Climate-driven shifts in rainfall regimes could alter the vertical distribution of water in soils, with potential consequences for the balance of deep- and shallow-rooted species. However, the extent to which trees and grasses compete for the same water source has remained unclear, particularly in tropical and subtropical savannahs.
Ricardo Holdo, of the University of Missouri, and Jesse Nippert examined the sensitivity of tree and grass transpiration to depth-specific variations in soil water content in a lowland savannah ecosystem in South Africa over the course of a growing season. Tree transpiration was inferred from measurements of sap flow, and grass transpiration from measurements of canopy temperature. The stable isotope signature of soil and xylem water was used to determine the source of the soil water. Tree transpiration proved to be positively correlated with soil water content at 40-cm depth, but largely insensitive to variations in soil moisture at shallower depths. In contrast, grass transpiration increased with soil water content at 5-cm depth, but was unaffected by variations at greater depths.
The differential sensitivity of trees and grasses to different sources of soil water adds to the evidence for depth-dependent niche partitioning among trees and grasses in savannah systems.