Spatial and temporal patterns of sexual dimorphism and sex ratio in Lindera benzoin L. (Lauraceae)

• Published in: The journal of the Torrey Botanical Society 2013-09, Vol.140 (3), p.280-299
• Main Authors: Cipollini, Martin L., Culberson, Joshua, Whigham, Dennis, Johnson, Kartee, Knight, Troy, O'Neill, Jay
• Format: Article
• Language: English
• Subjects: Botany; Flowering; Flowers & plants; Gender bias; Herbivores; Lauraceae; Lindera benzoin; Male animals; Plant growth; Plant reproduction; Plants; Population growth; Population size; Sex ratio; Sexual dimorphism
• ISSN: 1095-5674; 1940-0616
• DOI: 10.3159/TORREY-D-13-00013.1

The focus of this study is spatial and temporal variation in sex ratio and sexual dimorphism in the woody dioecious shrub Lindera benzoin (spicebush, Lauraceae). This species has been shown to express female-biased flowering sex ratios in a focal study population in Maryland. Sex ratio variation and sexual dimorphism in dioecious plants has attracted the focus of many short-term, single site studies, particularly in light of Fisher's (1930) elucidation of the selective pressures favoring the production of unbiased progeny sex ratios. As informative as such studies have been, long term, multi-site studies are necessary for full evaluation of the factors leading to differential expression of sex within and among populations. Female-biased sex ratios, relatively rare among plant species, are particularly difficult to explain when many studies, including ours of L. benzoin, show females to have higher costs of reproduction and lower modular growth rates suggesting a competitive disadvantage in comparison with males. This study focuses intensively on a single population in Maryland, and is augmented by flowering censuses, and plant size and growth estimates in populations at 11 nearby sites in Maryland, and four distant sites in Pennsylvania, Virginia and West Virginia. Data were collected at most sites in the early 1990s and again in 2010, so that long-term variation in sex ratio could be explored. Results show tendencies for overall female-bias, for female bias in small size classes and male bias in large size classes, for an increase in the proportion of males within populations over time, and for sex ratios to be biased toward females in relatively recently established populations but unbiased or male-biased in older populations. Sexual dimorphism favoring males was consistent across sites, but no sexual dimorphism in herbivory by mammals or by foliage feeding insects was detected. These field observations combined with data derived from a separate but concurrent garden study suggest that early establishment of female-biased sex ratios may occur via a faster early growth rate and possible better tolerance of herbivory by females. This model further suggests that early advantages of females are followed by post-reproductive demographic changes favoring males, resulting in a tendency for sex ratios to shift toward maleness over time.