Interaction between seed dormancy-release mechanism, environment and seed bank strategy for a widely distributed perennial legume, Parkinsonia aculeata (Caesalpinaceae)

Background and Aims Parkinsonia aculeata (Caesalpinaceae) is a perennial legume with seeds that have hardseeded (physical) dormancy and are potentially very long-lived. Seed dormancy is a characteristic that can both help maximize the probability of seedling establishment and spread the risk of recruitment failure across years (bet-hedging). In this study, dormancy-release patterns are described across the diverse environments in which this species occurs in order to test whether wet heat (incubation under wet, warm-to-hot, conditions) alone can explain those patterns, and in order to determine the likely ecological role of physical dormancy across this species distribution. Methods A seed burial trial was conducted across the full environmental distribution of P. aculeata in Australia (arid to wet-dry tropics, uplands to wetlands, soil surface to 10 cm deep). Key Results Wet heat explained the pattern of dormancy release across all environments. Most seeds stored in the laboratory remained dormant throughout the trial (at least 84%). Dormancy release was quickest for seeds buried during the wet season at relatively high rainfall, upland sites (only 3% of seeds remained dormant after 35 d). The longest-lived seeds were in wetlands (9% remained dormant after almost 4 years) and on the soil surface (57% after 2 years). There was no consistent correlation between increased aridity and rate of dormancy release. Conclusions The results suggest that physical dormancy in P. aculeata is a mechanism for maximizing seedling establishment rather than a bet-hedging strategy. However, seed persistence can occur in environmental refuges where dormancy-release cues are weak and conditions for germination and establishment are poor (e.g. under dense vegetation or in more arid micro-environments) or unsuitable (e.g. when seeds are inundated or on the soil surface). Risks of recruitment failure in suboptimal environments could therefore be reduced by inter-year fluctuations in microclimate or seed movement.