Does sex ratio bias and sexual dimorphism occur in Lindera benzoin L. (Lauraceae) prior to fruit production? 1

Abstract

Secondary (flowering) sex ratio of a wild population of the dioecious woody shrub Lindera benzoin (spicebush) in Maryland has been persistently female biased over a time span of ∼ 17 years. Female-biased secondary sex ratios are relatively uncommon in dioecious woody plants, and are not readily explained by differential costs of reproduction, which are generally higher in females. In fact, prior studies of reproductive adults in L. benzoin populations have shown higher growth rates in males. This leads to questions concerning the source of female-biased sex ratios in some populations. Using seeds collected from 15 maternal plants at the Maryland site in 2013, a potted plant study was conducted from 2014–2020 in which periodic flowering censuses were taken, and plant size parameters were recorded. This study improves upon an analogous common garden study conducted under highly competitive conditions that induced considerable preflowering mortality. In the current study, plants were grown in pots to eliminate root competition and in a loose array to minimize shoot competition; mortality was thus kept to ∼ 1% after seed germination. Like the source population, genet sex ratios in the current study were significantly female-biased, partly due to a trend toward female-biased sex ratios in the offspring of a number of maternal plants. Estimates of plant size were generally higher for females in 2018, just prior to the onset of significant fruit production, supporting earlier results suggesting that females may compensate for differential costs of reproduction that can manifest after fruiting begins. Even in the absence of biased primary sex ratios, formation of female bias in wild populations could involve development of a relatively larger size in the pre-fruiting stage that gives females a temporary advantage over males. This could lead to relatively greater survival and/or earlier flowering of female plants in the early stages of population establishment, and could thus explain observed patterns of sex ratio bias.