Water sourcing by trees in a mesic savanna: Responses to severing deep and shallow roots

The co-dominance of trees and grasses is a much-debated topic in savanna ecology. One of the proposed mechanisms facilitating the co-dominance of these two different life-forms is root niche partitioning. Specifically, the rooting zone is hypothesized to have two layers: grasses are thought to be the superior competitors for water in the upper soil horizons, whereas trees avoid competitive exclusion by sourcing water from soil layers beyond the vertical reach of grasses. We examined this two-layer hypothesis through a field experiment in which we severed deep and shallow roots of Terminalia sericea saplings, 6 weeks before the onset of the new growing season, using a novel technique. During the course of the growing season, physiological responses to the root severing were measured. Severing lateral (shallow) roots resulted in rapid abscission of the old leaves, smaller leaves after the onset of the following growing season and lower xylem pressure potentials – indicating higher water stress – compared to the other treatment groups. Effects of severing deep-growing taproots were small, demonstrating the importance of lateral roots for maintaining the trees’ water balance, notably in drier times. We have shown that saplings of the common and locally dominant savanna tree T. sericea manage to coexist with grasses without avoiding competition through spatial root separation. The results of our study therefore do not support the two-layer hypothesis for explaining tree–grass coexistence in this mesic savanna.

• Trees in savannas are thought to avoid competition with grasses by sourcing deep water. • To test this hypothesis, deep and shallow roots of Terminalia sericea were severed. • Severing deep roots had little effect; severing shallow roots induced severe water stress. • T. sericea is able to coexist with grasses without avoiding competition by spatial root separation.