Fire-related cues break seed dormancy of six legumes of tropical eucalypt savannas in north-eastern Australia

Abstract

This paper describes an assessment of the effect of exposure to fire-related cues (heat shock, smoke and nitrate) and the interactions between the cues on seed dormancy release of tropical savanna legumes in north-eastern Australia. Ten legume species were tested, comprising both native and exotic species. The ten species responded variously to the treatments. Brief exposure to temperatures between 80 and 100°C was found to break the seed dormancy of the native ephemeral herbs 'Chamaecrista mimosoides', 'Crotalaria calycina', 'Crotalaria montana', 'Indigofera hirsuta' and 'Tephrosia juncea', as well as the exotic ephemeral herb 'Crotalaria lanceolata'. Exposure to 80°C combined with treatment with a nitrate solution produced an additive effect on the germination of 'Chamaecrista mimosoides' and 'Crotalaria lanceolata'. However, the four species with the heaviest seeds, two exotic ephemeral herbs ('Chamaecrista absus' and 'Crotalaria pallida') and two native perennials ('Galactia tenuiflora' and 'Glycine tomentella') displayed no significant increase in germination with exposure to fire-related cues. Exposure to 120°C for 5 min produced seed mortality in all species tested. Two of the largest seeded species, 'Crotalaria pallida' and 'Galactia tenuiflora', displayed the lowest tolerance to heat shock, with seed mortality after exposure to 100°C for 5 min. These data indicate that fire can promote the germination of some tropical savanna legumes. As a proportion of seeds of each species displayed no innate dormancy, some germination may occur in the absence of fire, especially of exotic species.