Mast seeding by the dioecious temperate forest tree Dacrydium cupressinum (Podocarpaceae) in New Zealand

A 33-year series of data on seed production in Dacrydium cupressinum (rimu), a common wind-pollinated tree in the lowland and lower montane forests of New Zealand, shows mast seeding. Six of the 15 mast years followed immediately after other mast years. Warm temperatures in the summer of seedfall and low seed production two years previously are necessary, but not sufficient, conditions for the occurrence of a mast year. The interaction of seedfall in one year with that two years previously is linked to the long reproductive development period in rimu; seed ripens two years after the reproductive structures are initiated. This provides a clear example of competition between overlapping cohorts of reproductive structures in the same plant. Mast years showed higher percentages of sound seed as well as higher total seed crops per unit area. The two existing hypotheses which could account for mast seeding (predator satiation and resource limitation) are discussed. We suggest that resource limitation cannot account for observed cases of mast seeding and predator satiation is only one of a group of possible causes (efficiencies of scale). For example, wind pollination could also provide efficiencies of scale; thus we would predict that wind-pollinated trees are likely to be mast seeding. In rimu, wind pollination could be important, while predator satiation seems less likely as a selective pressure. In common with other New Zealand podocarp trees, rimu has bird-dispersed seeds. The prediction that mast seeding is unlikely to occur in trees with animal-dispersed seed (Silvertown, 1980) is not supported by the results of this study. The reason for this discrepancy appears to be that the bird species dispersing rimu seed are generalist feeders.