Fire synchronizes flowering and boosts reproduction in a widespread but declining prairie species

Abstract

Fire is an important determinant of habitat structure and biodiversity across ecosystems worldwide. In fire-dependent communities, similar to the North American prairie, fire suppression contributes to local plant extinctions. Yet the demographic mechanisms responsible for species loss have not been directly investigated. We conducted a 21-y longitudinal study of 778 individual plants of Echinacea angustifolia, a widespread perennial species with chronically limited mating opportunities, to explore how fire affects reproduction. In a large preserve, with management units on different burn schedules, we investigated Echinacea mating scenes, which quantify isolation from potential mates and overlap in the timing of flowering, to determine the extent to which fire influences the potential for sexual reproduction. We demonstrate that fire consistently increased mating opportunities by synchronizing reproductive effort. Each fire occurred during fall or spring and stimulated flowering in the subsequent summer, thus synchronizing reproduction among years and increasing the proximity of potential mates after a fire. Greater within-season flowering synchrony in postfire mating scenes further increased mating potential. The improved postfire mating scene enhanced reproduction by increasing pollination efficiency. Seed set in scenes postfire exceeded other scenes by 55%, and annual fecundity nearly doubled (88% increase). We predict the reproductive benefits of synchronized flowering after fire can alleviate mate-finding Allee effects, promote population growth, and forestall local extirpation in small populations of Echinacea and many other prairie species. Furthermore, the synchronization of flowering by burning may improve mating opportunities, reproduction, and the likelihood of persistence for many other plant species in fire-dependent habitats.