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.

1. 植物物种在响应土壤水分限制时，其协同变异的根系性状展现出广泛的可塑性谱系，变异范围从无性状变化到转向增强水分与养分获取的性状。明晰这类植物根系性状关联以及干旱条件下相关性状的可塑性，对于在未来更趋干燥的气候背景下维持牧场与草地生产力至关重要。
2. 本研究依托一项重复良好的野外操控实验，探究了4种C3与3种C4牧草的地下部对干旱的响应，以及此类响应能否解释极端冬春干旱期间及干旱后恢复阶段的地上部响应。我们提出如下假说：其一，C3与C4功能群（functional groups）对干旱的地下部性状响应存在差异——C3物种通过调整性状采取干旱规避策略（drought avoidance strategy），C4物种则转向干旱耐受策略（drought tolerance strategy）；其二，两类功能群的干旱后恢复均与地下部碳储备（carbon reserves）相关。
3. C3物种的根系性状与更高的比根长（specific root length）、氮浓度及可溶性糖浓度相关，而C4物种的根系性状则更多与更高的平均根直径、组织密度（tissue density）及淀粉浓度相关。干旱期间，C3与C4功能群的根系性状可塑性均提升了比根长、根系可溶性糖、根系氮的水平，并降低了根系组织密度的水平，我们将此解读为干旱规避策略的体现。干旱期间的地上部生产力与根系可溶性糖、根颈氮浓度呈正相关，与根系组织密度及根颈可溶性糖呈负相关；而干旱后恢复则与根系及根颈中的可溶性糖含量呈正相关。
4. 本研究结果凸显了明晰根系性状可塑性与干旱及干旱后恢复阶段植物生产力之间关联的重要性，这可为预测植物物种对未来气候变化的响应提供依据。