African savanna grasses outperform trees across the full spectrum of soil moisture availability

Models of tree-grass coexistence in savannas make different assumptions about the relative performance of trees and grasses under wet vs dry conditions. We quantified transpiration and drought tolerance traits in 26 tree and 19 grass species from the African savanna biome across a gradient of soil water potentials to test for a trade-off between water use under wet conditions and drought tolerance.We measured whole-plant hourly transpiration in a growth chamber and quantified drought tolerance using leaf osmotic potential (psi(osm)). We also quantified whole-plant water-use efficiency (WUE) and relative growth rate (RGR) under well-watered conditions.Grasses transpired twice as much as trees on a leaf-mass basis across all soil water potentials. Grasses also had a lower psi(osm) than trees, indicating higher drought tolerance in the former. Higher grass transpiration and WUE combined to largely explain the threefold RGR advantage in grasses.Our results suggest that grasses outperform trees under a wide range of conditions, and that there is no evidence for a trade-off in water-use patterns in wet vs dry soils. This work will help inform mechanistic models of water use in savanna ecosystems, providing much-needed whole-plant parameter estimates for African species.

Automated summary

This study found that there are differences in the performance of trees and grasses in wet and dry conditions in savannas. Grasses transpired twice as much as trees on a leaf-mass basis and had a lower leaf osmotic potential, indicating higher drought tolerance. These results provide insights into the water use mechanisms in savanna ecosystems and offer much-needed whole-plant parameter estimates for African species.