VARIATION BETWEEN YEARS IN POLLEN AND NUTRIENT LIMITATION OF FRUIT-SET IN BANKSIA-SPINULOSA

(1) This study investigated pollen and nutrient limitation of fruit-set in Banksia spinulosa var. neoanglica (Proteaceae) in natural sclerophyll forest during four flowering seasons from 1986 to 1989. (2) Fruit production differed fourfold among years indicating that the conditions determining fruit-set were variable. (3) Levels of cross-pollination, as estimated by visits of nectarivorous birds to inflorescences, differed threefold during the study. (4) In 1986, when natural levels of fruit-set were high, the number of fruits plant-1 was not increased by adding nutrients or cross-pollen, either separately or together. In 1988, when natural levels of fruit-set were lower than in 1986, fruit production plant-1 increased by 65% and 114% following the separate additions of nutrients and cross-pollen, respectively, and by 298% following the addition of both. (5) In 1989, plants that received additional nutrients in 1988 produced more fruits than did plants that did not receive nutrients. There was no such effect in 1987 following the 1986 experiment. (6) The extent of pollen and nutrient limitation of fruit-set may differ among years. Extra flowers may provide opportunities for plants to increase fruit-set when cross-pollination and nutrient levels are high.