Many plant species in historically fire-dependent ecosystems exhibit fire-stimulated flowering. While greater reproductive effort after fire is expected to result in increased reproductive outcomes, seed production often depends on pollination, the spatial distribution of prospective mates, and the timing of their reproductive activity. Fire-stimulated flowering may thus have limited fitness benefits in small, isolated populations where mating opportunities are restricted and pollination rates are low. We conducted a 6-y study of 6,357 Echinacea angustifolia (Asteraceae) individuals across 35 remnant prairies in Minnesota (USA) to experimentally evaluate how fire effects on multiple components of reproduction vary with population size in a common species. Fire increased annual reproductive effort across populations, doubling the proportion of plants in flower and increasing the number of flower heads 65% per plant. In contrast, fire’s influence on reproductive outcomes differed between large and small populations, reflecting the density-dependent effects of fire on spatiotemporal mating potential and pollination. In populations with fewer than 20 individuals, fire did not consistently increase pollination or annual seed production. Above this threshold, fire increased mating potential, leading to a 24% increase in seed set and a 71% increase in annual seed production. Our findings suggest that density-dependent effects of fire on pollination largely determine plant reproductive outcomes and could influence population dynamics across fire-dependent systems. Failure to account for the density-dependent effects of fire on seed production may lead us to overestimate the beneficial effects of fire on plant demography and the capacity of fire to maintain plant diversity, especially in fragmented habitats.
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Fire synchronizes flowering and boosts reproduction in a widespread but declining prairie species
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
- NSF-PAR ID:
- 10132124
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 117
- Issue:
- 6
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- p. 3000-3005
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Species that persist in small populations isolated by habitat destruction may experience reproductive failure. Self‐incompatible plants face dual threats of mate‐limitation and competition with co‐flowering plants for pollination services. Such competition may lower pollinator visitation, increase heterospecific pollen transfer and reduce the likelihood that a visit results in successful pollination.
To understand how isolation from mates and competition with co‐flowering species contribute to reproductive failure in fragmented habitat, we conducted an observational study of a tallgrass prairie perennial
Echinacea angustifolia . We quantified the isolation of focal individuals from mates, characterized species richness and counted inflorescences within 1 m radius, observed pollinator visitation, collected pollinators, quantified pollen loads on pollinators and onEchinacea stigmas, and measured pollination success. Throughout the season, we sampled 223 focal plants across 10 remnant prairie sites.We present evidence that both co‐flowering species and isolation from mates substantially limit reproduction in
Echinacea . As the flowering season progressed, the probability of pollinator visitation to focal plants decreased and evidence for pollen‐limited reproduction increased. Pollinators were most likely to visitEchinacea plants from low‐richness floral neighbourhoods with close potential mates, or plants from high‐richness neighbourhoods with distant potential mates. Frequent visitation only increased pollination success in the former case, likely becauseEchinacea in high‐richness floral neighbourhoods received low‐quality visits.Synthesis . InEchinacea, reproduction was limited by isolation from potential mates and the richness of co‐flowering species. These aspects of the floral neighbourhood influenced pollinator visitation and pollination success, although conditions that predicted high visitation did not always lead to high pollination success. These results reveal how habitat modification and destruction, which influence floral neighbourhood and isolation from conspecific mates, can differentially affect various stages of reproductive biology in self‐incompatible plants. Our results suggest that prairie conservation and restoration efforts that promote patches of greater floral diversity may improve reproductive outcomes in fragmented habitats. -
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Premise Variation in pollinator effectiveness may contribute to pollen limitation in fragmented plant populations. In plants with multiovulate ovaries, the number of conspecific pollen grains per stigma often predicts seed set and is used to quantify pollinator effectiveness. In the Asteraceae, however, florets are uniovulate, which suggests that the total amount of pollen deposited per floret may not measure pollinator effectiveness. We examined two aspects of pollinator effectiveness—effective pollen deposition and effective pollen movement—for insects visiting
Echinacea angustifolia , a composite that is pollen limited in small, isolated populations.Methods We filmed insect visits to
Echinacea in two prairie restorations and used these videos to quantify behavior that might predict effectiveness. To quantify effective pollen deposition, we used the number of styles shriveled per visit. To quantify effective pollen movement, we conducted paternity analysis on a subset of offspring and measured the pollen movement distance between mates.Results Effective pollen deposition varied among taxa.
Andrena helianthiformis , a Heliantheae oligolege, was the most effective taxon, shriveling more than twice the proportion of styles as all other visitors. Differences in visitor behavior on a flowering head did not explain variation in effective pollen deposition, nor did flowering phenology. On average, visitors moved pollen 16 m between plants, and this distance did not vary among taxa.Conclusions Andrena helianthiformis is an important pollinator ofEchinacea . Variation in reproductive fitness ofEchinacea in fragmented habitat may result, in part, from the abundance of this species. -
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Summary A recent study posited that fire in grasslands promotes persistence of plant species by improving mating opportunities and reproductive outcomes. We devised an investigation to test these predicted mechanisms in two widespread, long‐lived perennials. We expect fire to synchronize flowering, increase mating and boost seed set.
We quantified individual flowering phenology and seed set of
Liatris aspera andSolidago speciosa for 3 yr on a preserve in Minnesota, USA. The preserve comprises two management units burned on alternating years, allowing for comparisons between plants in burned and unburned areas within the same year, and plants in the same area across years with and without burns.Fire increased flowering synchrony and increased time between start date and peak flowering. Individuals of both species that initiated flowering later in the season had higher seed set. Fire was associated with substantially higher flowering rates and seed set in
L .aspera but notS .speciosa . InL .aspera , greater synchrony was associated with increased mean seed set.Although fire affected flowering phenology in both species, reproductive success improved only in the species in which fire also synchronized among‐year flowering. Our results support the hypothesis that reproduction in some grassland species benefits from fire.
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Abstract Flowering phenology can vary considerably even at fine spatial scales, potentially leading to temporal reproductive isolation among habitat patches. Climate change could alter flowering synchrony, and hence temporal isolation, if plants in different microhabitats vary in their phenological response to climate change. Despite the importance of temporal isolation in determining patterns of gene flow, and hence population genetic structure and local adaptation, little is known about how changes in climate affect temporal isolation within populations.
Here, we use flowering phenology and floral abundance data of 50 subalpine plant species over 44 years to test whether temporal isolation between habitat patches is affected by spring temperature. For each species and year, we analysed temporal separation in peak flowering and flowering overlap between habitat patches separated by 5–950 m.
Across our study species, warmer springs were associated with more temporal differentiation in flowering peaks among habitat patches, and less flowering overlap, increasing potential for temporal isolation within populations.
Synthesis . By reducing opportunities for mating among plants in nearby habitat patches, our results suggest that warmer springs may reduce opportunities for gene flow within populations, and, consequently, the capacity of plant populations to adapt to environmental changes.
