Sex ratios, fruit set and size-class structure in the threatened, gynodioecious, sand-dune species Pimelea arenaria (Thymelaeaceae) from New Zealand

Abstract

Pimelea arenaria sens. str. Cunn. is one of a small suite of native species that occur exclusively on unconsolidated sand dunes and in dune hollows of the North Island and Chatham Islands of New Zealand. It has become extinct at several beaches, and is currently listed in the Gradual Decline category of threatened plants. Eighteen populations of P. arenaria from throughout the North Island of New Zealand were investigated to determine sex ratios, fruit set and population size-class structures. Sex ratios were variable among the 18 study populations; the proportion of females was higher in populations in the northern half of the North Island (15.9–45.5%) than in populations from Kawhia southwards (0–12.7%). Females were absent from three south-western coastal populations. Although fruit set was relatively high, averaging 47% for female and 68% for hermaphroditic plants, recruitment failure was evident at most of the 18 sites surveyed. There was no evidence that sex ratios or fruit set were factors contributing to recruitment failure. Although most of the populations surveyed are not under immediate threat, lack of recruitment could affect population persistence in the long term.