Fire and legume germination in a tropical savanna: ecological and historical factors

Abstract

Background and Aims In many flammable ecosystems, physically dormant seeds show dormancy-break patterns tied to fire, but the link between heat shock and germination in the tropical savannas of Africa and South America remains controversial. Seed heat tolerance is important, preventing seed mortality during fire passage, and is usually predicted by seed traits. This study investigated the role of fire frequency (ecological effects) and seed traits through phylogenetic comparison (historical effects), in determining post-fire germination and seed mortality in legume species of the Cerrado, a tropical savanna–forest mosaic. Methods Seeds of 46 legume species were collected from three vegetation types (grassy savannas, woody savannas and forests) with different fire frequencies. Heat shock experiments (100 °C for 1 min; 100 °C for 3 min; 200 °C for 1 min) were then performed, followed by germination and seed viability tests. Principal component analysis, generalized linear mixed models and phylogenetic comparisons were used in data analyses. Key Results Heat shocks had little effect on germination, but seed mortality was variable across treatments and species. Seed mortality was lowest under the 100 °C 1 min treatment, and significantly higher under 100 °C 3 min and 200 °C 1 min; larger seed mass decreased seed mortality, especially at 200 °C. Tree species in Detarioideae had the largest seeds and were unaffected by heat. Small-seeded species (mostly shrubs from grassy savannas) were relatively sensitive to the hottest treatment. Nevertheless, the presence of physical dormancy helped to avoid seed mortality in small-seeded species under the hottest treatment. Conclusions Physical dormancy-break is not tied to fire in the Cerrado mosaic. Heat tolerance appears in both forest and savanna species and is predicted by seed traits (seed mass and physical dormancy), which might have helped forest lineages to colonize the savannas. The results show seed fire responses are better explained by historical than ecological factors in the Cerrado, contrasting with different fire-prone ecosystems throughout the world.

Post‐fire regeneration strategies in a frequently burned Cerrado community

“…Since fire regime is expected to shape post-fire regeneration strategies (Bond & Midgley, 2003;Keeley et al, 2012), we expect resprouting to be the main regeneration strategy in the Cerrado (Bond & Midgley, 2001;Clarke et al, 2015;Pilon et al, 2020) Zirondi, José et al, 2019). We also expect propagules to be tolerant to fire-related temperatures (Daibes et al, 2019), remaining ungerminated (yet viable) until the onset of the rainy season (Escobar et al, 2018). (Coutinho, 1978).…”

“…Heat shocks caused by fire have been reported to stimulate germination in several species of fire-prone ecosystems, such as those in Australia and in the Mediterranean region, mainly in hard-seeded species in which high temperatures may break physical dormancy, thus enabling germination (Auld & O'Connell, 1991;Williams et al, 2003;Reyes & Trabaud, 2009;Moreira et al, 2010;Moreira & Pausas, 2012). Nonetheless, heat shocks have shown little to no effect among species of South America, although most seeds were able to survive the exposure to high temperatures, which is considered a tolerance trait in face of fires (Jaureguiberry & Diaz, 2015;Le Stradic et al, 2015;Ribeiro et al, 2015;Fichino et al, 2016;Fidelis et al, 2016;Zupo et al, 2016;Gómez-González et al, 2017;Daibes et al, 2019;Zirondi, José, et al, 2019).…”

“…After fires, many woody and herbaceous species resprout from dormant buds (Coutinho, 1990;Medeiros & Miranda, 2008;Pausas et al, 2018;Pilon et al, 2020), but the role of post-fire seed germination as a regeneration mechanism in Cerrado remains unclear. Even though post-fire seed germination has been observed in grasslands and savannas that undergo surface fires (Overbeck & Pfadenhauer, 2007;Clarke et al, 2015;Ramos et al, 2019;Pilon et al, 2020), studies evaluating seed responses after heat shocks in the Cerrado have done so with few species (Ribeiro et al, 2013;Le Stradic et al, 2015;Fichino et al, 2016) or are restricted to certain clades (Ramos et al, 2016;Daibes et al, 2019;Zirondi, José, et al, 2019;Dairel & Fidelis, 2020), hindering the understanding of general postfire regeneration mechanisms at the community scale. Moreover, few studies addressing fire-related seed responses in the Cerrado have taken into account plant resprouting ability following experimental fires (but see Pilon et al, 2020).…”

What kind of seed dormancy occurs in the legume genus Cassia?

“…No results were found associating PY with another kind of dormancy, for example, with physiological dormancy (combinational dormancy). In the case of Chamaecrista, the second largest genus in this subtribe, the investigations also found PY as the only kind of dormancy occurring in these seeds [68][69][70][71][72] . The results found in the present study also corroborate with the evidence that impermeability to water is a prevailing trait in seeds of this subtribe, since PY was found in all Cassia species.…”

Do regeneration traits vary according to vegetation structure? A case study for savannas

“…One of the main hypotheses related to the evolution of dormancy states that dormancy is selected where seasonality and environmental uncertainty are high (Baskin and Baskin, 2014). Thus, we expect a larger proportion of dormant species in open savannas, where fire frequency, environmental unpredictability, and drought severity are greater than in closed savannas (Franco, 2002;Oliveira-Filho and Ratter, 2002;Rossatto et al, 2012;Salazar et al, 2012;Daibes et al, 2019; Figure 1). In contrast, higher environmental predictability in closed savannas should select non-dormant species because of the costs associated with the evolution of dormancy (Baskin and Baskin, 2014;Dayrell et al, 2017).…”