Abstract: Across historically fire-dependent ecosystems, many plant species flower vigorously after fire. In addition to increasing reproductive effort, synchronized post-fire flowering can enhance reproductive success by increasing mating opportunities and improving pollination. However, habitat loss and fragmentation could constrain the reproductive benefits of synchronized post-fire reproduction by reducing the pool of prospective mates and opportunities for pollination in small populations. Our objective in this study was to evaluate the potentially density-dependent effects of fire on mating opportunities and pollination efficiency. We hypothesized fire effects on reproductive effort would not vary with population size while fire effects on reproductive success would increase with population size due to increased mating opportunities in large populations. To test these hypotheses, we conducted a six-year study of 6357 Echinacea angustifolia (Asteraceae) individuals across 35 remnant prairies in western Minnesota (USA). We experimentally burned a stratified sample of 18 remnants with population sizes ranging from 3 to nearly 4000 sexually mature individuals. We quantified plant reproductive effort, spatial proximity to prospective mates, flowering phenology, pollination, and seed production in this obligately outcrossing species. We show that fire increased reproductive effort consistently across populations, leading to a 102 percent increase in the proportion of sexually mature individuals that flowered and a 65 percent increase in the number of flower heads per reproductive plant. However, the pollination benefits of synchronized post-fire flowering increased non-linearly with census population size, reflecting density-dependent effects of fire on spatiotemporal mating opportunities. Populations with fewer than 20 sexually mature individuals experienced no significant increase in pollination or seed production after fire. Above this threshold, fire increased spatiotemporal mating opportunities, leading to a 24 percent increase in the proportion of florets receiving compatible pollen and a 71 percent increase in annual seed production. In conclusion, this study reveals density-dependent effects of fire on plant reproduction. Previous studies investigating fire effects on plant reproduction have emphasized the density-independent effects of fire on reproductive effort. Our findings suggest the density-dependent effects of fire on pollination largely determine plant reproductive outcomes and may influence demographic rates across historically fire-dependent systems that have experienced extensive habitat loss and fragmentation.
