The ecological significance of canopy seed storage in fire-prone environments: a model for non-sprouting shrubs

1 A comprehensive data set on age, survival and reproduction for the non-sp routing (fire-killed) shrub Banksia hookeriana, encompassing 13 years of me asurements at 15 sites in south-western Australia, and including 10 fires, was used to parameterize a computer model to investigate optimum plant life -history strategies in a fire-prone environment. Parameter ranges encompass ed life-history information for other non-sprouting Banksia species from th e same region. 2 The relationship between fire interval and level of canopy seed storage ( serotiny) was analysed to identify the circumstances under which serotiny i s favoured, and what degree of serotiny maximizes potential population grow th rate. In addition to deterministic versions of the model, stochasticity in fire interval and conditions for recruitment were analysed. 3 The deterministic model indicated a maximum finite rate of natural increa se (lambda = 1.15) when the fire interval was 16 years and all seeds were r etained on the plant until fire occurred. Although the model failed to pred ict the intermediate degrees of serotiny present in nature, it supported th e optimum fire interval predicted from canopy seed bank dynamics. 4 Changes to biological attributes associated with timing of reproduction a nd longevity shifted the optimum fire interval and estimated rate of popula tion growth, but did not alter the conclusions concerning serotiny. Althoug h shorter seed longevity and increased rates of predation and/or decay redu ced the value of serotiny, even very low levels of canopy seed storage incr eased species fitness under intermediate fire frequencies (10-20 years). 5 If the probability of inter-fire recruitment and survival was increased, optimum growth shifted from strong serotiny under a regime of frequent fire (< 20 year interval), to weak (or no) serotiny where the interval between successive fires was long ( > 40 year interval). 6 Stochasticity around mean fire interval resulted in intermediate to stron g (but not complete) serotiny being predicted as optimal once the CV for fi re interval approached 100%, This result is interpreted as a bet-hedging st rategy whereby spontaneous release of some seeds during the inter-fire peri od permits recruitment on rare occasions where fire interval approaches or exceeds the species longevity.