Growth & Development

Botanists find that some herbaceous use fire as a signal to prepare for boosted flowering in the following year

In the prairies of Minnesota, Liatris aspera is almost a real-life phoenix
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You might think of wildfire as something that reduces plants to ash. Yet, in the prairies of North America, Lea Richardson and Stuart Wagenius have found that Rough Blazing StarLiatris asperadoesn’t just survive fires. It thrives on them.

Liatris aspera. Image: Crazytwoknobs / Wikimedia

Fire is a threat to plant life, but so too are other plants. Left unmolested, woody species may outcompete herbaceous species over time. However, a fire attacks all plants, and it can also open new opportunities by clearing ground. Over time, the species that prosper will be the plants capable of responding to these opportunities.

Richardson and Wagenius examined flowering phenology and seed set of Liatris aspera and Solidago speciosa on a preserve in Minnesota, USA. The plants are both perennial plants in the Asteraceae family, the same family as daisies or sunflowers. They are also self-incompatible, meaning they have to receive pollen from a partner. So not only must they build flowers, they have to activate them at the same time as any potential partner.

The authors write that the preserve’s advantage is that it has managed plots, burned in alternating years. “The alternating burn schedule allowed us to compare burned and unburned plants both within years and within units. Both species were dormant and persisting belowground during the prescribed burns. During a dormant-season burn, the aboveground biomass combusts without killing the belowground parts of perennial plants, so plants exhibit effects of fire in the subsequent flowering season.”

The plants differed in how they responded to fire. After burning, L. aspera flowered more while S. speciosa did not. But while the quantities of flowering differed, both changed the timing of flowering. In both cases, flowering started earlier in the year following burning.

L. aspera has a particularly interesting response to fire. After fire, its flowering season was earlier and longer than usual, with more flowerheads. It also doubled its seed set.

The authors conclude: “We provide strong evidence that fire affects flowering time in two perennials and improves reproductive success in one species, consistent with the mechanism that fire-stimulated flowering improves mating opportunities as posited by Wagenius et al. (2020). Because we have evidence of fire-stimulated increases in reproductive success for multiple species, the next step is to gauge the extent to which these increases in reproduction affect population dynamics.”


Richardson, L.K., Wagenius, S., 2021. Fire influences reproductive outcomes by modifying flowering phenology and mate‐availability. New Phytologist.


21 Dec 2021. Revised to replace an image of (probably) Liatris spicata with an image of Liatris aspera. Thanks to Dr Dan Carter @dryspikesedge for spotting this.


  1. An increase in flowering in Liatirs aspera following fire is not a newly documented observation. It was reported upon in the literature as early 1948 and again in 1981.

  2. It looks like they were testing the the “mate finding Allee effect” and quantifying seed production after fire. This is from their article “Wagenius et al. (2020) proposed a mechanism by which fire affects reproductive outcomes. They propose that lack of fire exacerbates existing mate-finding Allee effects, leads to reproductive failure and possibly reduces population growth. Mate-finding Allee effects occur when mating and reproduction increase with population size or density (Gascoigne et al., 2009). ” and “we devised a minimally adequate natural experiment to test this second mechanism proposed in Wagenius et al. (2020), and to quantify the extent to which fire affects flowering phenology and influences reproductive outcomes in two widespread prairie species.”

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