Secondary branching in Wollemia nobilis (Araucariaceae), an unexpected occurrence

Abstract

Context Until now, Wollemia nobilis was thought to differ from all other members of the Araucariaceae in having unbranched primary branches, except following some instances of traumatic damage to the branch apex. This paper reports on the serendipitous discovery of abundant secondary branching on seedling Wollemi pines. Aims I document the occurrence of secondary branches from axillary meristems, along the whole length of annual growth cycles of primary branches. Methods The abundance and location of secondary branching was observed, photographed and documented in over 90 nursery seedlings being grown for sale across two growing seasons. Key results Secondary buds and both secondary and tertiary branches occurred on 1-year-old primary branch segments on numerous seedlings being grown under standard commercial-nursery conditions. Conclusions Wollemia is capable of secondary and tertiary branch development without traumatic injury to the primary or secondary branch apex, thus confirming an earlier report that all or nearly all leaf axils in Wollemia may contain viable axillary meristems. This contradicts previous expectations that secondary branches will not develop without prior apical trauma to primary branches. Implications Secondary branching, as reported here, opens the door to further assessment of the fossil record for specimens that may broaden the concept of Wollemia and possibly its former distribution. The occurrence of these secondary branches should allow a reconsideration of the characters that distinguish the genus from other members of the family. They also broaden the understanding of the evolutionary potential of the species.