A comparative assessment of water use by Acacia longifolia invasions occurring on hillslopes and riparian zones in the Cape Agulhas region of South Africa

The detrimental impacts of invasive alien plants on ecosystems and water resources have raised concerns in arid and semi-arid countries like South Africa where the average precipitation is approximately 500 mm/yr, which is below the world average of around 860 mm/yr. Several studies have examined the effects of invasive alien plants such as the Australian Acacias on the water resources. However, few studies have quantified the differences in water use between hillslope and riparian Acacia longifolia invasions. A. longifolia is one of the aggressive invader species in South Africa even on hillslopes that contribute substantially to runoff generation. Therefore, the encroachment of invasive alien plants has the potential to reduce runoff, thereby adversely affecting the available water downstream. This paper aims to; 1) compare transpiration rates of A. longifolia growing on hillslopes and along riparian areas, 2) establish the key drivers for water use by this species, and 3) estimate the hydrological impacts of the invasions at the catchment scale in the Heuningnes catchment, in the Western Cape Province of South Africa. Transpiration by the trees was measured using the heat pulse velocity sap flow method. Automatic weather stations and soil moisture sensors were used to monitor weather and soil water content variations at each site. The results showed that, at the stand level the riparian A. longifolia transpired two times more water (∼596 mm/yr) than on the hillslope (∼242 mm/yr). During years with above average rainfall, the water use rates by the invasions was estimated to be ∼579 mm/yr on the hillslope and could be as much as ∼1348 mm/yr at the riparian site. Thus, the hypothesis that riparian trees use more water than invasions on non-riparian areas was accepted in this study. At the catchment scale (740 km2), the estimated water use by the invasions was 20.5 Mm3. Clearing of all the invasions in the study catchment is likely to make 17 Mm3/yr of water available. Hence the clearing of A. longifolia along the riparian corridors should be prioritised as this will lead to water savings at the catchment scale.