Causes of synchronous flowering in Astragalus scaphoides, and iteroparous perennial plant

Many plant populations reproduce synchronously over time. Hypotheses to explain reproductive synchrony have typically been based on evolutionary advantages of synchrony, such as increased pollination or decreased predation with increasing numbers of flowering plants, often referred to as economies of scale. We tested for decreased herbivory, increased fruit set, and decreased predation as evolutionary mechanisms driving synchronous reproduction in an iteroparous perennial plant, Astragalus scaphoides, at three semiarid sagebrush steppe sites in intermountain valleys of Idaho and Montana, USA. Reproduction was synchronous at all sites and significantly bimodal at two sites, but it was never as variable as distinctly mast-seeding plant species. Plants in all populations experienced at least one economy of scale: decreased inflorescence herbivory, increased fruit set, and/or decreased weevil seed predation with increasing flowering plant density. At two of three sites, these relationships were strong enough to increase expected geometric mean seed set. However, among-site differences in benefits and strength of synchrony were not concordant. In particular, the site with the strongest economy of scale and highest coefficient of variation (cv) was the least bimodal. We suggest that different mechanisms drive cv and bimodality in this species, both of which contribute to overall synchrony. In addition to reflecting direct fitness benefits, synchrony may be at least partly an indirect consequence of individual-level reproductive constraints. Recent models have demonstrated that individual-level constraints can make bimodal, alternate-year flowering likely.