For over 100 years trees have been assumed to use deep roots to avoid competition with grasses.  This ‘two-layer’ hypothesis has been used to describe the basic structure and function of savanna ecosystems, but the difficulties of belowground research have prevented direct tests.  

Results/Conclusions

Using a depth-controlled tracer study in Kruger National Park, South Africa, we show that trees foraged almost exclusively from 20 cm depths, over wide areas, and from within their own storage tissues.  Grasses, in contrast, foraged from throughout the soil profile (0-120 cm) over narrow areas.  These results fill a fundamental gap in our understanding of plant-plant and soil-plant-atmosphere interactions in an ecosystem that covers over one-sixth of earth’s land surface.