Why is mast seeding in Chionochloa rubra (Poaceae) most extreme where seedpredation is lowest?

Mast seeding (highly variable population seed crops among years) is common in the New Zealand flora, and is especially evident in high elevation popul ations of Chionochloa (snow tussocks). However, mast seeding is less pronou nced at low elevations in New Zealand plants, including C. rubra. Past stud ies of high elevation Chionochloa populations showed that the main benefit of mast seeding is the satiation of pre-dispersal insect seed predators. We therefore predicted that, to be able to set seed regularly, either C. rubr a should suffer less predation than other Chionochloa species, or low eleva tion populations of C. rubra should experience lower seed predation than hi gher elevation populations. To test these predictions we surveyed seed pred ation in C. rubra and neighbouring C. pallens and C. flavescens at 22 New Z ealand sites spanning over 1000 m in altitude. Total seed predation was not significantly lower in C. rubra (48%) than in C. pallens (22%) and C. flav escens (54%). Seed predation significantly increased with decreasing altitu de, in direct contradiction to our prediction. This reveals a reproductive paradox: why do low elevation populations of C. rubra not show more pronoun ced mast seeding? We discuss three possible answers: (1) C. rubra has not f ormed altitudinal ecotypes with different critical temperatures for heavy f loral induction; (2) the seed predators at low elevation are harder to sati ate; or (3) low elevation populations suffer stronger disadvantages from ma sting through high intraspecific competition and/ or density dependent seed ling mortality. We suggest that this paradox may also occur in many other m asting plants with wide altitudinal ranges, that predator satiation will of ten break down at the lower altitudinal limit of the plant, and that seed p redation may limit the lower elevational range of some masting plant specie s.