Patterns of seed longevity and dormancy in obligate seeding legumes of box-ironbark forests, south-eastern Australia

Current fuel loads and distribution suggest that fire events are infrequent and of a low intensity in the regenerated dry sclerophyll forests of the Victorian box-ironbark ecosystem. However, many box-ironbark species possess traits consistent with fire-cued regeneration. It is unclear the degree to which human disturbance may have altered fire regimes in these forests. The infrequent and low-intensity fire regime suggested by current fuel dynamics may pose a threat to the persistence of fire-cued species. Obligate seeders such as those of the Fabaceae and Mimosaceae, common in box-ironbark understoreys, may be particularly vulnerable if inter-fire intervals exceed seed longevity. This study used seed burial trials to examine seed dormancy and longevity in five legume species to explore their capacity to regenerate under an infrequent, low-intensity fire regime. All species displayed dormancy and longevity patterns consistent with other south-east Australian legumes. Before burial, dormancy levels were high for all species (98-100%). After 3 years, storage under in situ and ex situ conditions, dormancy in Pultenaea prostrata remained at pre-burial levels with virtually no seed becoming non-dormant. Over time, some Acacia seed became non-dormant under both in situ and ex situ storage, with the pattern varying among species. Longevity also varied between species. Variation in the dormancy and longevity patterns observed in these obligate seeder legumes suggests two strategies: (i) releasing a portion of soil-stored seed from dormancy during the inter-fire period to permit inter-fire recruitment; and (ii) retaining most soil-stored seed as dormant during the inter-fire interval. Both strategies represent potential weaknesses under a long fire interval regime. The first relies on dormancy release translating to successful recruitment and requires ongoing inter-fire input into the soil seed bank. The second relies on seed longevity exceeding the inter-fire interval. Whether either is more suitable to coping with long-term infrequent fire requires long-term monitoring.