Fire influences reproductive outcomes by modifying flowering phenology and mate‐availability

Abstract

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 and Solidago 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 not S. speciosa. In L. 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.

Fire effects on plant reproductive fitness vary among individuals, reflecting pollination‐dependent mechanisms

“…Fire influences the reproduction of herbaceous plants in fire‐prone habitats worldwide (Ehrenreich and Aikman, 1963; Lamont and Downes, 2011; Pyke, 2017; Wagenius et al, 2020; Zirondi et al, 2021; Richardson and Wagenius, 2022). Fire stimulates flowering of many perennial species, increasing both the density of flowering plants and reproductive effort per flowering plant (Hulbert, 1988; Hartnett, 1990, 1991).…”

“…Total annual maternal fitness (hereafter “total reproductive fitness”) is constrained by the number of stems, flowers, and fruits, as well as by successful pollination. Because fire has the potential to both boost resources (Hulbert, 1988) and alter pollination outcomes (García et al, 2018; LoPresti et al, 2018; Ne'eman et al, 2000; Wagenius et al, 2020; Richardson and Wagenius, 2022), fire may affect total reproductive fitness via both resource‐dependent mechanisms and pollination‐dependent mechanisms. While fires can have a variety of negative impacts on plants and pollinators (Ne'eman et al, 2000; Gornish, 2013; Carbone and Aguilar, 2017, 2021; LoPresti et al, 2018), fires may have positive effects on plant communities, particularly in fire‐prone grasslands with fire‐stimulated flowering (García et al, 2016; Fidelis and Zirondi, 2021; Morris et al, 2021; Richardson and Wagenius, 2022).…”

Estimating phenological sensitivity in contemporary vs. historical data sets: Effects of climate resolution and spatial scale

“…Limiting the herbarium data set to only specimens with nearby weather station data resulted in reduced power to detect phenological shifts and did not improve estimates of phenological sensitivity. The large geographic and temporal scope of herbarium specimens may result in underestimates of phenological sensitivity in comparison to local observational or experimental studies, regardless of climate data source, potentially because herbarium data sets average responses across many populations and/or reflect geographic variation in the importance of other potential phenological cues, such as snowfall (Bjorkman et al, 2015), winter temperatures (Cook et al, 2012; Zettlemoyer et al, 2021), photoperiod (Meng et al, 2021), precipitation (Cui et al, 2017), fertilization (Wang and Tang, 2019), or burning (Richardson and Wagenius, 2022). However, pruning herbarium data sets too strongly on the basis of weather station availability can exacerbate biases in geographic sampling or sample size limitations.…”

Prescribed burning has limited effects on the population dynamics of rare plants

“…Populations increasing in size after burning could be the result of fewer competitors, particularly woody plants that prevent light from reaching understory species. Burning could also benefit rare species via other mechanisms such as synchronizing flowering years to increase seed production (Richardson & Wagenius, 2022;Wagenius et al, 2020) and nutrient deposition (Boerner, 1982;Christensen, 1973Christensen, , 1977. Synchronized flowering has been shown to be a particularly important mechanism for prairie species (Richardson & Wagenius, 2022;Wagenius et al, 2020) and thus might be important for Asclepias viridiflora, Cirsium hillii, Isoetes butleri, and Sarracenia purpurea, which all responded positively to burns in short-and/or long-term datasets.…”

“…Burning could also benefit rare species via other mechanisms such as synchronizing flowering years to increase seed production (Richardson & Wagenius, 2022;Wagenius et al, 2020) and nutrient deposition (Boerner, 1982;Christensen, 1973Christensen, , 1977. Synchronized flowering has been shown to be a particularly important mechanism for prairie species (Richardson & Wagenius, 2022;Wagenius et al, 2020) and thus might be important for Asclepias viridiflora, Cirsium hillii, Isoetes butleri, and Sarracenia purpurea, which all responded positively to burns in short-and/or long-term datasets. Similarly, burning releases nutrients from leaf litter into the soil, which could benefit herbaceous species (Glasgow & Matlack, 2007).…”