- Title
- The optimisation of Eucalyptus regeneration practices for improved survival, growth and uniformity in South African pulpwood plantations
- Creator
- Hechter, Ullrich
- Subject
- Eucalyptus -- Regeneration -- South Africa
- Subject
- Forests and forestry -- Economic aspects
- Subject
- Forests and forestry
- Date Issued
- 2024-12
- Date
- 2024-12
- Type
- Doctoral theses
- Type
- text
- Identifier
- http://hdl.handle.net/10948/68862
- Identifier
- vital:77135
- Description
- Commercial forestry plantations in South Africa play an important role in the economy of the country, contributing 1.2% towards the gross domestic product. Currently plantation forests occupy 1.1% (1.2 million hectares) of the South African land surface, of which 75 000 hectares are re-established each year. Eucalypts constitute 43% of planted area, of which 88% is grown for pulpwood. Achieving high tree survival (>90%) is important in terms of optimising rotation-end yield. The industry origin of a 90% survival benchmark is unclear, although company procedures incorporate this as the minimum threshold in terms of re-establishment success. Past research indicates that most mortality occurs within a narrow period post-establishment and is often associated with substandard re-establishment practices and/or a stressed micro-environment. An improved understanding is needed about the various mitigation measures needed to minimize mortality during eucalypt re-establishment. Before making decisions related to mortality mitigation measures, comprehensive data are required as to their commercial applicability as well as outcomes from multiple trials that accurately quantify any impacts on tree survival and financial return. The overall purpose of this dissertation was the optimisation of Eucalyptus re-establishment practices for improved survival, growth and uniformity in South African pulpwood plantations. To achieve this, five inter-linked objectives were determined. The first objective was to highlight the most important factors contributing to increased mortality in eucalypt plantations during re-establishment. This was achieved through conducting a literature review. Citations were ranked in terms of credibility, with the importance ratings (derived from the literature sources) applied to the different factors affecting survival and growth during eucalypt re-establishment. Of the various factors impacting early eucalypt mortality, water stress and planting stock quality were considered highly important. The manner and quality of site preparation (soil and slash), planting practices (planting depth included), timing of planting (during dry, hot periods), various post-planting operations (incorrect fertiliser placement or herbicide drift) and insect pests and diseases also contribute to mortality, but to a lesser extent. These factors cannot be considered in isolation due to the complex interactions that exist between them and determining the primary causes of mortality can be elusive, especially as their impacts tend to be additive by nature. The second objective was to link survival to silvicultural treatments, site-related physiographic factors and climatic variables in South Africa. This was achieved by conducting an integrated analysis of 43 Eucalyptus trials. Of the seven re-establishment practices considered, watering, planting depth and fertiliser application were significant, with plant size, pitting method, residue management and insecticide application were not significant. However, when environmental variables were included within the analyses, there were significant site x treatment interactions for planting depth, plant size, residue management and fertiliser application. This highlights the importance of taking site related factors into consideration when interpreting the causes of mortality. The third objective was to determine the interaction between planting density and mortality on Eucalyptus growth, uniformity and financial yield at rotation end in South Africa. This was carried out to verify whether planting at different densities may be used as a preventative (before planting) mitigation measure. One trial was used to answer four keys sub-objectives: 1) The impact of three planting densities (1 102, 1 500, 1 959 SPH) with no mortality on yield at rotation-end; 2) The impact of mortality (0%, 10%, 20%, 30%, 40%) on rotation-end yield; 3) The quantification of tree performance when planting at a higher density and accepting a certain degree of mortality; and 4) The financial impact of different planting densities and mortality on rotation-end profit. Higher planting densities resulted in smaller individual trees, but with an increase in stand level performance. At rotation-end, lower mortality (0% and 10%) had significantly higher volumes ha-1 than the higher mortality (30% and 40%). Planting at higher densities (1 722 and 1 959 SPH) and accepting a certain degree of mortality resulted in non-significant differences for volume at rotation-end compared to the fully stocked 1 500 SPH treatment. A higher SPH resulted in a higher yield, but with an increase in estimated establishment/tending and harvesting costs. In contrast, an increase in mortality and/or lower SPH (in the absence of mortality) resulted in more variable stand growth, together with an increase in estimated machine harvesting productivity and reduced costs. Irrespective of SPH, the higher the mortality the greater the loss of income, with the best profit within each treatment related to full stocking (0% mortality). Within the higher panting densities, the profit gained following low mortality (10 and 20%) was similar to that of no mortality (0%), indicating that higher mortality may be tolerated when planting at higher densities, confirming the 90% survival threshold the industry aims to achieve post-establishment. The fourth objective was to determine if silviculture intervention (blanking at 1, 2 and 3 months or coppicing and interplanting at 6 months) will result in acceptable eucalypt stocking, if mortality is higher than 10% (remedial mitigation measure). Data from a re-establishment trial were analysed to determine which of the mitigation measures performed best in terms of stocking and growth. Coppicing and interplanting with larger plants was not a viable option as a mitigation measure for mortality as most of the coppice shoots have died. This may have been a result of frost. Although high re-establishment costs may be incurred, disaster clearing to waste followed by replanting is an option if mortality is unacceptably high (as opposed to leaving the stand as is). The results of this objective confirm that blanking as the current Best Operating Practice is still appropriate in South African forestry (i.e., try to have survival >90% and blank as soon as possible to retain >90% of stems). Blanked plants do contribute to volume, but for this to occur, blanking should be carried out within 4 weeks after planting to gain maximum benefit. In addition, it highlights the importance of implementing remedial mitigation measures to achieve >90% survival so as to gain maximum benefit. Using the outcomes from objectives 1-4, the fifth objective focussed on the development of a decision support system (DSS) for implementation of mitigation measures to improve survival within commercial eucalypt pulpwood plantations in South Africa. Improved survival starts with the implementation of good re-establishment practices and good quality planting stock. Mitigation measures for poor survival can be implemented either prior to re-establishment (before mortality occurs) or post re-establishment (after mortality has occurred). If poor survival still occurs after the implementation of good silviculture practices and pre-re-establishment mitigation practices (planting at higher densities), one should consider the various options available in terms of post re-establishment mitigation practices (remedial practices) such as blanking or replanting if mortality is high. Overall, the outcomes from this dissertation provide benchmark data and derived information as to the necessity for various mortality mitigation options within the commercial forestry sector in South Africa. In addition, the DSS will assist with decision making in terms of implementing the best silviculture practices and mitigation measures for improved survival during eucalypt re-establishment in South African pulpwood plantations.
- Description
- Thesis (PhD) -- Faculty of Science, School of Natural Resource Science & Management, 2024
- Format
- computer
- Format
- online resource
- Format
- application/pdf
- Format
- 1 online resource (190 pages)
- Format
- Publisher
- Nelson Mandela University
- Publisher
- Faculty of Science
- Language
- English
- Rights
- Nelson Mandela University
- Rights
- All Rights Reserved
- Rights
- Open Access
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | SOURCE1 | HECHTER, U.pdf | 3 MB | Adobe Acrobat PDF | View Details Download |