Herb hydraulics: variation and correlation for traits governing drought tolerance and efficiency of water transport

Hydraulic traits dictate plant response to drought, thus enabling better understanding of community dynamics under global climate change. Despite being intensively documented in woody species, herbaceous species (graminoids and forbs) are largely understudied, hence the distribution and correlation of hydraulic traits in herbaceous species remains unclear. Here, we collected key hydraulic traits for 436 herbaceous species from published literature, including leaf hydraulic conductivity (K_leaf), water potential inducing 50 % loss of hydraulic conductivity (P₅₀), stomatal closure (P_close) and turgor loss (P_tlp). Trait variation of herbs was analyzed and contrasted with angiosperm woody species within the existing global hydraulic traits database, as well as between different growth forms within herbs. Furthermore, hydraulic traits coordination was also assessed for herbaceous species. We found that herbs showed overall more negative P_close but less negative P_tlp compared with angiosperm woody species, while P₅₀ did not differ between functional types, regardless of the organ (leaf and stem). In addition, correlations were found between K_leaf and P₅₀ of leaf (P_50leaf), as well as between P_close, P_50leaf and K_leaf. Within herbs, graminoids generally exhibited more negative P₅₀ and P_tlp, but lower K_leaf, relative to forbs. Within herbs, no clear pattern regarding hydraulic traits-climate relationship was found. Our analysis provided insights into herb hydraulic, and highlighted the knowledge gaps need to be filled regarding the response of herbs to drought.