Browsing by Author "Erasmus, Justin"

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    The effect of planting density on Pinus patula stem form, wood properties and lumber strength and stiffness
    (Stellenbosch : Stellenbosch University, 2016-03) Erasmus, Justin; Wessels, C. Brand; Stellenbosch University. Faculty of Agrisciences. Dept. of Forest and Wood Science.
    ENGLISH ABSTRACT: Faster growth and reduced harvesting age are causing a reduction in the stiffness of structural lumber from South African-grown pine plantations. A number of studies have shown the positive effects of high planting densities as a tool to improve the mean modulus of elasticity (MOE) of structural lumber. The objective of this study was to investigate the effect of planting density on stem form, wood properties and the strength and stiffness of structural lumber of young Pinus Patula – the most important and extensively planted commercial softwood in South Africa. In the first part of this study, four different planting density treatments (403, 1097, 1808 and 2981 stems/ha) from an 18-year old P. patula spacing trial located in Mpumalanga, South Africa were sampled non-destructively. Stem slenderness, stem curvature, and the dynamic modulus of elasticity (MOEfak) were measured on 171 standing trees. Increment cores were removed from 40 trees for measurement of density, microfibril angle (MFA) and ring width using the Silviscan 3 technology. Planting density had a significant effect on stem curve with the lowest planting density having the highest mean stem curve. Planting density also had a highly significant effect on stem slenderness. The MOEfak increased greatly with increases in planting density. MFA was significantly influenced by both planting density and year ring number and the interaction between them. The mean MFA at similar ring numbers decreased significantly from the 403 stems/ha treatment toward the higher planting densities (1808 and 2981 stems/ha). Planting density had a limited effect on wood density. MFA seems to be the mechanism through which the tree compensates for the instability caused by a high slenderness ratio. Density, on the other hand, did not correlate with slenderness at all and was probably mostly influenced by environmental and growth factors. In the second part of this study, a total of 37 trees from two commercial compartments, planted at different densities, were processed into 71 logs, cant-sawed into lumber, and tested for static MOE, modulus of rupture (MOR), density, and warp. The first compartment was 18 years old, planted at 1334 stems/ha and thinned to 827 stems/ha at age 11. The second compartment was 17 years old, planted at 1667 stems/ha and was unthinned. Lumber from the 1667 stems/ha compartment had a mean MOE of 8967 MPa compared to a mean MOE of 7134 MPa for the 1334/827 stems/ha compartment. Based on this evidence and results from previous studies, it seems as if planting density has a large effect on the stiffness of young P. patula lumber and that planting density may be used as a practical management intervention to increase the stiffness of lumber.
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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.