Root structure and water-use diversity of Kalahari savanna woody plant communities

Background/Question/Methods: The root zone of savannas is often conceptualized as two homogeneous layers with shallow-rooted grasses and deep-rooted trees, but recent findings on water-use characteristics and drought response suggest that the root structure of coexisting woody plants may be more diverse. Using compressed air, we excavated individual trees and shrubs and mapped their coarse root systems on a three-dimensional grid system up to 1.5 m depth. We excavated four woody savanna species at three sites spanning a climate gradient on the Kalahari Transect in southern Africa.

Results/Conclusions: Overall, species was more important than site in determining rooting depth, distribution with depth, and lateral relative to vertical extent, as well as small-scale characteristics of root system structure such as root taper rate and sinuosity. The species excavated fell into two groups that coexisted across the climate gradient. Acacia mellifera and Terminalia sericea had straight roots in a laterally-extensive system that was shallow relative to the aggregate root profile for woody plants at the sites. Boscia albitrunca and Ochna pulchra had sinuous roots that were mostly concentrated beneath the canopy and were more prevalent in deep than near-surface soil layers, departing from the conventional model of decreasing root abundance with depth. The shallow-rooted species had small taproots, though it is unlikely that they reached the water table. Deep- and shallow-rootedness appear to correlate with other characteristics such as growth form (tree or shrub) and drought deciduousness.