Fire promotes reproduction in the fragmented mating scene of Echinacea angustifolia
Historically, the once expansive grasslands of North America burned regularly but habitat fragmentation and changes in land use have reduced the frequency of fires in prairie remnants. In an effort to conserve what little prairie remains, land managers often use prescribed fire to maintain native plant communities. Aside from reducing the abundance of woody vegetation and preventing the spread of exotic species, observational evidence suggests spring and fall burns stimulate summer flowering in many prairie species. However, the influence of fire on plant reproductive fitness and demography in prairies has not been rigorously studied. We examined effects of fire on the reproduction and demography of Echinacea angustifolia (Asteraceae), the narrow-leaved purple coneflower, a long-lived native forb distributed widely across grasslands in North America. Pollen limitation and reproductive failure threaten the long-term persistence of fragmented Echinacea populations in the predominantly agricultural landscape of western Minnesota but we hypothesize that fire enhances plant reproductive fitness by increasing reproductive effort and mating opportunities. To characterize the relationship between fire and fitness, we monitored the annual survival and reproduction of more than 500 Echinaceaindividuals located within a regularly burned prairie remnant between 1996 and 2014. The 38 hectare remnant was segregated into two management units, each burned four times since 1996 at roughly five-year intervals. We then conducted longitudinal demographic analyses to evaluate effects of fire on survival and reproduction and characterized the relationship between fire, reproductive isolation, and seed set.
Flowering rates in burned units were invariably higher than unburned units both within and among years. Between 40 and 80 percent of adult Echinacea plants flowered in burned units while 10 to 35 percent flowered in unburned units. Similarly, flowering plants in burned units produced more flowering heads (1.7 to 3.1 heads per plant) than plants in unburned units (0.7 to 1.6 heads per plant). Apart from increasing reproductive effort, burning also reduced the isolation of plants from potential mates contributing to increased seed set. All evidence points to a strong influence of fire on plant reproductive effort, mating opportunities, and fitness in this Echinacea population. We discuss the potential importance of periodic burning for successful reproduction in native prairie plant populations.