Fire‐survival strategies of first‐year acacia seedlings

Abstract Savannas—defined by discontinuous tree cover and continuous grass cover—are maintained by factors such as drought, fire and herbivory which impose bottlenecks on tree populations. Despite evidence that young plants are particularly vulnerable to fire, surprisingly little is known about the traits that enable first‐year seedlings to survive fire. This limits our understanding of the mechanisms underlying tree establishment and savanna dynamics more generally. We conducted a full‐factorial common garden experiment to measure the separate and combined impacts of fire, drought and herbivory on rates on 3‐ to 6‐month‐old seedlings of 12 ‘ Acacia ’ (sensu lato) species in the genera Vachellia and Senegalia . Results demonstrate an overwhelming impact of fire on mortality, enhanced by drought, but the impact of herbivory was not detectable. Stem thickness was the best predictor of topkill and survival, with threshold diameters of 9.56 mm (95% CI 9.51–9.59) for well‐watered and 11.52 mm (95% CI 10.88–12.69) for droughted plants, above which seedlings had favourable odds of avoiding topkill by fire and surviving. The ability of seedlings to survive after being topkilled by fire varied by species and was linked to the drought treatment and the existence of stem bases below‐ground. Thus, fire represents a major demographic bottleneck for first‐year acacia seedlings, even when seedlings are established and have sufficient access to water. Synthesis: Acacia seedlings (11 of 12 species) acquire some ability to survive grass fires in their first year, either by resisting topkill with thickened stems and/or by regrowing from below‐ground stems. Our results suggest that small differences in these fire‐survival strategies at this crucial life stage translate to large differences in savanna physiognomy and species composition.