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

Abstract

Mast seeding (highly variable population seed crops among years) is common in the New Zealand flora, and is especially evident in high elevation populations of Chionochloa (snow tussocks). However, mast seeding is less pronounced at low elevations in New Zealand plants, including C. rubra. Past studies 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. rubra should suffer less predation than other Chionochloa species, or low elevation populations of C. rubra should experience lower seed predation than higher elevation populations. To test these predictions we surveyed seed predation in C. rubra and neighbouring C. pallens and C. flavescens at 22 New Zealand 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. flavescens (54%). Seed predation significantly increased with decreasing altitude, in direct contradiction to our prediction. This reveals a reproductive paradox: why do low elevation populations of C. rubra not show more pronounced mast seeding? We discuss three possible answers: (1) C. rubra has not formed altitudinal ecotypes with different critical temperatures for heavy floral induction; (2) the seed predators at low elevation are harder to satiate; or (3) low elevation populations suffer stronger disadvantages from masting through high intraspecific competition and or density dependent seedling mortality. We suggest that this paradox may also occur in many other masting plants with wide altitudinal ranges, that predator satiation will often break down at the lower altitudinal limit of the plant, and that seed predation may limit the lower elevational range of some masting plant species.