Angiosperm tree species in temperate regions are broadly divided into diffuse-porous and ring-porous species based on their xylem anatomy. Diffuse-porous species show very little distinction between the diameter of vessel elements in early versus late wood, while ring-porous species have a bimodal distribution of vessel diameters associated with large, early season vessels and small late season vessels. These anatomical differences result in differences between these two kinds of tree species in stem water transport capacity and in the vulnerability to drought-induced cavitation. However, it is not clear if diffuse-porous and ring-porous species show differences in leaf hydraulic traits. Water transport resistance in leaves accounts for 30%-80% of the total hydraulic resistance of the whole-plant water transport pathway, and relatively few studies have focused on leaf hydraulics owing to methodological barriers; hence, elucidating the differences between diffuse-porous versus ring-porous species in leaf hydraulics and in leaf hydraulic trait coordination with stem hydraulic traits can be helpful in demonstrating the differences between diffuse-porous and ring-porous tree species in plant water use, geological distribution, leaf phenology and ecological adaptation.
We compared hydraulic traits of branches and leaves in three diffuse-porous deciduous tree species (Populus tomentosa, Platanus hispanica, Prunus serrulata) and three ring-porous deciduous tree species (Robinia pseudoacacia, Albizia julibrissin, Fraxinus chinensis) growing in northwestern China. Branch and leaf water transport capacity was evaluated by their maximum hydraulic conductivities (K_max), and the hydraulic vulnerability was evaluated with P50, which corresponds to the branch or leaf water potential at 50% loss of maximum hydraulic conductivities. For stems, P50 was inferred from the vulnerability curves generated by air injection or bench dehydration method. For leaves, the curves were constructed by measuring hydraulic conductance (K_leaf) in leaves rehydrated from a range of water potentials (ψ_leaf). K_leaf was measured by assessing kinetics of ψ_leaf relaxation upon leaf rehydration.
The results showed that branch cross-sectional area-based maximum specific conductivities (K_s-max) of the ring-porous species were greater than those of the diffuse-porous species. Ring-porous species were more vulnerable to cavitation (P50 _branch) than diffuse-porous species and a tradeoff relationship was evident between K_s-max and P50_branch in branches. No differences were found between leaf water transport capacities (K_l-max) or the vulnerability to hydraulic dysfunction (P50_leaf) in the two species types, and there was no tradeoff relationship between K_l-max and P50_leaf. In the three diffuse-porous species, leaves were more vulnerable than branches to water stress-induced dysfunction, but in the ring-porous species, branches were more vulnerable than leaves. Pearson correlation analysis indicated that there was no correlation between branch and leaf hydraulic traits (K_max and P50) in the six investigated woody species. These results suggest that in our study: (1) diffuse-porous and ring-porous species diverged mainly in branch but not in leaf hydraulics, so that leaf hydraulics alone cannot be used to explain the differences between these two tree types in ecological function and adaptation. (2) Branch and leaf hydraulic traits were relatively independent and may be correlated with branch and leaf structure, respectively.