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

1 Based on a parameter set describing mean life-history attributes for the resprouting shrub Banksia attenuata in south-western Australia, the relatio nship between fire interval and extent of canopy seed storage (serotiny) wa s analysed. Deterministic and stochastic computer models were used to try t o identify the circumstances under which serotiny is favoured, and what deg ree of serotiny maximizes the estimated finite rate of natural increase ii. ) for species which, in addition to serotiny, have the capacity to resprout vegetatively. 2 The deterministic model indicated a maximum finite rate of natural increa se (lambda = 1.015) when the fire interval was 13 years and all seeds were retained on the plant until fire occurred (i.e. complete serotiny). Populat ions were predicted to decline if the fire interval exceeded about 40 years . 3 Changes to biological attributes associated with timing of reproduction s hifted the optimum fire interval but did not change the overall pattern in relation to serotiny. Reduction in the probability of adults resprouting af ter fire also shifted the optimum. so that as probability of resprouting de clined, optimum fire interval increased. 4 Increasing stochasticity around the mean fire interval (CV > 50%) under r egimes of frequent fire (fire interval < 20 years) led to strong, but not c omplete, serotiny as the optimum strategy(lambda = 1.009 +/- 0.001). I fire was less frequent, and the probability of inter-fire recruitment was simil ar to that of post-fire recruitment. then the optimum strategy was zero ser otiny. 5 The probability of seedling recruitment was low after most fires due to l ow seed availability and high seedling mortality over the first summer. Sto chastic weather (variation in summer rainfall! resulted in a higher rate of population growth (lambda = 1.019) than under constant average weather con ditions, emphasizing the importance of rare recruitment events in the popul ation behaviour of resprouting species. 6 Resprouters were able to cope with more frequent fire than non-sprouters, but there was no evidence to support the contention that they are also fav oured under regimes of infrequent fire.