Xylem Hydraulics of Two Temperate Tree Species with Contrasting Growth Rates

Hydraulic functionality is crucial for tree productivity and stress tolerance. According to the theory of the fast-slow economics spectrum, the adaptive strategies of different tree species diverge along a spectrum defined by coordination and trade-offs of a suite of functional traits. The fast- and slow-growing species are expected to differ in hydraulic efficiency and safety; however, there is still a lack of investigation on the mechanistic association between tree growth rate and tree hydraulic functionality. Here, in a common garden condition, we measured radial growth rate and hydraulic traits in a fast-growing (Populus alba L. x P. berolinensis Dippel) and a slow-growing tree species (Acer truncatum Bunge), which are both important tree species for afforestation in northern China. In line with the contrasts in radial growth rate and wood anatomical traits at both the tissue and pit levels between the two species, stem hydraulic conductivity of the Populus species was significantly higher than that of the Acer species, but the resistance to drought-induced xylem cavitation was the opposite. A trade-off between hydraulic efficiency and safety was observed across the sampled trees of the two species. Higher water-transport efficiency supports the greater leaf net photosynthetic carbon assimilation capacity of the Populus species and hence facilitates fast growth, while the conservative hydraulic traits of the Acer species result in a slower growth rate but enhanced drought tolerance.