The ecological significance of canopy seed storage in fire-prone environments: a model for non-sprouting shrubs
Enright, N.J., Marsula, R., Lamont, B.B. and Wissel, C. (1998) The ecological significance of canopy seed storage in fire-prone environments: a model for non-sprouting shrubs. Journal of Ecology, 86 (6). pp. 946-959.
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1 A comprehensive data set on age, survival and reproduction for the non-sprouting (fire-killed) shrub Banksia hookeriana, encompassing 13 years of measurements 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 encompassed life-history information for other nonsprouting Banksia species from the same region. 2 The relationship between fire interval and level of canopy seed storage (serotiny) was analysed to identify the circumstances under which serotiny is favoured, and what degree of serotiny maximizes potential population growth 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 increase (λ = 1.15) when the fire interval was 16 years and all seeds were retained on the plant until fire occurred. Although the model failed to predict the intermediate degrees of serotiny present in nature, it supported the optimum fire interval predicted from canopy seed bank dynamics. 4 Changes to biological attributes associated with timing of reproduction and longevity shifted the optimum fire interval and estimated rate of population growth, but did not alter the conclusions concerning serotiny. Although shorter seed longevity and increased rates of predation and/or decay reduced the value of serotiny, even very low levels of canopy seed storage increased 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 strong (but not complete) serotiny being predicted as optimal once the CV for fire interval approached 100%. This result is interpreted as a bet-hedging strategy whereby spontaneous release of some seeds during the inter-fire period permits recruitment on rare occasions where fire interval approaches or exceeds the species longevity.
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