Root system of Terminalia sericea shrubs across rainfall gradient in a semi-arid environment of Etosha National Park, Namibia

Abstract

Unlike any other biome, savannas consist of both trees and grasses in a persistent and equitable manner. This unique mixture of two different life forms attracted scientific scrutiny for decades. Embedded in this body of research studies is bush encroachment that affects many overgrazed savannas. Despite scientific efforts, however, the underlying factors that shape the functioning of this biome and govern bush encroachment remain largely elusive. Terminalia sericea is one such encroaching woody plant at the expense of herbaceous layers in overgrazed areas of southern Africa. Recent studies indicated a potential link between rainfall amount and opportunistic rooting systems of encroaching species for harvesting near-surface soil water. This study was thus carried out to specifically test this theory in Etosha National Park across a rainfall gradient. A total of 31 T. sericea shrubs were excavated to examine their rooting systems. Results show that although all plants surveyed showed a shallow root system, only two out of 15 shrubs in the drier section of the park exhibited a taproot. In contrast, half of the 16 shrubs studied in the wetter part displayed a taproot. This suggests that T. sericea shrubs employ a rooting strategy attuned to local climatic conditions. A differential competition between woody plants and the herbaceous layers for soil water is thus variably enhanced, which might act as one of the feeding mechanisms for bush encroachment. Future ecological models of the savanna ecosystems should therefore recognise variation in the rooting characteristics of this, and possibly other bush encroaching plants, in response to precipitation.