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Summary 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 (Ψ 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 Ψ 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. 

Abstract Root‐based functional traits are relatively overlooked as drivers of savanna plant community dynamics, an important gap in water‐limited ecosystems. Recent work has shed light on patterns of trait coordination in roots, but less is known about the relationship between root functional traits, water acquisition, and plant demographic rates. Here, we investigated how fine‐root vascular and morphological traits are related in two dominant PFTs (C₃trees and C₄grasses from the savanna biome), whether root traits can predict plant relative growth rate (RGR), and whether root trait multivariate relationships differ in trees and grasses. We used root data from 21 tree and 18 grass species grown under greenhouse conditions, and quantified a suite of vascular and morphological root traits. We used a principal components analysis (PCA) to identify common axes of trait variation, compared trait correlation matrices between the two PFTs, and investigated the relationship between PCA axes and individual traits and RGR. We found that there was no clear single axis integrating vascular and morphological traits, but found that vascular anatomy predicted RGR in both trees and grasses. Trait correlation matrices differed in trees and grasses, suggesting potentially divergent patterns of trait coordination between the two functional types. Our results suggested that, despite differences in trait relationships between trees and grasses, root conductivity may constrain maximum growth rate in both PFTs, highlighting the critical role that water relations play in savanna vegetation dynamics and suggesting that root water transport capacity is an important predictor of plant performance in the savanna biome.