Root trait shifts towards an avoidance strategy promote productivity and recovery in C3 and C4 pasture grasses under drought

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 water and nutrient acquisition. Knowledge of root trait correlations and associated trait plasticity under drought is crucial to sustaining grassland production, including rangelands and pastures, under future, drier climates. Here, we investigated below-ground responses to drought in four C3 and three C4 grasses and whether these explain above-ground responses during extreme cool-season drought and post-drought recovery in a well-replicated field experiment. We hypothesised that (a) C3 and C4 functional groups differ in their below-ground trait responses to drought: C3 species shift their traits in line with a drought avoidance strategy and C4 species shift towards drought tolerance strategies; (b) post-drought recovery is associated with below-ground carbon reserves in both functional groups. Root traits in the C3 functional group differed from C4 by having higher values for specific root length, nitrogen and soluble sugar concentrations, while C4 root traits were associated with higher mean diameter, tissue density and starch concentrations. Drought-induced plasticity in root traits in both functional groups was evidenced by increased values for specific root length, soluble sugars and nitrogen concentrations, and reduced values for tissue density, which we interpret as a drought avoidance strategy. Increasing root soluble sugars in C3 and C4 grasses and lower root tissue density in C4 species under drought were associated with greater above-ground productivity. Above-ground biomass recovery post-drought was predicted by root and crown soluble sugar concentrations and root longevity across both plant functional groups. These findings highlight the importance of understanding links between root trait plasticity and plant productivity during and following drought for predicting plant species' responses to changes in a future climate. Read the free Plain Language Summary for this article on the Journal blog.