Adaptation to climate change of dioecious plants: does gender balance matter?

Since the 1877 publication of Charles Darwin's The Different Forms of Flowers on Plants of the Same Species, interest in understanding the fundamental biological processes of sexual and gender dimorphism in dioecious plant species has continued. Dioecy is found in 157 flowering plant families and 7.5% of flowering plant genera, about 6% of the 240,000 angio-sperm species (Renner and Ricklefs 1995). Dioecious plants provide an excellent opportunity for examining the trade-offs in resource allocation related to plant reproduction (Cepeda-Cornejo and Dirzo 2010). Natural selection (acting within phy-logenetic, physiological and ecological constraints) shapes patterns of resource allocation, balancing the costs and benefits associated with these trade-offs, resulting in the evolution of life-history strategies maximizing fitness. Male and female individuals play different roles in the reproductive biology of a dioecious species and hence have very different resource demands imposed upon them (Thomas and LaFrankie 1993). The selection pressures presented by different resource demands could in turn lead to the evolution of sexual dimorphism (Meagher 1984), the separation of male (androecium) and female (gynoecium) plant characters that are not directly related to gamete production. Plants are predicted to allocate more resources to the sexual function that has the greater marginal rate of return (Charnov 1982). Resources that could be used for the production of new leaves, stems and roots are shifted to the production of flowers , fruits and seeds. Males and females of many dioecious species differ from one another in terms of their life histories, involving differences in photosynthetic performance (Dawson and Geber 1999) water use (Rowland and Johnson 2001), plant phenology (Delph 1999) and herbivory tolerance (Cornelissen and Stiling 2005). The theory of ecological causation explains differentiation in secondary characteristics between sexes by postulating that males and females have adapted to different ecological niches (Shine 1989). Spatial segregation of the sexes associated with microhabitat differences is common in dioecious tree species for which sexes are generally distributed such that females are more common in high-resource microsites and males are more common in low-resource microsites (Dawson and Bliss 1989). Because segregation of the sexes may be adaptive, global change and subsequent changes to resource availability and allocation pattern may change the sex ratio and reproductive success of many dioecious species (Hultine et al. 2008). Land clearing and environmental pollution may lead to the subdivision of continuous habitats into fragmented patches, eventually leading to loss of biodiversity. Dioecious plants are expected to …