Doctoral Degrees (Forest and Wood Science)

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Browsing Doctoral Degrees (Forest and Wood Science) by Author "Erasmus, Justin"

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    Modelling the effect of stand density management and environmental variables on Pinus patula wood properties
    (Stellenbosch : Stellenbosch University, 2020-03) Erasmus, Justin; Wessels, Brand; Drew, David M.; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.
    ENGLISH ABSTRACT: Approximately one million hectares of Pinus patula has been planted worldwide, mainly in southern and eastern Africa and accounts for roughly half of the total softwood plantation area in South Africa. Improved growth rates and shorter rotation ages of these forest resources have caused an increase in the proportion of juvenile wood and a decrease in the stiffness of lumber, which often does not comply with the requirements for structural use. The growing space of trees has been shown to influence wood properties and may be a useful management intervention to improve stiffness properties. The financial b enefits of th ese sh ort ro tation systems also means that they are likely to persist into the future. An understanding of the properties of wood within the juvenile zone is therefore increasingly important. The objectives of this study were thus (1), to examine the effect of tree spacing expressed as stand density, particularly at stand establishment, on the stiffness of wood and important wood properties which are known to influence wood stiffness and (2), to study the development of wood in young trees as affected by selected environmental factors. The study was based on four experiments. In the first e xperiment wood increment cores were non-destructively removed from a total of 171 trees from four different planting density treatments from an 18-year old Pinus patula spacing trial. The wood density, microfibril a ngle (MFA) a nd r ing width were measured using Silviscan3 technology. In a second experiment, two commercial Pinus patula stands which were subjected to different stand density management regimes, were destructively sampled and 37 trees were processed into lumber of which the modulus of elasticity (MOE) and modulus of rupture (MOR) were measured. In the third experiment, 46 trees from a spacing trial was also destructively sampled and processed into lumber of which MOE and MOR were measured. The last experiment was a controlled greenhouse potting trial where the temperature, water supply and leaf nitrogen/potassium ratio (N/K) were measured and compared to the MFA and density of 168 trees over their first/second year of growth. Bending test results on lumber from trees from commercial stands showed that, compared to a number of stands with typical stand density regimes, only lumber processed from a higher stand density (1667 stems ha􀀀1) conformed to the requirements for structural use. The MOR values were however adequate across all management regimes. The MOE of lumber from a spacing trial showed that only the most closely spaced trees (2981 stems ha􀀀1) had lumber which conformed to requirements for structural grades. MFA, varied from roughly 30° at the pith to 7° at the bark, and along with wood density and knot characteristics, was able to explain over 70% of the variation in lumber MOE. The increase in lumber MOE with closer spacing was due to a combined effect of decreases and increases in both MFA and density respectively, and a restriction of the juvenile core. MFA and density were both significantly influenced by tree spacing, decreasing and increasing with closely spaced trees respectively. The differences in growth rate due to tree spacing could not fully capture the effects of spacing, which had an independent effect on wood properties. Environmental variables also had a significant effect on growth and cell properties of Pinus patula. The stiffness of the young saplings was significantly influenced by leaf mass and by water supply and N/K. Based on these results it seems as if wood stiffness is linked to foliar biomass. Stand density seems to have great potential as a management intervention to improve the cell and wood properties controlling the stiffness of South Africangrown Pinus patula lumber at final harvest. Low levels of water supply and N/K can also increase the stiffness of wood within early cambial ages.