Fall annual, winter annual, and spring annual life history strategies are well known among annual plants. Less well known is a twice-flowering life history strategy in which a plant germinates and flowers in the fall, overwinters as a rosette, and flowers a second time in the spring. This life history strategy has been documented in a population of Capsella bursa-pastoris in Nebraska, where twice-flowering individuals coexist with individuals that exhibit single-flowering strategies. The existence of this unusual life history strategy raises the question of what conditions in a plant’s abiotic and biotic environment favor a twice-flowering strategy for plants that germinate in the fall. We address this question with a mathematical model that uses dynamic energy budget ideas to predict relationships between environmental parameters and the relative fitness of each of the competing strategies.
The primary result of this theoretical study is a graph that predicts the optimal strategy in terms of two key dimensionless parameters. One of these parameters represents the expected amount of carbon capture for the spring growing season, relative to the amount of carbon in a seed. The other parameter represents either the actual or the expected amount of carbon capture for the fall growing season, depending on whether or not one assumes the possibility of plasticity that allows the life history decision to respond to the degree of fall growth. As one might guess, greater expected spring carbon capture favors a winter annual strategy, while greater fall carbon capture favors a fall annual strategy. The model also predicts the existence of a critical value for the expected spring carbon capture such that the fall annual strategy is never optimal when the critical value is exceeded. In this case, the twice-flowering strategy is optimal if the fall carbon capture is sufficiently high. We conjecture that the twice-flowering strategy occurs only among ruderals inhabiting environments, such as farmland, that can support a dense plant population and suffer frequent disturbance.