ObjectiveThis paper intends to explore the axial variation of the anatomical structure of wood sapwood of Catalpa bungei, in order to deepen our understanding of the water conduction mechanism in broadleaved tree species, as well as to provide theoretical basis for the cultivation of C. bungei plantation, the protection of this precious tree species and the utilization of its wood.

MethodThree trees of C. bungei were selected, sapwood samples were collected from tree height at 0, 1.3, 3.8, 6.3, 8.8 and 11.3 m, respectively. Meanwhile, sapwood area was measured, transverse and longitudinal sections of wood blocks were prepared. The anatomical traits of xylem such as vessel lumen diameter, vessel frequency were observed with light microscopy. Relationships among wood anatomical traits were tested by correlation analysis and analysis of variance, and axial changes of wood anatomical traits were analyzed by linear regression.

Result(1) Earlywood vessel lumen diameter and vessel density do not change significantly with tree height, however, earlywood vessel lumen diameter decreased with height while vessel density had an opposite trend. Maximum vessel lumen diameter of early- and latewood, latewood vessel lumen diameter and pit membrane diameter varied significantly with tree height, which decreased with tree height. (2) Both sapwood area and hydraulic vessel diameter decreased significantly with tree height. (3) Both sapwood area and pit membrane diameter were significantly positively correlated with the hydraulic vessel diameter.

ConclusionAxial variance of hydraulic structure of C. bungei is manifested in three aspects: sapwood area, vessel related traits and pit membrane properties. Growth ring of C. bungei was obvious, and the early and late vessel lumen diameter wood differed greatly with bigger variation of early wood than that of late wood. Axial variation of the maximum vessel lumen diameter, sapwood area and pit membrane diameter are significant for this tree species whereas axial variation of the vessel density is not significant. Taken together, wood at the base of C. bungei owned relatively large and sparse vessels and higher proportion of sapwood area compared to many small vessels and lower proportion of sapwood area in wood at the upper stem, and this architecture is an optimized structural design for the long-distance water transport function during xylem evolution, leading to reduction of embolization risk and improvement of efficiency and safety of water transport.