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 nobiliswas anatomically investigated. Wollemia nobilis is probablythe only extant tree species that produces only first-orderbranches and where all branches are cleanly abscised. An investigationwas carried out to see if these unusual features might influencebranch-base xylem structure and water supply to the foliage. Methods: The xylem was sectioned at various distances along the branchbases of 6-year-old saplings. Huber values and relative theoreticalhydraulic conductivities were calculated for various regionsof 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 % (average28 %) of the theoretical hydraulic conductivity of themore distal branch xylem. Wollemia nobilis had extremely lowHuber values for a conifer. Conclusions: The branch-base xylem constriction would appear to facilitatebranch abscission, while the associated Huber values show thatW. nobilis supplies a relatively large leaf area through a relativelysmall diameter ‘pipe’. It is tempting to suggestthat the pronounced decline of W. nobilis in the Tertiary isrelated to its unusual branch-base structure but physiologicalstudies of whole plant conductance are still needed.