Nonrandom Patterns of Fruit Set in Banksia spinulosa (Proteaceae): Interovary Competition Within and Among Inflorescences

In Banksia spinulosa var. neoanglica (Proteaceae) not all flowers and inflorescences are equally likely to produce fruit. Fruit set is low on the apex of inflorescences, and most inflorescences with small supporting stems do not produce fruit. Resource limitation was examined as a possible determinant of this fruiting pattern. All flowers had two apparently functional ovules, indicating that B. spinulosa is not andromonoecious. Ovules from the apical third of inflorescences were smaller than ovules from the mid and basal thirds of inflorescences. Also, ovule size decreased with decreasing stem diameter of inflorescences. However, small ovules were fertile. Experimental removal of flowers from the basal one-third and two-thirds of inflorescences increased fruit set of apical flowers by 71% and 152%, respectively. Similarly, fruit set of inflorescences with small stems increased twofold when inflorescences with large stems were prevented from producing fruit. These results indicate that resources can be reallocated both within and among inflorescences and that fruiting patterns are primarily a consequence of interovary competition occurring at both these levels. Anatomical constraints may also be important because the increases in fruit set in the experimental treatments did not completely compensate for the flowers and inflorescences that were removed. Low fruit: flower ratios in B. spinulosa may allow plants to increase reproductive output when nutrients are abundant and to compensate for the effects of floral predators.