Browsing by Author "Juba, Roderick"
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- ItemDetermining the biomass and nutrient content of the invasive Acacia mearnsii and Eucalyptus camaldulensis trees in Fynbos riparian zones(Stellenbosch : Stellenbosch University, 2020-12) Juba, Roderick; Jacobs, Shayne Martin; Le Maitre, David C.; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.ENGLISH ABSTRACT: Alien tree invasion into South African riparian zones remains a significant threat to water resources and biodiversity. While much has been done to limit the spread of invasive alien trees, there are still great strides to be made in successful management of extant invasive alien stands and in reducing the potential for future invasions and re-invasions. This dissertation sought to improve our understanding of stands of Acacia mearnsii (Black wattle) and Eucalyptus camaldulensis (River red gum) in riparian zones of the Western Cape of South Africa. Information is presented on the extent of invasion in selected catchment areas and its implications for rehabilitation of such sites post-clearing. Additionally, biomass products in the form of wood chips and biochar have been trialled to determine its efficacy as soil amendment agents in agriculture. Sites were selected along the upper to middle Breede River, where stands of either A. mearnsii or E. camaldulensis were present in high and low densities. Trees were sampled destructively for biomass and nutrient determinations. From different subsets of trees of the same invasive species, wood chips and biochar were produced and used in greenhouse-based experiments on the agricultural crops Brassica napus (canola) and Triticum aestivum (wheat). Samples were grouped along density extremes, for which allometric models for biomass and nutrient contents showed divergent patterns throughout. Biomass and nutrient data were used to determine the potential for nutrient export from sites during large-scale clearing operations. It is widely accepted that nutrient accumulation through invasion could impair ecosystem recover. However, it is also estimated here that a high-density site of A. mearnsii could store approximately 110 t ha-1 of AGB, which could contain up to 46 kg of P, 1200 kg of N, and 63 tonnes of C. Similarly, a high-density site of E. camaldulensis which has approximately 92 t ha-1 of aboveground biomass (AGB), could store up to 531 kg of N, 78 kg of P, and 52 tonnes of C.These nutrients can be lost through large-scale biomass removal (including loss of C storagecapacity) and most are likely to significantly impact recovery of affected ecosystems. The effectof short-term storage on the allelopathic potential of wood chips was also tested during this studyon wheat and canola. For canola, fresh chips of A. mearnsii promoted shoot growth more thanany of the other treatments. For wheat, however, fresh chips of E. camaldulensis were best suitedfor shoot growth. Allelopathic effects in AGB components of both invasive species were recordedin leachate from photosynthetic components and are minimal in stem material. It is suggestedthat the bulk stemwood is safest for soil applications and inclusion of other components may beappropriate in smaller quantities. With biochar, it was shown for both crop species that the additionof fertiliser is critical for shoot development. It is suggested that either or both of these biomassproducts be tested in an ecosystem rehabilitation context. This study shows the need for a more effective means of invasive alien tree management that considers all the various aspects of invasion in South Africa. Large-scale removal of IAPs in South Africa presents several opportunities, but could also pose significant future challenges if it is not done with consideration for natural processes and cycles. It is proposed here that the current invasive alien tree management models be re-evaluated to include new information such as soil nutrient enrichment and export, potential use of biomass and biomass products, the place of invasion as both a driver and a consequence of ecological disturbance, and the need for an integrated ecosystem management framework.