An anatomical assessment of branch abscission and branch-base hydraulic architecture in the endangered Wollemia nobilis.

•Background and Aims The branch base xylem structure of the endangered Wollemia nobilis was anatomically investigated. Wollemia nobilis is probably the only extant tree species that produces only first order branches and where all branches are cleanly abscised. We wondered if these unusual features might influence branch base xylem structure and water supply to the foliage.•Methods The xylem was sectioned at various distances along the branch bases of 6 yr old saplings. Huber values and relative theoretical hydraulic conductivities were calculated for various regions of the branch base.•Key Results The most proximal branch base featured a pronounced xylem constriction. The constriction had only 14-31% (average 21%) of the cross sectional area and 20-42% (average 28%) of the theoretical hydraulic conductivity of the more distal branch xylem. Wollemia nobilis had extremely low Huber values for a conifer. •Conclusions The branch base xylem constriction would appear to facilitate branch abscission while the associated Huber values show that W. nobilis supplies a relatively large leaf area through a relatively small diameter ‘pipe’. It is tempting to suggest that the pronounced decline of W. nobilis in the Tertiary is related to its unusual branch base structure but physiological studies of whole plant conductance are still needed.