Root structure of shrub encroaching plants in the African savannas: insights from Terminalia sericea (Burch. ex dc) across a climate gradient in the Kalahari Basin

Abstract

The competitive exclusion of grasses by shrubs in the African savannas is an elusive phenomenon. The popular concept, Walter’s two-layer hypothesis is still inconclusive. This concept suggests that trees and shrubs in the savannas develop deeper roots to avoid competition with grasses. This study was designed to test this hypothesis by investigating the root system of T. sericea, one of the common encroaching species in the Kalahari Basin. Using direct excavation method, 39 shrubs were randomly excavated across the Kalahari Basin.Results revealed contrasting rooting strategies by T. sericea under varying climatic conditions. Drier areas exhibit largely lateral roots, whereas moist sites were dominated by dual root systems. These findings are not consistent with the existing framework which argues that savanna shrubs are essentially deeper rooted. Instead, results support an emerging hypothesis that certain savanna shrubs opportunistically adapt their root systems in response to the prevailing environmental constraints such as water availability A shrub such as T. sericea with lateral roots abundantly deployed in shallow soil depth points to a direct competition exclusion with grasses in the Kalahari Basin. It is probable that the occurrence of shrub encroachment by T. sericea is a manifestation of this competitive interaction, contrary to the root niche partitioning hypothesis. Future savanna models need to be cognizant of the variation in savanna shrubs roots system architecture and its potential implications on tree-grass coexistence and competition.