Comparative seed biology and co‐existence of two fynbos shrub species

Abstract. Fire‐prone fynbos communities include numerous species which are difficult to distinguish in terms of conventional niche axes since they are morphologically similar and share the same broad life‐history traits. These species compete for space after each fire and lottery theory predicts that those with dissimilar per capita regeneration would not co‐exist in the long‐term. We studied seed production, soil seed bank dynamics, germination and post‐fire seedling establishment and mortality of two obligate reseeding, morphologically similar shrubs which co‐occur in dune fynbos throughout the southwestern Cape. Passerina paleacea produced 30 x more plump seeds and maintained soil seed banks between 1.5 x (pre‐dispersal) and 2.6 x (post‐dispersal) times larger than Phylica ericoides. Seeds of both species showed a high degree of dormancy although germination was stimulated by direct fire‐related cues for Phylica only. Approximately 4 x more Passerina seedlings than Phylica seedlings were counted at the end of the winter germination period after a mild autumn fire. However, seedling mortality in the first five months of the dry summer was more than twice as high for Passerina than for Phylica. We suggest that the long‐term co‐existence of these two species does not result from similar post‐fire regeneration success but rather from population instabilities arising from differential regeneration in relation to fire regime. The results predict that hot fires followed by dry summers would favour Phylica over Passerina as the former has fire‐stimulated germination and greater drought tolerance; cool fires followed by moist summers would favour Passerina.