Dense roots with low specific root length underpin performance of dryland restoration species under two types of drought

Trait‐based approaches may improve plant performance in the restoration of drought‐prone landscapes. However, because plants use diverse strategies to cope with drought, traits that underpin performance may differ by drought context. In addition, plants may alter traits via phenotypic plasticity to cope with drought, but whether these adjustments improve plant performance during drought remains unknown. We completed a greenhouse experiment using five grass and three forb species to: (1) assess how individual traits and multivariate and bivariate trait relationships respond to moderate and severe drought; (2) identify which traits or trait axes are associated with plant performance under both drought conditions; and (3) determine which species are good candidates for restoration in drought settings. Plants reduced root diameter and increased root tissue density and dry matter content in response to drought. Despite these phenotypic adjustments, plasticity was not an important predictor of plant performance under either drought scenario. Rather, we found that species with high root tissue density and low specific root length (i.e., the length of roots per unit mass) performed best under both moderate and severe drought conditions. We also observed an unexpected decoupling of aboveground and belowground traits; conservative root traits (i.e., high root dry matter content) were coupled with acquisitive leaf traits (i.e., high specific leaf area). Our results suggest that dryland species may exhibit unique aboveground–belowground trait relationships and that species with dense roots and low specific root length may be particularly good candidates for restoration in increasingly arid ecosystems.