Stable isotope evidence of plant water sources for alien and indigenous riparian trees in headwater streams, South Africa

Abstract

Vegetation can strongly influence the hydrology of headwater catchments, particularly where alien species invade riparian zones in grassland-savanna ecosystems. Stable isotopes (δ18O and δ2H) were used to investigate water sources and seasonal water-use patterns of alien and indigenous riparian trees along a headwater stream. Rainwater, groundwater, streamwater, soil and plant xylem water were sampled over two years during wet and dry seasons. Bayesian mixing models revealed diverse water-use strategies, with riverbank trees primarily relying on streamwater, while trees further from the channel used a combination of soil water and ground- or streamwater. Results indicated that the dominant alien species increased its reliance on groundwater during the dry season. This study provides new insights into the hydrology and ecohydrology of a poorly studied headwater catchment and demonstrates the value of stable isotope techniques for informing catchment-scale hydrological modelling and predicting the impacts of large-scale vegetation change, including afforestation and restoration.