On the relationship between root economics and plant hydraulic traits

Drought stress constrains the productivity of terrestrial ecosystems and distribution of plant species. To withstand drought stress, plants have evolved a wide range of water-use strategies. Despite the critical function of roots in whole-plant water regulations, drought strategies have been primarily studied from an aboveground perspective. Our knowledge of how fine-root traits are related to aboveground hydraulic traits remains limited. Here, we compiled a global dataset comprising five aboveground plant hydraulic traits (the xylem water potential at 50% loss of hydraulic conductivity [P50], the water potential at turgor loss point [πtlp], maximum xylem conductivity per unit sapwood area [Ks], the leaf-to-sapwood area ratio [Al:As], and wood density [WD]) associated with water-use strategies and the four fine-root traits from the root economics space (mean root diameter [MRD], specific root length [SRL], root tissue density [SRL], and root nitrogen concentration [RN]). We then investigated how and to what extent the global diversity in ecological strategies of four root economics space traits relate to aboveground hydraulic traits contributing to drought resistance. We found a slight trend towards acquisitive woody species with higher RNC and lower RTD having lower drought resistance in aboveground tissues (less negative P50 and πtlp and higher Ks and Al:As). Outsourcing woody species with thicker roots displayed a tendency towards slightly higher drought resistance (more negative P50 and πtlp and higher Ks and Al:As) than thin-rooted woody species. The weakness of these relationships highlights that aboveground plant adaptations to drought might be largely independent from classical axes of fine root ecological strategies. This could indicate a decoupling between above- and belowground strategies of drought adaptations, or that other root traits could provide more efficient adaptations to drought stress and be more strongly coordinated with aboveground hydraulics.