Determining maturity and population structure in Macrozamia parcifolia (Zamiaceae), a threatened Australian cycad

Abstract

There is difficulty assigning maturity to non-arborescent (trunkless) cycad species and as a consequence in determining the mature–immature structure of populations, which is important for their management, particularly for those under threat. The aim of this investigation was to find a reliable and simple method to determine maturity for the threatened, non-arborescent cycad Macrozamia parcifolia P.I. Forst. & D.L. Jones, and to incorporate this information into a population structure. Measurements were taken from tagged plants on four quadrats in eucalypt-dominated open forest in south-eastern Queensland, Australia. Using a single time-point dataset of three variables associated with the longest mature leaf, basal petiole width coupled with several years of coning evidence was found best at distinguishing mature plants. Choice of this variable and the threshold point to class non-coning plants as mature or immature was through a classification-tree model using a binary recursive partitioning process, the tree being pruned to identify the best variable and threshold point via a cross-validation process. This simple, reliable method to determine maturity was still effective when using a single time-point dataset for coning evidence. The method can be applied to other threatened, non-arborescent cycads, which could aid in their conservation management. The structure of M. parcifolia population was bimodal. The mode that encompassed immature plants was broadly reverse-J shaped, indicating younger immature plants had highest mortality. Reasons for the bimodality are possibly complex, but could simply highlight a non-lineal relationship of basal petiole width with plant age.