Doctoral Degrees (Forest and Wood Science)
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- ItemProperties of wood-plastic composites made from alien invasive tree waste and recycled low-density polyethylene for interior use in social housing(Stellenbosch : Stellenbosch University, 2022-11) Mohammed, Abubakar Sadiq; Meincken, Martina; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.ENGLISH ABSTRACTS: Low-cost wood plastic composites (WPCs) were developed from the entire biomass of invasive trees and recycled low-density polyethylene. The aim was to produce affordable building materials for low-cost social housing in South Africa. Both raw materials are regarded as waste materials and the subsequent product development adds value to these resources, while simultaneously reducing the waste stream. The entire biomass of Acacia saligna salvaged from clearing operations was utilised as received, without any prior processing to minimise associated processing costs. Low-grade recycled low-density polyethylene (rLDPE) was used as the matrix to make the WPCs without any additives as an additional cost-cutting measure. The WPCs were at three different biomass content (50, 60 and 70 wt%), with two wood particle sizes (0.3 and 0.5 mm), and at different hot press times (10 and 30 min) and temperatures (150 and 180 °C) to obtain boards with optimum mechanical, physical and insulation properties for use in social housing in South Africa. The water absorption, dimensional stability, moduli of elasticity (MOE) and rupture (MOR), tensile strength and modulus were found to be better at longer press times and higher temperatures for all blending ratios. An increased biomass content and particle size were positively correlated with water absorption and thickness swelling and inversely related to MOR, tensile strength and density due to discontinues plastic matrix formation. A heat flow meter (HFM) and an acoustic impedance tube were designed and built to measure the thermal transmission and sound transmission loss (STL) properties of the WPCs. Boards from larger sized particles generally showed lower conductivity. The boards pressed with 70 wt% biomass and lower press temperature and time had the lowest heat conductivity value (0.044 W/m‧k), which can be attributed to higher incidence of lattice defects and vacancies due to a higher proportion of thermally modified biomass. The boards pressed with 50 wt% biomass at 150 °C for 10 min possessed a higher mass/stiffness ratio, which resulted in improved STL via reflection and absorption of acoustic energy in the mid-frequencies (500-2000 Hz). Boards produced with 60 wt% biomass exhibited the best noise attenuation level (8-14 dB) and thermal insulation with a heat conductivity around 0.048-0.056 W/m‧k, making them best suited as insulation boards to improve living conditions in Reconstruction and Development Programme (RDP) houses. The low proportion of non-woody biomass (leaves and bark) in the WPC formulation did not entirely inhibit mould growth on the WPC surfaces immediatly, even though the growth was slowed across all samples in no particular order. The fire tests showed that the WPCs composed with 70 wt% biomass pose the lowest fire hazard as they possess higher flammability and self-ignition temperatures and are also the most thermally stable with the highest melt-deflection temperature. This study demonstrates the feasibility of utilising low-grade recycled polyethylene and whole-tree biomass of A. saligna to manufacture WPC boards, without the need for pre-processing, or the addition of expensive compatibilisers, to produce boards with properties that satisfy the minimum South African Bureau of Standards (SABS) requirements for interior wall or ceiling cladding.
- ItemMeasuring and modelling dynamic moisture loss of pulpwood during rail transport in South Africa(Stellenbosch : Stellenbosch University, 2022-11) Morkel, Russel George; Ackermann, Pierre Alexander; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.ENGLISH ABSTRACT: The unit of planning in the South African forest industry is volume (m3). The unit of trade and financial reporting for pulpwood, however, is weight (tonnes). As living trees invariably contain more water than wood, this disparity has necessitated a detailed understanding of moisture loss with age-after-felling, to better predict m3/tonne conversion factors (for yield estimation, forest valuation and stock control purposes). All moisture loss research to date, has been conducted in a static setting, where low-speed ambient winds come to the processed wood in a forested environment. No research has assessed the possibility of incremental weight loss induced by the slipstream of a fast-moving freight train. This is not necessarily an academic oversight, more a lack of context. Few countries rail large quantities of debarked, small diameter, wet pulpwood traded by weight. Furthermore, although natural airflow is a primary driver of moisture loss, its quantity and quality are uncontrollable, and its effect in a forest setting often inconsequential. The objective of this research was to measure and model the dynamic weight loss of wet pulpwood, during a 317 km rail journey, on one of the world's largest dedicated timber trains. The research included scaled wind-tunnel experiments and operational trials using a wagon-mounted continuous-weighing platform and mobile automatic weather station. The simplified and more controlled wind-tunnel simulations recorded a statistically significant 6.6% step-down (p<0.001) in the treatment payload weight caused by the slipstream of the train. The treatments lost 114% more moisture than the comparable static controls. Time, temperature, relative humidity, wind speed and wind temperature accounted for 87% of the weight loss of the hardwood treatments over the entire period. The complex and less controllable operational methods recorded smaller slipstream induced weight losses. The modernized operational method treatments lost on average 2.84% more than the static controls (a relative difference of 346%). The treatment regression models showed that time, temperature, relative humidity and wind accounted for between 86% to 96% of the variation in weight loss. The preponderance of nighttime travel (characterized by low temperatures and high relative humidity levels) limited the effect of the slipstream on incremental moisture loss.
- ItemGrowth Response of Brachystegia longifolia to Copper Mining Pollution-Induced Heavy Metal Toxicity(Stellenbosch : Stellenbosch University, 2022-10) Mulenga, Charles; Meincken, Martina; Clarke, Catherine E.; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.ENGLISH ABSTRACT: Mining-induced heavy metal dispersion often contaminates the soil in forests surrounding copper mines. Heavy metals are highly toxic to plant health at elevated levels depending on many factors, including species. However, the impact of copper mining-induced metal toxicities on the productivity of miombo forests is not well known. This study investigated the degree of forest soil contamination, bioavailable proportions of heavy metals, uptake and their subsequent effect on the growth and wood quality of Brachystegia longifolia, one of the commercially important miombo woodland species growing in a mining environment. The effect of copper mining activities on B. longifolia was studied as a function of distance and wind direction around an active copper mine in Mufulira, Zambia. Four sampling plots were demarcated along each transect, stretching 19 km upwind and 12 km downwind from the mine and trees were sampled from 50 m radius sample plots. Wood discs and crown leaves free of visible growth abnormalities were collected to form the biomass samples, as well as topsoil. Portable X-ray fluorescence was used to analyze the total concentration of Mn Fe, Cu and Zn in sieved soil and ashed leaves. The degree of soil contamination was analyzed using the geoaccumulation and enrichment factors, while the bioaccumulation potential of the studied elements in B. longifolia was assessed by the bioaccumulation factor. Furthermore, the bioavailable proportions of trace elements were analyzed by ICP-AES. The annual ring widths were measured and the relative tree growth was determined using the mean annual increment (MAI) across the sampling plots. The growth-limiting trace elements and stress conditions were then identified using the growth-response curve with the relative tree growth as a dependent variable. Furthermore, wood cell dimensions, wood density and elemental composition were analyzed from selected annual rings that included dry and wet years based on the mean annual temperature and mean annual precipitation, respectively. The degree of soil contamination illustrated that the forest soils around Mufulira are strongly polluted by heavy metals dispersed from the copper mine. Soil contamination extended up to 7 km upwind and the entire sampled 12 km stretch downwind. The sample plot at 2 km downwind was the most polluted site accounting for 296 mg/kg, 2337 mg/kg, 1101 mg/kg and 109 mg/kg accumulation of Mn, Zn, Fe and Cu in B. longifolia leaves, respectively. In contrast, only 3196 mg/kg, 154 mg/kg, 516 mg/kg and 55 mg/kg of Mn, Zn, Fe and Cu, respectively, were reported on the furthest unpolluted site upwind 19 km from the mine. Significant decreases in the concentration of Fe and Cu were observed with increasing distance from the mine, whereas the Zn and Mn content increased significantly. Furthermore, it was also observed that B. longifolia leaves accumulated Zn (x15) and Mn (x38) more than the soil throughout all sample plots. The radial growth of B. longifolia on all contaminated sites was significantly reduced compared to the control site. A multivariate analysis attributed the reduction in tree growth to Fe and Cu toxicities and it confirmed that B. longifolia trees accumulate and tolerate Zn and Mn. The decrease in radial expansion results from a significant reduction in wood cell dimensions and proportions on polluted sites. It was further observed that Cu and Fe toxicity increases the basic wood density of B. longifolia and compromises the capacity of wood tracheary elements to transport water and nutrients. This study demonstrates that copper mining-induced heavy metal forest soil pollution and the subsequent metal stress affects the growth and productivity of B. longifolia. It shows that Cu and Fe are highly toxic to this tree species, although it can accumulate and tolerate elevated levels of Zn and Mn. Fe and Cu toxicities alter the anatomical characteristics of B. longifolia wood, affecting the sustainable utilization of this tree species and possibly other miombo species naturally growing in heavy metal polluted environments.
- ItemIndustrial forestry compartment characteristics: the effect on end of rotation processes and the forestry supply chain(Stellenbosch : Stellenbosch University, 2022-03) Ackerman, Simon Alexander; Talbot, Bruce; Astrup, Rasmus; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.ENGLISH ABSTRACT: Industrial plantation management aims to produce timber at the end of its rotation of a desired size to maximise the volume on the site, through the application of intensive silviculture. These silvicultural operations maximise this volume primarily through maintaining an acceptable level of tree size and tree size distribution uniformity. With tree size strongly related to timber harvesting productivity, uniformity of the trees on the stand should also benefit these end of rotation processes. For this reason, a series of experiments were undertaken to test the effect of tree size uniformity of mechanised cut to length (CTL) harvesting. These were firstly, to understand the fine scale effect of tree-by-tree mechanised CTL harvesting productivity and overall costs and cost sensitivity for two different sized harvesting machines (medium sized Ponsse Beaver and larger Ponsse Bear). Secondly, to explore the consequences of varying tree distributions on the productivity of mechanised CTL harvesters by comparing the marginal and weighted mean productivity at clear-felling age. Lastly, to investigate the systems effect of mechanised CTL harvesting of marginal first thinning trees by a combination harvester-forwarder machine in a fully selective thinning operation. In the first investigation, a long-term data set of harvesting machine data were collected and analysed using a non-linear mixed effects modelling approach to model the productivity for each of the different sized machines applied to the machine-based tree data. Results showed clear differences between the productivity of these two machine sizes and a clear decline of productivity when these machines reached their technical tree size limit. In terms of machine costs, the higher capital cost machine, the Bear, was only slightly greater to that of the Beaver. This was due to the high productivity of the Bear in the tree sizes harvested by this machine. Cost sensitivity further reinforced this; with the Bear’s capital cost being offset by the machine’s productivity, even at high interest rates. This investigation showed that both machines could effectively operate in any tree size, although one could infer that the smaller machine is not technically suited to large trees, and these could damage the machine in the long term. The second investigation used the models and data from the first investigation and compared these marginal productivity results to a weighted mean productivity when using the tree distribution rather than assuming a uniform distribution of trees in a stand. This investigation showed the disparity between using a mean tree size to predict harvesting productivity vs the tree size distribution. This disparity equates up to 8 m3 PMH-1 for the smaller Ponsse Beaver. This component highlights where remote sensing technology can be used to measure stand tree distributions more accurately than current enumeration techniques. The third part of the study investigated the application of a small-scale combination harvester forwarder in small and poor form first thinning trees. The investigation highlighted the challenges faced with these small and poorly formed trees that often the standing tree volume is not effectively translated to timber extracted to roadside. This necessitated a quantification of this timber ‘waste’ and an adjustment of the harvesting component’s productivity to take this into account. Further to this, since the machine forms a harvesting and forwarding system (the harvesting and forwarding functions cannot operate at the same time), the systems productivity was calculated, with the overall productivity limited by the least productive function. This final experiment aligned the previous two, where heterogeneous tree size effect of CTL harvesting is quantified. The overall objectives of the study were met, and it was found that tree size uniformity does affect CTL harvesting productivity. To this end, this needs to be matched against investments in more intensive silviculture. The study also highlighted the connection with the need for accurate stand characterisation to efficiently predict tree metrics for efficient timber harvesting.
- ItemInvestigating the photosynthetic and hydraulic trade-off during drought recovery in eucalypts(Stellenbosch : Stellenbosch University, 2022-04) Saunders, Alta; Drew, David M.ENGLISH ABSTRACT: Stomatal regulation plays a vital role in maintaining the water status of a plant by minimising water loss, however, decreases in stomatal conductance can lead to reductions in carbon uptake. The stomata balance a trade-off between water loss and carbon gain. The hydraulic system and stomatal conductance are closely linked but play opposing roles within a plant. The hydraulic system ensures that there is a sufficient water supply to leaves, while stomatal conductance regulates the loss of water from the leaves. During periods of drought, reductions in hydraulic conductance due to embolism formation can be seen, however stomatal regulation can help reduce embolism formation or prevent runaway cavitation during drought. Understanding how plant hydraulics and stomatal regulation influence production rates is becoming more important to model stomatal responses in a changing climate, especially for Eucalyptus species which is often grown in drought prone environments. Plants use a wide range of strategies to reduce or mitigate the negative impact of embolism formation, with this study focusing on the strategies utilised by commercially significant Eucalyptus hybrids. Two Eucalyptus hybrids, E. grandis X camaldulensis (GC) and E. urophylla X grandis (UG), where subjected to a drought-recovery treatment where they were periodically droughted. During the experimental period the stomatal responses, together with transpiration rates, photosynthetic capacity and biomass allocation was measured. Hydraulic measurements together with CT-scan imaging was also utilised to determine whether these plants can recover lost hydraulic pathways post-drought, and what the underlying mechanism for this might be. During the study two distinct hydraulic strategies were observed. GC was more resistant to embolism formation compared to UG, however GC showed lower levels of hydraulic recovery after rewatering. The drought responses could also be split into a resilient v. a resistant response, with the more resilient hybrid, UG, maintaining stomatal conductance throughout drought periods running the risk of hydraulic failure, however with the ability to recover lost hydraulic pathways through refilling post- drought. This is in comparison with the resistant strategy seen in GC, where water loss during drought was minimised, however this also reduced carbon uptake and production. From this study the stomatal responses could also be accurately modelled using a gain-risk model that assumes stomata optimise the trade-off between water loss and carbon gain. It was however clear, that the strategy used by plants need to be considered when using a gain-risk model, with the gain-risk model performing better for the Eucalyptus hybrid that utilised a resistant strategy, since plants that uses a resilient strategy will maintain stomatal conductance during drought, regardless of the hydraulic risk. The use of alternative models was also investigated during this study, with Machine Learning models being able to accurately predict stomatal responses on a global scale. Major increases in tree mortality are predicted due to changes in climate. To properly predict these changes, accurate models of plant responses to water limitations and other environmental changes are therefore becoming increasingly important for effective forest management. Understanding how hydraulic traits and stomatal regulation covary, can help model drought-induced tree mortality in a changing climate.