Memory of past fire regime enhances plant developmental response to fire

1. Fire is a severe disturbance to plants that may vary considerably in frequency over time. Plasticity in post-fire persistence and flammability is important in coping with temporal variation in fire occurrence, but how plastic changes to current conditions are affected by prior exposure to fires remains unknown. Here we investigate whether populations of resprouting savanna grasses exposed to differing experimental fire regimes continue to show phenotypic divergence after being grown in a common environment.2. Cuttings taken from four savanna grass species in experimental plots that were either annually burnt or unburnt for >35 years, were re-established in a common environment for one year. Traits relating to persistence and flammability were then measured before plants were burnt during the fire season, and their regrowth tracked. To determine if there was genetic differentiation between the populations subjected to differing fire regimes, restriction-site associated genome sequences were compared for a subset of individuals.3. After a year in the common environment, the historic fire frequency influenced traits relating to flammability, flowering and below-ground investment, but not regrowth rate after fire. In comparison with historically unburnt plants, those previously subjected to high fire frequency were more flammable, which could increase opportunities for recruitment and regeneration after frequent fires. Annually burnt plants also produced more inflorescences and invested a greater proportion of biomass belowground in comparison to plants protected from fires, suggesting that high fire frequency selects for a greater capacity for recruitment and resprouting. Within species, plant flammability and regrowth were tightly linked. We found no consistent genetic differences between the two populations, suggesting that trait differences arose from phenotypic plasticity and not selection.4. Synthesis. The development of four perennial grass species is influenced by prior exposure to fire, independent of the current environmental conditions, suggesting a ‘memory’ for fire. This priming of developmental plasticity may improve the fitness of individual plants when exposed to fires in future. Heightened flammability in frequently burnt plants, and the positive relationship between flammability and regrowth, are consistent with the idea that flammability may be an adaptive trait in fire-prone environments.