Canopy seed bank dynamics and optimum fire regime for the highly serotinous shrub, Banksia hookeriana

1 Banksia hookeriana is a serotinous nonsprouting shrub restricted to the fire-prone northern sandplain scrub-heaths of Western Australia. Evidence of the natural fire frequency to which the species is best adapted was sought through analysis of its seed demography using a computer model of canopy seed bank dynamics based on field data. 2 Plants began flowering at 3-4 years of age but storage of viable seeds in the canopy was rare for plants < 5 years. Infructescences (cones) contained a mean of 12 follicles, holding two seeds of which 62% were estimated to be viable. 3 Cone (and seed) production increased with plant age to 15 years, but then fluctuated widely around a mean of 17 cones per year. By this time the average plant had an estimated canopy seed bank of 700 viable seeds comprising seeds 1-10 years old. 4 Seeds were lost from the canopy seed bank through spontaneous follicle rupture (i.e. in the absence of fire), granivory by insect larvae, and decay. Most follicles remained closed for at least 5 years and up to 50% of follicles were still closed after 10 years. Insect granivores destroyed 18% of seeds in I-year-old cones, and continued to destroy seeds in older cones at a rate of 2-3% per year. 5 The seed bank model predicted continued seed accumulation in the canopy to at least age 25 years (1400 seeds), but by this time more viable seeds (approximate to 2200) had been lost to spontaneous opening, granivory and decay than were then stored on the plant. 6 Shannon-Weiner diversity (H') of seed ages in the canopy seed bank increased with plant age, while evenness (V') decreased at first (to plant age 17 years) and then increased. 7 On a time-scale of many generations, the likelihood of successful recruitment of individuals in the next generation (on average), occurs for a fire frequency of 15-18 years. At this frequency resource wastage in this low-nutrient environment would be minimized. 8 Recent fires have recurred within 7-11 years and indicate a human impact on the natural fire regime which may ultimately threaten the species.