Spatial and temporal variation in tree seed production and dispersal in a New Zealand temperate rainforest

Spatial and temporal variation in tree seed production is an important driver of the population dynamics of trees and of mammalian and avian seed consumers. Many studies have documented strong synchrony in production of intermittent large tree seed crops (masting), with cascading effects on the food webs of seed consumers and their predators. We used inverse modeling to characterize spatial and temporal variation in seed production and dispersal by four dominant tree species (two angiosperms and two conifers) over 8 years in southern temperate rainforests of New Zealand. In contrast to expectations from masting theory, there was little evidence of synchrony across species in years of high seed production, and only weak evidence in support of the expectation that temporal variation in seed production within species was strongly bimodal. Contrary to expectation from allometric scaling rules, there was no increase in reproductive effort once tree size (DBH) exceeded a minimum threshold (22–29 cm DBH) in the two angiosperm species. In the conifers, the minimum estimated size threshold for seed production was much higher (42–56 cm DBH), and in one of the species increased faster than linearly with biomass above the threshold size, indicating that the very largest trees in the conifer populations dominated seed production. Of the two species that occurred commonly on both fertile alluvial sites and less fertile uplifted marine terraces, the angiosperm species had higher per capita seed production on the fertile sites, while the conifer had higher per capita seed production (during seed years) on the less fertile sites. Local seed rain from all four species declined steeply with increasing distance from a parent, with peak local dispersal within 6 m of parent trees. This is not surprising given that the species are predominately gravity or animal dispersed with poor adaptations for wind dispersal. A substantial fraction of the input of seeds of all four species could not be attributed to local parent trees, consistent with longer distance dispersal by animals.