Fire‐triggered flowering is the dominant post‐fire strategy in a tropical savanna

Abstract

Questions:In fire-prone ecosystems, fire can enhance the flowering and fruiting of many species, a strategy assumed to be well represented in savanna. Despite this, there are surprisingly few studies assessing how prevalent fire-stimulated flowering is. Thus, we asked: (a) are there differences in the reproductive phenology of Cerrado plants between recently burned and unburned areas; (b) how does fire affect the speed of flowering and how does this differ between growth forms; and (c) what are the post-fire flowering (PFF) strategies of Cerrado species and is there evidence for high proportions of obligate PFF? Location: Open savannas (campo sujo in the Cerrado) in Central Brazil (Reserva Natural Serra do Tombador -RNST, 13°35-13°38' S and 47°45'-47°51' W).Methods: We established six plots, three recently and frequently burned (FB) and three excluded from fire for six years (E). In all treatments, the number of species flowering and fruiting was counted every 15 days for three months, and then at six, nine and 12 months after fire. We also counted the number of reproductive and vegetative shoots in 10 subplots (1 m × 1 m) per plot.Results: Approximately 66% of species studied were fire-stimulated, with half of these only flowering after fire (obligate PFF). Fire-enhanced flowering was rapid, with the clearest differences between burned and unburned plots seen in the first 30 days, and up to three months after fire, where there were up to two times more species flowering in the FB than E areas. Conclusions:The extremely high proportion of PFF species, at least five times that reported for heathlands and other shrub communities, highlights the role that shortinterval fire regimes have in savanna ecosystems, selecting for resprouting life forms and PFF dominance, particularly in herbaceous species. Rapid post-fire reproduction may be a strategy to disperse large quantities of seed into an environment with a small recruitment window.

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

“…Fire appears to stimulate flowering in many plant species in fire‐prone habitats (Ehrenreich & Aikman, 1963; Pemble et al ., 1981; Hartnett, 1990; Peter, 2002a,b; Pilon et al ., 2018; Zirondi et al ., 2021), as we found in L . aspera (Table 1).…”

“…We suspect that fire affects S. speciosa differently than L. aspera in part because S. speciosa is more likely to flower each year and produces stems every year regardless of flowering status, whereas L. aspera flowers infrequently with drastic differences in aboveground structure and biomass between vegetative and flowering years. Fire appears to stimulate flowering in many plant species in fire-prone habitats (Ehrenreich & Aikman, 1963;Pemble et al, 1981;Hartnett, 1990;Peter, 2002a,b;Pilon et al, 2018;Zirondi et al, 2021), as we found in L. aspera (Table 1). Synchronized flowering, which we conceptualize as a continuous trait, at least in perennial prairie species, may be an adaptive trait, as posited for masting species (Janzen, 1971;Bogdziewicz et al, 2020Bogdziewicz et al, , 2021Kelly, 2021).…”

“…Many perennials produce more flowers after a burn (i.e. fire increases reproductive allocation) (Ehrenreich & Aikman, 1963; Pemble et al ., 1981; Hartnett, 1990; Peter, 2002a,b; Pilon et al ., 2018; Zirondi et al ., 2021), but reproductive outcomes such as seed production are usually unknown. Annual reproductive fitness depends on both allocation of resources to reproduction and on pollination.…”

“…Evidence that competition drives local extinction in prairies where fire is suppressed comes from observational community‐level studies (Bowles & Jones, 2013; Alstad & Damschen, 2016; Alstad et al ., 2016). Other studies have recognized the importance of fire for stimulating flowering in many prairie species (Ehrenreich & Aikman, 1963; Pemble et al ., 1981; Hartnett, 1990; Peter, 2002a,b) and in other fire‐prone systems such as the cerrado (Pilon et al ., 2018; Zirondi et al ., 2021). Building on these observations, Wagenius et al .…”

Fire stimulates grass flowering in the Cerrado independent of season

“…Grasses also respond very fast to dry‐season fires by resprouting and allocating reserves to flowering, with their flowering peak being just 30 days post fire, with ca. 30% of the species only flowering when fire occurs (Zirondi et al, 2021). However, such studies analyzed post‐fire grass flowering only after dry‐season fires and information is still lacking about the effects of fire given different seasons, on grass phenology.…”