Browsing by Author "Van Zitters, Monique"
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- ItemEvaluating the effect of active riparian rehabilitation initiatives and investment on native vegetation recovery, in the Berg-Breede catchment, Western Cape, South Africa(Stellenbosch : Stellenbosch University, 2021-12) Van Zitters, Monique; Esler, Karen J.; Rebelo, Alanna; Mingo, Jason; Stellenbosch Universty. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.ENGLISH ABSTRACT: Human-induced ecosystem transformations such as those driven by land-use change and biological invasions, have accelerated in the 20th century, affecting rivers and associated riparian ecosystems. To recover ecosystem functioning, the Berg and Breede River Rehabilitation Programme has undertaken active (alien clearing and revegetation) and passive (alien clearing and spontaneous succession) rehabilitation of riparian habitat at small scales (<2 ha) in the Berg and Breede Catchments. To provide an evidence-based approach to scaling up interventions, a monitoring programme is required to track progress and monitor success indicators. Four to five years after initial interventions, I investigated the impact of rehabilitation on vegetation composition at 11 sites along the Berg and Breede Rivers. I aimed to determine whether there is a difference in native vegetation recovery (rehabilitation success) between active and passively rehabilitated sites and assess the effect of abiotic variables and rehabilitation investment on success (defined here as native vegetation recovery). Vegetation surveys were conducted on five, 5x5m, plots per active and passive site, in spring and summer (n=220 plots). Abiotic (soil, geomorphological and climatic) variables were measured at each site. A cost calculator was constructed to account for irrigation, planting and labour costs for each site associated with active rehabilitation (n=11). Results demonstrated that native species cover, richness and diversity were significantly higher following active rehabilitation after alien tree clearing. However, four to five years post- rehabilitation, there was no significant difference in alien cover, species richness or diversity between active and passive rehabilitation sites, with alien pioneer grass and herbaceous weed species present in both active and passively rehabilitated sites. Soil and geomorphological variables were the same across active and passive sites, suggesting that rehabilitation success resulted due to active measures taken and not because of abiotic conditions. However, soil (organic carbon, depth, bulk density, Ca, Mg, P, Na) and geomorphological variables (bank slope, buffer zone length) were related to various native and alien vegetation indices. Total species richness (alien and native species) correlated positively with the number of dry days, isothermality, annual precipitation, precipitation of the driest and wettest month and the precipitation of the wettest, driest, warmest and coldest quarter. Total species richness was negatively correlated to annual mean temperature, temperature seasonality, maximum temperature and mean temperature (driest and warmest quarter). Total species diversity was correlated with the same suite of climatic variables, apart from annual mean temperature. Evenness was negatively correlated with the mean temperature (coldest quarter). The correlations between climatic variables and vegetation indices, suggests that rehabilitation practitioners should work with fine-scale climate data during the project planning phase to ensure optimal conditions for planting and establishment. The mean total cost of active rehabilitation per site was approximately ZAR33 per m2 year-1 and investment variables (plant, labour, irrigation) demonstrated no correlation to rehabilitation success. However, planting was negatively related to total and alien species richness (rehabilitation failure indicators). Monitoring vegetation recovery is essential specifically native and alien vegetation indices, abiotic and hydrological variables and rehabilitation costs over longer periods (>3 years) post-rehabilitation to understand lag effects. I conclude that this small-scale active riparian rehabilitation significantly improved native vegetation community composition locally, and results provide initial insights into the effect of abiotic conditions and investment on rehabilitation efforts. The documentation of riparian rehabilitation projects is rare, and this research therefore sets a benchmark and is valuable towards guiding monitoring, evaluation and costing of riparian rehabilitation.