Plants in fire-prone environments are often characterized by biological attributes that increase the probability of survival and/or recruitment following disturbance by fire. Among these are the ability of individuals to regrow from protected buds either above- or below-ground (resprouting), fire-stimulated flowering and seed production, and fire-triggered germination of seeds stored either in the soil or in the plant canopy (Gill 1981). Since species that resprout after fire commonly co-exist in fire-prone environments (e.g. mediterranean-type shrublands) with those that are killed by fire (non-sprouters), a frequent question concerns the circumstances under which one or other strategy is favoured (e.g. Keeley & Zedler 1978; Keeley 1986; Hilbert 1987). Beyond this simple dichotomous division into resprouters and non-sprouters, fire-prone communities may contain an array of (sometimes congeneric) species showing consistent variation in the amount of resprouting after fire, the extent to which seeds are stored in the soil or on the plant, and in other biological attributes (e.g. longevity, age to reproduction, level of seed production).
It is important to determine the biological attributes that influence demographic behaviour and to establish which attributes confer the greatest fitness advantage on a species in relation to particular fire regimes. As well as revealing information on the selective pressures acting on species in fire-prone environments, the answers aid conservation and management of species in relation to human-imposed fire regimes.
This paper presents a computer model of population dynamics for a non-sprouting woody perennial in a fire-prone environment, and the results of simulations which sought to reveal the optimum level of on-plant seed storage (serotiny) in relation to fire interval. Serotiny refers to the retention of seeds in closed fruits or cones within the crown for more than 1 year and is common in species from a number of important families in fire-prone areas of Australia (Proteaceae, Myrtaceae, Cupressaceae, Casuarinaceae), South Africa (Proteaceae, Bruniaceae) and North America (Pinaceae) (Lamont et al. 1991a). It can be contrasted with two alternative fates of seeds: storage in a soil seed bank, or release of short-lived seeds once mature (no seed storage). Seed release in serotinous species is normally triggered by fire and seeds are short-lived after release, germinating during the first favourable period (Cowling et al. 1987). Species can range from weakly serotinous, where most seeds are released spontaneously (in the absence of fire) within a few years of production, to strongly serotinous, where most seeds are held in the canopy for many years (e.g. >10 years in some species of Pinus and Banksia).
The ecological and evolutionary significance of serotiny seems clear: where fire is the most common cause of ecosystem disturbance, cued release of canopy-stored seeds by fire (and their germination in the first growing season after fire) maximizes plant age and therefore seed available for the next generation by the time of the next fire. Seeds released, and seedlings established, later during the inter-fire period have a lower probability of survival due to competition with their parents and other established plants (Cowling & Lamont 1987). They will be younger and will thus have a smaller canopy seed bank when fire recurs, and so are less fit (sensuStearns 1992). It would therefore appear that very strong serotiny should represent the optimum strategy for perennial plants in fire-prone environments. However, a range of degrees of serotiny may be encountered within the same plant community (e.g. Cowling et al. 1987; Enright & Lamont 1989a,b) or even within the same species. In the latter case, variations have been found in relation to environmental gradients, e.g. lower serotiny in less fire-prone parts of the species’ geographical range for Banksia attenuata in Australia (Cowling & Lamont 1985), and in relation to time since last fire, e.g. inter-fire recruits show lower serotiny than immediate post-fire recruits of Pinus banksiana in boreal forests of southern Quebec (Gautier et al. 1996). Lamont et al. (1991a) note that some fire regimes and growth forms apparently foster incomplete serotiny, but that ‘degree of serotiny has received little formal treatment, and the empirical subtleties of such relationships remain largely unexplored’.
The model described here is based on field data collected over 13 years from 15 sites and including 10 different fires (up to two at the same site) for B. hookeriana Meissn. (Proteaceae), a non-sprouting woody shrub of mediterranean-type shrublands in south-western Australia (Enright & Lamont 1989a, 1992a,b; Enright et al. 1996). Nevertheless, the model is likely to have a more general application since parameter values can readily be changed to match the known life-history attributes of other serotinous, non-sprouting perennial plants. By basing the model and its parameter values on those measured over an extended period of time (and at a number of sites) for a real plant species the model described here avoids many of the criticisms levelled at population models. For example, Cousens (1995) argues that the results of simulation studies for hypothetical species are often determined largely by the structure of the models themselves and provide little insight into real population dynamic behaviour and, further, that many models based on real data are flawed since the data are derived from glasshouse experiments or from density studies that exclude stochastic year-to-year variations in factors unrelated to density (e.g. weather).