Hydraulic traits are coordinated but decoupled from carbon traits in herbaceous species

Plant hydraulic traits primarily define the water regulation strategy, thus enabling a better understanding of vegetation structure, function and dynamics under varying hydro-environments. Despite being intensively documented in woody species, the variation and correlation of hydraulic traits across herbaceous species remain largely understudied. Here, we report on the leaf hydraulics of nine herbs with contrasting growth forms (graminoid and forb). Traits quantifying drought resistance, including leaf water potential thresholds triggering xylem embolism (P_x), stomatal closure (P_gs) or leaf turgor loss point (P_tlp), and minimum conductance (g_min), together with leaf gas exchange, morphological traits and biomass allocation, were measured on pot-grown plants. In addition, an in situ dry-down was imposed on four representative species, with leaf gas exchange, water potential and level of xylem embolism being continuously monitored during dehydration to determine the dynamics of stomatal closure and leaf xylem embolism. We found that the studied graminoids tended to be more drought tolerant than forbs, although the difference in hydraulic safety margin for stomatal closure (HSM_st) did not differ significantly between these growth forms. Across species, P_x was coordinated with P_gs and P_tlp, but was decoupled from gas exchange traits, including maximum photosynthetic rate and stomatal conductance. Furthermore, no correlations were found between hydraulic traits and specific leaf area or the ratio of aboveground to belowground biomass. For plants that experienced in situ dehydration, stomatal closure always preceded the onset of xylem embolism in leaves. Moreover, species exhibited a distinct stomatal regulation strategy during the dehydration despite belonging to the same growth form. Our findings contribute to the understanding of herb hydraulics, which will inform prediction on the dynamics of grassy ecosystems by providing traits data and guiding the classification of plant functional types in ‘grassy’ ecosystems. Read the free Plain Language Summary for this article on the Journal blog.