercial thinning on longitudinal and radial variation in wood properties. Stiffness and basic density were measured longitudinally at 2.5 m intervals from the base to 20 m height in the tree and radially at a fixed height of 2.5 m. Longitudinally, wood properties varied more within the tree than they did as a result of thinning. Stiffness was lowest at the bottom of the tree and the highest stiffness was located from 7.5 to 15 m of height. Thinning to 300 trees ha\\(^{-1}\\) had no effect on stiffness in the bottom of the tree but led to reduced stiffness in upper logs. Similarly, trees in thinned stands had slightly lower basic density, however, longitudinally, basic density increased with tree height. While thinning negatively impacted wood quality traits, this silvicultural treatment results in larger sized trees therefore bigger and more valuable logs can be obtained from the upper sections of the tree where basic density and stiffness is higher. For this reason, the benefits thinning brings to maximising wood growth can outweigh any loss of wood quality. Thinning resulted in significant radial change in wood properties and the thinning effect was apparent soon after the thinning treatment. While thinning results in lower wood quality, it also makes wood properties less variable radially which is better for solid wood processing. Given that wood products require different threshold requirements for size and quality, the information in this study demonstrates how commercial thinning can affect stiffness and basic density and this may significantly influence log allocation and processing strategies. The previous chapters focused on the effects of one silvicultural treatment (thinning) on wood quality traits. The fourth study (Chapter 5) examined combined fertiliser and thinning silvicultural treatments on growth rates and wood quality on two different sites. One site had fertiliser application before and after thinning while the other site had fertiliser application after thinning only. Tree height and diameter were measured from age 3 to 19 years and non-destructive drilling resistance and acoustic wave velocity were used to assess standing tree wood properties at age 19 years. Basic density and stiffness were not affected by fertiliser addition to the thinning treatment and fertiliser, in addition to thinning, had no effect on radial variation in basic density at 19 years of age. The difference in tree diameters on both sites was observed only due to thinning, resulting in larger trees with lower stiffness. The results suggest that there is no benefit in terms of basic density, stiffness and growth in applying fertiliser in addition to the thinning for timber grown for solid-wood production on longer rotations. Work presented in this thesis demonstrates significant within-tree variation in basic density and stiffness and that silvicultural management of Eucalyptus nitens plantations is an important tool for manipulation of wood quantity and quality that defines both suitability and value for different solid wood products. The most suitable approach for solid wood production would be to thin early (non-commercially) immediately after pruning to maximise individual tree growth without loss of wood quality. Fertiliser application in combination with commercial thinning is unlikely to result in greater individual tree volume at the end of rotation whereas earlier or non-commercial thinning would be a better option. Thinning results in a relatively small reduction in basic density and stiffness and high variation in basic density and stiffness within the tree suggests that it would be worthwhile segregating logs based on wood quality to maximise solid wood product value.