Impact of bush fire on germination of some West African acacias

Bush fire is a widespread and recurrent feature of the African dry savannas and contributes largely to changes in the composition of vegetation communities. However, the impact on seed regeneration in local species has received little attention. This study evaluates the effects of managed bush fire on seed viability and germination capacity for 10 species of West African acacia (sensu lato), taking into consideration fire intensity (related to the quantity of combustible biomass) and seed location (in, on or above the soil) at the time of the fire. The results indicate that the behavior of different species is related to their taxonomic position. Acacia species belonging to the sub-genus Aculeiferum are characterized by their lack of integument inhibition. The consequences of the passage of fire, depending on the intensity of heat shock, are limited to seed survival: they either retain viability or die. Seeds from the subgenera Acacia and Faidherbia albida have a hard integument that needs to be scarified in order for germination to ensue. In our study, we show that on the whole, heat shocks cannot scarify the integument of the seeds without resultant lethal damage to their embryos. This is true regardless of seed status (naked or protected by seed pods), seed location (in, on or under the soil) and fire intensity. The only species to manifest an improvement in germination capacity, under certain selective experimental fire conditions, and in relation to the non-scarified control are A. raddiana, A. seyal and A. sieberiana. Nevertheless, germination remains substantially inferior to that obtained after scarification by removal of a small piece of integument. It thus seems conceivable to conclude that, contrary to what takes place in Mediterranean climate ecosystems and the wet tropics, fire does not encourage the in situ germination of different Acacia species. This finding implies that in dry savanna areas, it is not possible to regard fire as an accessory to reforestation or natural regeneration of these species. (Résumé d'auteur