Water transport from stem to stomata: the coordination of hydraulic and gas exchange traits across 33 subtropical woody species.

Coordination between sapwood specific hydraulic conductivity (Ks) and stomatal conductance (gs) has been identified in previous studies, however, coordination between leaf hydraulic conductance (Kleaf) and gs, as well as between Kleaf and Ks is not always consistent. This suggests that there is a need to improve our understanding of the coordination among hydraulic and gas exchange traits. In this study, hydraulic traits (e.g., Ks and Kleaf) and gas exchange traits, including gs, transpiration (E), and net CO2 assimilation (Aarea) were measured across 33 co-occurring subtropical woody species. Kleaf was divided into two components: leaf hydraulic conductance inside the xylem (Kleaf-x) and outside the xylem (Kleaf-ox). We found that both Kleaf-x and Kleaf-ox were coordinated with gs and E, but the correlation between Kleaf-ox and gs (or E) were substantially weaker, and that Ks was coordinated with Kleaf-x, but not with Kleaf-ox. In addition, we found that Ks, Kleaf-x and Kleaf-ox together explained 63% of the variation in gs and 42% of the variation in Aarea across species, with Ks contributing the largest proportion of explanatory power, whereas Kleaf-ox contributing the least explanatory power. Our results demonstrate that the coordination between leaf water transport and gas exchange, as well as the hydraulic linkage between leaf and stem, were weakened by leaf outside-xylem hydraulic conductance. This highlights the possibility that water transport efficiencies of stem and leaf xylem, rather than that of leaf tissues outside the xylem, are important determinants of stomatal conductance and photosynthetic capacity across species.