Seed Bank and Population Dynamics of Banksia cuneata: The Role of Time, Fire, and Moisture

Abstract

Banksia cuneata is a large floriferous shrub or tree confined to seven small populations in southwestern Western Australia. The stand at the study site consisted of groups of even-aged, fire-sensitive plants, suggesting recruitment is usually dependent on recurrent fire. Following a 4-yr juvenile period, annual fruit set increased exponentially, yielding 11,700 two-seeded follicles per plant over the subsequent 19 yr. Two major constraints restricted the number of seeds available for the next generation: low fruit set relative to flower production (1.5%) and decay of canopy-held follicles after 10 yr. There was no predation of reproductive parts and only minor abortion, senescence, and prefire release of seeds. Most of the 17,100 viable canopy-stored seeds in the 23-yr-old cohort were released within 24 h of a hot autumn fire, whereas wet/dry cycles were required following milder fires. Less than 5% of seeds germinated and only 0.1% of these survived the first summer drought. Seedlings transplanted to moister sites and/or watered regularly over summer were much larger and more likely to survive to the next winter than the controls. We conclude that population numbers are not limited by the size and dynamics of the canopy seed bank but by the weather pattern following fire-induced seed release.