Mast seeding over 33 years by Dacrydium cupressinum Lamb. (rimu) (Podocarpaceae) in New Zealand: the importance of economies of scale

A 33-year series of data on seed production in Dacrydium cupressinum Lamb. (rimu), a common wind-pollinated tree in the lowland and lower montane forests of New Zealand, shows marked mast seeding. Mast years showed higher percentages of sound seed as well as higher total seed crops per unit area. The percentage of sound seed (a measure of pollination success) was bimodally distributed, allowing mast years to be objectively defined. At least 6 of the 15 mast years followed immediately after other mast years. Low seed production and cool temperature 2 years previously and warm temperatures in the summer of seedfall are necessary for the occurrence of a mast year. The interaction of seedfall in one year with that 2 years previously is linked to the long reproductive development period in rimu and provides an example of competition between overlapping cohorts of reproductive structures in the same plant. The seedfall data also suggest that male and female reproductive effort is synchronized in rimu, which is dioecious. Of the two existing hypotheses which could favour mast seeding, resource matching is incompatible with observed data while predator satiation is only one of a group of possible economies of scale. For example, wind pollination could also provide economies of scale, as appears to be the case in rimu.