Demographic costs of masting for a Bornean rain forest tree species, Scaphium macropodum
Masting — the synchronous production of large fruit crops of conspecific trees among years — is a life history strategy in SE Asian tree species. While the reproductive biology of masting is relatively well described, the demographic consequences of masting are poorly understood. Theoretically, irregular production of seeds (masting) is less advantageous than annual fruiting for population growth, even if seed production is proportional to fruiting interval. This is related to the mortality risk between fruiting events, which needs to be compensated by a disproportional increase in seed production in order to maintain the same population growth. To assess the demographic costs of masting for Scaphium macropodum displaying masting behavior, population dynamics of this species in a tropical rain forest in West Kalimantan, Indonesia was studied using a stochastic matrix model. Our stochastic matrix model analyses of different fruiting frequencies revealed a long-term population growth rate of 1.002 for a situation in which fruiting occurs every 16 years. The average masting frequency in the region is higher than this frequency, and we therefore expect populations of our study species to be sustained. Stochastic elasticity analysis suggested that population growth rate was rather insensitive to the level and/or frequency of seed production while it was highly sensitive to adult survival. Stochastic population growth rates were barely reduced when fruiting intervals were prolonged and a proportional increase in seed output was simulated. Hence, if irregular reproduction is combined with increased seed output during fruiting events, the demographic costs of masting may be fully compensated.