The Interaction between site, harvest residue management and plant stock quality on eucalyptus transplant survival, growth and uniformity in Kwazulu-Natal, South Africa

Thesis (PhD)--Stellenbosch University, 2021.

ENGLISH ABSTRACT: Most eucalypt plantations are managed forshort rotations (6 to 8 years) and established across regions with varying water and nutritional stresses. The design and implementation of silvicultural management techniques should be adapted to the specific production and protection objectives, which are strongly governed local edapho-climatic conditions, water resources management, and protection against soil erosion, diseases, pests, bushfires, strong winds and alien species (Goncalves et al., 2017). Although a number of studies have reviewed factors such as planting stock quality and harvest residue management, and their interactive responses, few have quantified the interaction over different sites and the effect on early survival, growth and uniformity through to full rotation. Documented work, specifically in South Africa, focusing on the interaction of mulch residue and differing planting stock of different plug cavity volumes is scarce. Costa et al. (2004); Guarnaschelli et al. (2003)and Guarnaschelli et al. (2006) all confirm that eucalypt seedlings exposed to water stress preconditioning, do experience morphological adjustments whereby total biomass, leaf area and shoot:root biomass ratio, as well as certain physiological dynamics, can be associated with drought hardening. A combination of substandard nursery plant quality and inconsistent planting practices have been associated with poor survival and sub-optimal growth in eucalypt pulpwood plantations of South Africa and attributed to morphological differences found within plant stock. Plantation forestry is entirely sustainable under conditions of good husbandry, but only where wasteful and damaging practices are avoided. Our understanding of the mechanisms underpinning growth response to harvest residue mulching in hardwood eucalypt plantations remain limited. Forest residue retention is well understood to be an important tool in the sustainability of production forestry; however, it is but one component of a number that are just as critically important.Gonçalves et al. (2008)and Stape et al. (2002) report that under water and nutritional stress, residues retained on certain sites canincrease nutrient availability. This is strongly associated with reduced nutrient and organic matter losses, and the maintenance of important soil physical properties such as porosity, permeability, infiltration and aeration. Du Toit (2003) reports that plantation management operations carried out during the inter-rotational period (harvesting, residue management and certain silviculture operations) have a major impact on the productivity and long-term sustainability of forest stands, especially where short rotations are applied. Retaining residues following harvesting is thusan important strategy for sustaining the productivity of subsequent rotations; however the threat of fire on sites where residues have are retained can be catastrophic with financial losses running into millions of Rands. Four field trials were planted in the Midlands of KwaZulu-Natal Province, South Africa, to understand the interaction of residue treatments, plant stock size, nursery conditioning protocols and the application of insecticides on survival, growth and uniformity of E.dunnii (seedling) and E.gxn (clone),up to clear fell age. It was hypothesised that a combination of mulching, combined with the planting of larger,more robust plants,would ultimately result in better stocked and more vigorously growing out-plantings, with the goal of maximising utilisable timber availability at the end of the rotation. Although burning is a valuable and viable residue management tool, efforts to augment with alternative residue management techniques that are just as productive and beneficial in terms of long-term site sustainability are essential. There is a management aversion to harvest residue mulching due to the high cost; however, experience has shown that there are benefits to be garnered, with fire protection being a potentially strong additional point. The environmental benefits of this technology are promising and the potential gains measurable in the long term.With ever shrinking plantable areas, the environmental and legal pressures placed on obtaining water permits and the growing impacts of climate change, understanding the driver so fearly survival, growth and uniformity at the commercial scale are the cornerstone to our business survival. Trial results from this research endorsed the importance of plant quality but this factor can never be viewed in isolation when examining long-term field performance, as nursey plant size, residue management and insecticide applications are all equally important silvicultural inputs, specifically in their early interactions up to canopy closure. Main effects of the treatments tested that were strongly significant at final rotation, included site, in terms of explaining differences in mean annual increment (MAI), stocking (Spha) and height. Furthermore, stocking was significantly different among residue management treatments. The application of insecticide (main effect) explained significant differences in stocking but mean annual increment did not differ significantly among the insecticide treatments. The best performing treatments across all sites included a combination of a large transplant root plug volume (105 cm3), planted on slash spread treatment(MAI = 24.8 m3ha-1), whilst the worst performing was a standard plug (60 cm3) planted on a burn treatment (MAI = 22.6 m3ha-1). Relative differences in mean basal area (BA) between burning and mulch, showed initial gains up to 3 years, but this dissipated to zero by full rotation. Transplant root plug volume revealed early gains up to at 3 months but this declined to zero by full rotation. Increases in BA were more specific at the individual trial level,with early plug volume response (1 to 3 years) driven by genotype, but disappeared at full rotation. Small relative gains accrued by not applying nursery hardening up to 1 year, but thereafter were negligible for the duration of the trial.In summary, relative differences between treatments all showed promising early gains in basal area growth from 3 months, but these all declined to around zero by 4 years and thereafter remained constant through to full rotation. The results from this study showed that silviculture factors such as transplant plug volume, residue management, nursery hardening and insecticide application, all applied simultaneously at planting,were most responsive (statistically significant differences in basal area growth) from planting to approximately 3.5 years. Thereafter, their individual and combined effects became increasingly difficult to explain or isolate as either main effects or interactions with the additive response of three to four interacting silviculture treatments only able to explain less than 15% of variability after canopy closure, even where significant difference exist. This result was in agreement with several of the key references cited in this study. Nursery plants raised in large volume cavities (105 cm3) did not require hardening in the nursery where as the standard plug volume (60 cm3)benefited from a gradual reduction in irrigation. Positive gains from 1 to 3 years for root plug volume, residue management, insecticide application and hardening lost significance with time to form only part of an interaction as site effects and intraspecific competition became more dominant. 5of the trial.In summary, relative differences between treatments all showed promising early gains in basal area growth from 3 months, but these all declined to around zero by 4 years and thereafter remained constant through to full rotation. The results from this study showed that silviculture factors such as transplant plug volume, residue management, nursery hardening and insecticide application, all applied simultaneously at planting,were most responsive (statistically significant differences in basal area growth) from planting to approximately 3.5 years. Thereafter, their individual and combined effects became increasingly difficult to explain or isolate as either main effects or interactions with the additive response of three to four interacting silviculture treatments only able to explain less than 15% of variability after canopy closure, even where significant difference exist. This result was in agreement with several of the key references cited in this study.Nursery plants raised in large volume cavities (105 cm3) did not require hardening in the nursery where as the standard plug volume (60 cm3)benefited from a gradual reduction in irrigation. Positive gains from 1 to 3 years for root plug volume, residue management, insecticide application and hardening lost significance with time to form only part of an interaction as site effects and intraspecific competition became more dominant. Stocking and MAI showed very different responses to silvicultural treatments,but for explainable reasons. Stocking proved highly responsive to early silvicultural treatments, and less so to site.A plausible reason for this observation is that mean water deficits were moderate (<100 mm y-1on average)across all site types tested and stocking differences were more affected by silvicultural inputs, either main or interactive effects, over the full rotation. However,basal area growth and MAI, although initially highly influenced by both silviculture inputs and edapho-climatic factors, became increasingly more responsive to the effects of water deficits and nutrient capital driven by intraspecific competition with age.

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Doctoral