Data and code from: Fire-survival strategies of first-year acacia seedlings

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 three- to six-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 favorable
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 belowground.
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 (eleven of twelve 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.