Evolutionary ecology of mast-seeding in temperate and tropical oaks (Quercus spp.)

Abstract

Mast-seeding is the synchronous production of large seed crops within a population or community of species every two or more years. This paper addresses three non-mutually exclusive hypotheses explaining the evolution of mast-seeding in temperate tree species, especially the genus Quercus: (1) mast-seeding is a consequence of mast-flowering which evolves to increased pollination efficiency in mast-flowering years; (2) mast-seeding has evolved as an anti-predator adaptation by which large seed crops during mast years satiate the seed predators and allow survival of some of the seeds; (3) selection on seed size by habitat can indirectly affect the evolution of masting if trees with large seeds require more time to accumulate reserves to mature those seeds. I find support for the pollination hypothesis in several windpollinated temperate tree species but not oaks. However, oaks show evidence favoring the predation and seed size hypotheses. I then develop a model to illustrate the relationships among the three hypotheses in their effects on the evolution of masting. Finally, using data from herbaria and Floras, the influence of selection via flowering, fruiting, and seed size in the evolution of masting in tropical oaks is discussed. I conclude that the need for a supra-annual cue to synchronize flowering and fruiting as well as the larger seed size found in many tropical oak species should contribute to the evolution of masting to a greater extent than seen among temperate oaks.