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. E vidence of the natural fire frequency to which the species is best ada pted was sought through analysis of its seed demography using a comput er model of canopy seed bank dynamics based on field data. 2 Plants be gan flowering at 3-4 years of age but storage of viable seeds in the c anopy 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 t o 15 years, but then fluctuated widely around a mean of 17 cones per y ear. 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 f ollicles remained closed for at least 5 years and up to 50% of follicl es were still closed after 10 years. Insect granivores destroyed 18% o f seeds in I-year-old cones, and continued to destroy seeds in older c ones at a rate of 2-3% per year. 5 The seed bank model predicted conti nued seed accumulation in the canopy to at least age 25 years (1400 se eds), but by this time more viable seeds (approximate to 2200) had bee n lost to spontaneous opening, granivory and decay than were then stor ed on the plant. 6 Shannon-Weiner diversity (H') of seed ages in the c anopy seed bank increased with plant age, while evenness (V') decrease d at first (to plant age 17 years) and then increased. 7 On a time-sca le of many generations, the likelihood of successful recruitment of in dividuals in the next generation (on average), occurs for a fire frequ ency of 15-18 years. At this frequency resource wastage in this low-nu trient environment would be minimized. 8 Recent fires have recurred wi thin 7-11 years and indicate a human impact on the natural fire regime which may ultimately threaten the species.