Proposed mechanism for the origin of sodic patches in Kruger National Park, South Africa

摘要

The accumulation of sodium on the footslopes of granitic catenas in semi-arid southern Africa leads to the formation of sodic patches. Sodic patches are ecologically important for nutrient accumulation, predator evasion and wallowing, but they are often perceived as derelict lands because of vegetation denudation and low aesthetic quality. This negative perception, by both ecologists and tourists, often leads to ill-advised management and 'rehabilitation' measures. In Kruger National Park, sodic patches occur at the riparian-upland boundary and hence the processes originating in both systems may contribute to their origin. The upland-based catena and riparian-based evapotran-spiration models were used to explore the hypothesis that these soils originate from both catena and evapotranspi-ration-driven hydraulic processes. The models predict vegetation zonation dictated by a salt tolerance gradient and an increase in sodic patch area over time as a result of progressive salt accumulation. Vegetationstructure and hillslope morphology across the riparian-upland boundary and analysis of change in patch configuration over a 50-year aerial photographic record were used to test the hypotheses. Sodic patches have unique vegetation, occur on the intermediate positions of hillslopes, have increased in area three-fold over 50 years and are encroaching into the riparian zone. This behaviour is consistent with the assertion that these patches originate from both catena and evapotranspiration processes. Results imply a dynamic aspect of sodic patches, which have been previously viewed as static landscape features in pedogenic time scales. However, we show that they change over relatively smaller time scales meaning that their management should be commensuratewith this dynamic nature.Therefore, sodic patches should be managed under a scheme that incorporates their small-scale expansion and recognizes their ecological importance.