What are the deepest-rooted trees in the world doing down there? Edmund February, Eleanor Shadwell and Klaudia Shactschneider. In the last two decades there have been at least two publications presenting a global synthesis of plant rooting depth. Despite these publications and a general understanding that plant rooting primarily revolves around access to resources there is still very little understanding as to what the roots of the deepest-rooted species in the world are doing down there. From excavations it has been demonstrated that Boscia albitrunca (68 m) and Vachellia erioloba (60 m) from the Kalahari and, Prosopis juliflora (53 m) from Mexico are the three deepest rooted trees in the world. Working on Vachellia erioloba, V. haematoxylon and Prosopis sp. in the Kalahari we ask what these roots are doing down there and are these trees able to adapt to any competition for resources. We do this using δ2H and δ18O values of both groundwater and stem water as well as xylem pressure potentials and δ13C values of leaves. In all the ephemeral rivers in the southern Kalahari the dominant tree species is, Vachellia erioloba, a keystone species. Potentially competing for water in these rivers, park authorities, in the Kgalagadi Transfrontier Park, are pumping water for tourist accommodation from the Auob and Nossob rivers and in the Kuruman Prosopis sp is aggressively invading. Our results show that competition for water with Prosopis sp results in a very low water table of 56 m and severe canopy dieback and even death of A. erioloba, in the Kuruman. In the Auob and Nossob V. erioloba and V. haematoxylon are using the same water that is being pumped by the park and that Prosopis sp is using. In the Auob where water abstraction is relatively low V. erioloba and V. haematoxylon are not affected with the water table relatively high (38-46 m). In the Nossob where abstraction is much higher the V. erioloba have adapted to the lower available water with smaller heavier leaves with less negative δ13C values and marginally but not significantly more negative XPP's. The water table is also much lower (49-59 m). These results would suggest that park authorities have to carefully monitor their water extraction to avoid negatively impacting on the trees in the rivers and that an aggressive program of eradication has to be followed to remove Prosopis sp.
