Ph.D Desk: What Are We Still Missing About Fire and the Soil Seed Bank?
When we think about fire in a forest, it is easy to imagine heat as the main culprit. But fire leaves behind more than heat. Smoke carries chemical signals, while ash changes the physical and chemical environment of the soil. Together, heat, smoke and ash can influence whether buried seeds survive, remain dormant, germinate or die. The problem is that we still do not fully understand how these components work together, particularly in tropical forests.
One important research gap is the limited study of smoke and ash in tropical rainforest soil seed banks. Research at Shasha Forest Reserve in southwestern Nigeria demonstrated that simulated heat could dramatically reduce soil seed-bank density and species richness (Akinyemi, Oseni & Oke, 2019). Seed density fell from about 1,521–3,321 seeds m⁻² to 261–468 seeds m⁻², while species richness declined from 11–22 species to only 2–5 species. Yet this study concentrated on heat.
That leaves an obvious question: What would happen if the same soil were exposed to smoke or ash? Studies elsewhere suggest that the answer may be very different. Read et al. (2000), for example, found that smoke and heat could influence soil seed-bank germination, while Enright et al. (1997) demonstrated independent effects of heat, smoke and ash on seedling emergence. However, responses vary considerably among species.
A second gap concerns interactions among fire components. Does smoke enhance the effect of heat? Does ash cancel it out? Or can two fire cues combine to produce an entirely different response? Evidence suggests that these interactions are not necessarily additive. Bargmann et al. (2014) found that smoke and ash sometimes produced negative interactions, while other species showed positive interactions. This means testing heat, smoke and ash individually may not adequately reproduce what happens during an actual fire.
A third gap is fire intensity and soil depth. Seeds buried at different depths experience different temperatures and exposure durations. The survival of a seed may therefore depend not simply on whether a fire occurs, but on the precise combination of temperature, duration and burial depth. Experimental studies often use fixed temperatures that may not represent the highly variable conditions of natural fires.
There is also a major temporal gap. A fire does not end when the flames disappear. Soil seed banks may change for months or years afterwards. The global synthesis by Shi et al. (2022), based on 539 paired comparisons, showed that fire effects vary with fire severity, ecosystem type and the time elapsed after burning. The authors specifically identified the shortage of long-term studies and research on repeated fire as important limitations.
Finally, we need more species-specific research in African tropical forests. Global evidence indicates particularly negative fire effects on tropical forest seed banks, but tropical Africa remains poorly represented. We still know relatively little about which Nigerian species are heat-sensitive, smoke-responsive, ash-tolerant or capable of surviving repeated fire.
The next generation of research should therefore move beyond simply asking, “What does fire do to the seed bank?” The better question is: “Which component of fire, at what intensity, duration, soil depth and combination, affects which species and for how long?”
Answering that question could transform our understanding of forest regeneration, post-fire succession, seed-bank ecology and fire management in tropical forests.