Vegetation responses to invasive alien plant clearing along the Sabie River in and adjacent to the Kruger National Park

Show simple item record Morris, Taryn Lee 2009-03-25T07:32:55Z 2009-03-25T07:32:55Z 2009-03-25T07:32:55Z
dc.description.abstract Alien plant invasions are a global problem and have widespread ecological and economic impacts. River ecosystems are highly prone to invasion by alien plants due to their dynamic hydrology and the ability of water to efficiently transport alien plant propagules. This, together with continuous access to water and nutrient resources, facilitates invasions. The recognized impact of woody alien species in reducing scarce water resources in South Africa inspired the inception of the Working for Water (WfW) programme. This programme aims to increase future water yields through the large scale clearing of invasive alien plants (IAPs), while simultaneously addressing poverty alleviation. Since its inception, the programme has been operating under the assumption that ecosystems would “self repair” after the removal of IAPs, but little monitoring has occurred to determine if this is in fact the case. In the Kruger National Park (KNP) invasive alien species have been declared one of the greatest threats to ecosystem biodiversity. Riparian zones are the most invaded systems due to the continuous influx of alien propagules from the poorly managed upper reaches of the major river catchments, combined with the capacity of river systems to facilitate invasions. WfW have executed the majority of the active clearing operations on an ongoing basis in the KNP for several years. However, little monitoring has occurred to determine the efficacy of control operations or the response of both native and alien vegetation to the removal of IAPs. Thus, the major aims of this project were to assess the efficacy of the WfW programme in clearing invasive alien plants in the KNP, and to assess the extent of natural ecosystem recovery after clearing operations. Vegetation from twelve sites along the Sabie River within and adjacent to the KNP ( Mpumalanga, South Africa) was sampled before and after an annual clearing operation by WfW. Results were compared with additional data derived from several historical studies in the area to assess the longer-term efficacy of WfW. The occurrence of dense stands of IAPs comprising up to 97% of the vegetation density along the Sabie River within the KNP were likely a result of the significantly elevated rainfall in the respective growth season. These dense stands of IAPs were associated with a decline in several diversity measures of native vegetation with a basal stem diameter > 1 cm (P < 0.05). However similar negative associations were not observed with understorey vegetation (P > 0.05). The annual clearing operations by WfW were highly effective and reduced invasive alien plant densities from 3508 ± 1113 plants per ha to 343 ± 156 plants per ha (P = 0.002), translating to an average reduction of ± 80%. After clearing, indigenous vegetation density increased (P = 0.02), despite the reduced rainfall received in that year’s growing season. Herbaceous and graminoid growth forms showed the greatest increase in previously densely invaded transects. In general, alien species richness has increased slightly in the KNP over the last ten years. However, the invasion intensities have remained exceptionally low, even in the face of several large disturbance events that are usually associated with a sharp increase in alien plant invasions. This is more than likely attributable to the continuous annual clearing operations by WfW in the KNP. Continuous clearing acts to effectively limit the establishment and spread of many IAP species despite the ever present threat of invasion from upstream. Furthermore, the continuous clearing of IAP stands in the KNP ensures that stands are relatively short-lived, preventing long lasting negative impacts on the ecosystem. Removal of IAP species reduces their disproportionate competitive influence and facilitates the natural re-establishment of native vegetation. This study presented important information with regards to the vital need for assessment and monitoring of WfW operations. It also provided KNP management with valuable results pertaining to research and management objectives. en
dc.language.iso en en
dc.title Vegetation responses to invasive alien plant clearing along the Sabie River in and adjacent to the Kruger National Park en
dc.type Thesis en

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