A river health assessment of selected Southwestern Cape rivers : riparian vegetation index (RVI) application, evaluation and alternatives

[摘要] ENGLISH ABSTRACT:This study focuses on riparian vegetation condition at sites along three nvers within thesouthwestern Cape, South Africa namely the Lourens, Hout Bay and Palmiet Rivers. The RiparianVegetation Index (RVI) is used as a tool to assess riparian vegetation condition. Data from thisstudy will be used in State of Rivers reporting for the River Health Programme in South Africa.This study highlights sites of high conservation status with regards to vegetation condition andindicates sites of concern due to poor vegetation condition. Deviations from expected vegetationcondition scores at certain sites along the rivers have highlighted certain flaws in the RVI method.Further, the repeatability and validity of the RVI method is tested. The method is tested forvariability and validity across disturbance gradients at sites along the Eerste River. Results indicatethat the RVI method produces repeatable results. The sub-index, Recruitment of IndigenousRiparian Species (RIRS), is the most variable in terms of assessor scores, while StructuralIntactness (SI) is the most reproducible sub-index. This has important implications for the futuredevelopment of the method, which should place more emphasis on reproducible sub-indices withinthe final RVI formula and should standardise those sub-indices that are highly variable. Statisticalanalysis indicates that the method does not reflect the perceived biological status of the vegetation.This may be due to the specified sampling duration, which should be increased in order to reflectmore meaningful results.Criticisms of and difficulties with the RVI method as well as suggestions to improve the method aredealt with and an alternative method to the Riparian Vegetation Index is explored. The FloristicQuality Assessment (FQA) method, used to determine vegetation integrity in Illinois USA, isadapted to assess riparian vegetation condition at sites along the Lourens, Hout Bay and PalmietRivers. Coefficients of conservatism, determined for the flora observed in the assessment, attemptto categorise species according to their response to levels of habitat degradation. These values areincorporated into the Floristic Quality Index (FQI) formula, which yields the final floristic qualityassessment results. Correlation analyses indicate that RVI and FQA results are strongly correlatedalthough the RVI at this stage best represents the perceived biological status of vegetation at sites.The FQA method, or a statistically modified alternative, may be a suitable and more objectivealternative to the RVI method.

[发布机构] Stellenbosch University