Recruitment and Mortality in Populations of Banksia grandis Willd. In Western Australian Forest

Abstract

This paper examines seed release and seed dispersal; recruitment in relation to fire and logging; mortality of seedlings in relation to their age; and the dynamics of one population from November 1979 to March 1983. Dehiscence of follicles is caused by wetting and drying of the infructescence, so that most seeds fall to the ground during autumn in the Mediterranean climate of south-western Australia. Seeds are wind- dispersed, settling downward from the parent tree: 90% of seed falls within the first year. There is large variation in annual recruitment of seedlings, reflecting large annual differences in the number of infructescences produced. Fires of various intensities or logging do not enhance recruitment. Seedling mortality is greatest during the first summer after germination; this may be related to the rudimentary development of the lignotuber. A population of 1890 individually marked plants was studied for 3.5 years. Over twice as many deaths took place in summer and autumn as in winter and spring, and 90% of deaths were of plants less than 20 cm tall. At the conclusion of the study, total population size, through recruitment of three cohorts of seedlings, had increased by 7%. This indicates how important control of reproduction would be in achieving long-term reduction in the abundance of B. grandis, which harbours the pathogenic fungus Phytophthora cinnamomi.