The interaction between vegetation and near-surface water in a wetland system, Stellenbosch, South Africa

[摘要] ENGLISH ABSTRACT: Understanding the responses of individual plant communities to variations innear-surface water levels and to water quality is a step towards determining the critical orimportant factors applicable to a Rapid Wetland Assessment System.This thesis describes and discusses factors associated with wetland plantcommunities, with an attempt to predict changes in a wetland system. This study wasinitiated with a primary aim of establishing the relationship between plant communitiesand the variation in near-surface water levels in areas occupied by various plantcommunities in the Middelvlei wetland system at Stellenbosch. A second aim was toassess whether water quality had an influence on the plant communities.Seven plant communities are identified and described from this particular wetlandsystem using standard Braun-Blanquet techniques (Typha capensis Reedswamp; Cyperustextilis Sedgeland; Pennisetum macrourum Grassland; Juncus effusus Sedgeland;Cyperus longus Sedgeland; Cliffortia strobilifera Shrubland and Populus canescensForest). The Typha capensis Reedswamp community is found in the wettest parts of thewetlands, with a fluctuation in water table from 0.10 m above surface during the wetseason to 0.43 m below surface during the dry season. The Populus canescens Forest isactively invading the wetland replacing the wetland species by modifying the wetlandhydrological condition. Water samples from 35 wells, collected on a monthly basis over11 months, are used to assess sodium, magnesium, potassium, calcium, nitrate andphosphate, pH, redoxs potencial and dissolved oxygen levels in each community, overfour seasons.Both multivariate analysis (ANOVA) and regression tree analysis (CART) areapplied to evaluate differences between communities or groups of plant communities on aseasonal basis. Direct gradient analysis (CCA) is used to determine the relationshipbetween plant communities and environmental variable gradients.A wide variation in water quality condition between plant communities is present.The Typha capensis Reedswamp community is associated with low nutrient levels(phosphates and nitrates) in all seasons. The Cyperus textilis Sedgeland is associated with low levels of nitrates and high phosphate levels. The Juncus effusus Sedgeland displaysthe highest phosphate concentration, occurring in summer, while low nitrate levels occurin this community during all the seasons.Dissolved oxygen in the near-surface water in this wetland is at very lowconcentrations, and has no significant difference between communities. It plays no majorrole in determining the occurrence and distribution of the plant communities.Most of the water chemical constituents measured in this study are the result ofmultiple complex relationships, with constituent variations occurring differently betweencommunities. A remarkable seasonal distinction in the chemical constituents in differentcommunities is present.Despite the complex nature of the relationships between plant communities andenvironmental factors, the low species diversity levels through the tendency for singlespecies dominance and the strong association of these communities with particularenvironmental variables, the combination of these factors all add value to the use ofwetland vegetation as a good tool to indicate wetland condition. An effort to understandwetland plant communities in relation to determining environmental factors wouldpromote the use of plant communities as user-friendly tools for wetland monitoring andassessment.

[发布机构] Stellenbosch University