Physico-chemical impacts of terrestrial alien vegetation on temporary wetlands in a sclerophyllous Sand fynbos ecosystem

Temporary depression wetlands form a characteristic feature of the wet-season landscape in the south-western Cape region of South Africa, yet they remain largely unstudied. We hypothesized that the loss of sclerophyllous Sand fynbos habitat around these temporary wetlands causes in-wetland physico-chemical changes. We expected to observe a decrease in the concentration of humic substances in wetlands and a corresponding rise in surface water pH as fynbos cover decreases around wetlands, in this case owing to alien vegetation invasion. A set of 12 differentially invaded temporary depression wetlands within a Sand fynbos ecosystem in Cape Town were repeatedly sampled during the 2009 wet season. Multivariate gradient analysis techniques revealed associations between fynbos cover bordering wetlands and various physico-chemical and biotope constituents within wetlands. Univariate linear regression models reported strong and temporally consistent negative relationships between terrestrial fynbos vegetation cover and wetland pH and positive relationships with humic content. Results indicate that replacement of Sand fynbos with alien vegetation causes a reduction of humic input to wetlands, which in turn has knock-on effects on other wetland environmental constituents such as pH. These findings are expected to be applicable to any wetland where surrounding humic-rich sclerophyllous vegetation is replaced by a non-sclerophyllous agent.