The Role of Age-Structure in the Optimal Germination Fraction of Seeds

Abstract

When a plant living in an unpredictable environment produces seeds, its ideal strategy might not be to have all seeds germinate at once. What is the germination fraction that will maximize a plant's fitness, allowing the plant to hedge its bets in case of a poor year? To answer this question, in his now classic 1966 paper, Dan Cohen worked out the optimal annual germination fraction for seeds in a fluctuating environment. His result was elegant, but relied on a series of simplifying assumptions. We focus here on two specific assumptions. Namely, Cohen assumed that the population consists of annual plants, which have no adult age-structure, and that a seed's age has no effect on whether or not it will germinate. We review empirical and theoretical papers that have considered what happens when one or both of these assumptions is relaxed. More than a half-century has passed since Cohen's landmark study, and yet the existing literature has yet to provide theoretical solutions for perennial species as Cohen was able to do for annuals. Empirical studies have addressed short-lived perennials with minimal seed dormancy. However, perennials with longer lifespans and significant dormancy, such as many species of trees, are left largely unexplored. In this light, we conclude our review with ideas that we hope will encourage future research on both theoretical and experimental aspects of this important problem.