Root trait responses to drought and warming

1. Plant species show a broad spectrum of plasticity in their covarying root traits in response to soil water limitation, ranging from no change to shifts towards traits that enhance the acquisition of water and nutrients. Knowledge of these plant root trait correlations and associated trait plasticity under drought is crucial to sustaining pasture and grassland productivity under future, drier climates. 2. Here, we investigated belowground responses to drought in four C3 and three C4 pasture grasses and whether these explain aboveground responses during extreme winter and spring drought, and post-drought recovery, in a well replicated field manipulation experiment. We hypothesised that: 1) C3 and C4 functional groups differ in their belowground trait responses to drought: C3 species shift their traits in line with a drought avoidance strategy and C4 species shift towards drought tolerance strategies; 2) post-drought recovery is associated with belowground carbon reserves in both functional groups. 3. Root traits in C3 species were associated with higher values for specific root length, nitrogen and soluble sugar concentrations, while those in C4 species were more associated with higher mean diameter, tissue density and starch concentrations. During drought, plasticity in root traits in both C3 and C4 functional groups increased values for specific root length, root soluble sugars, root nitrogen, and reduced values for root tissue density, which we interpret as a drought avoidance strategy. Aboveground productivity during drought was related positively with root soluble sugars, crown nitrogen concentrations and negatively with root tissue density and crown soluble sugars, and post-drought recovery was correlated positively with soluble sugars in roots and crowns. 4. These findings highlight the importance of understanding links between root trait plasticity and plant productivity during drought and post-drought recovery for predicting plant species’ responses to changes in a future climate.