Mast fruiting in a large tropical African legume tree species and the mineral nutrient limitation hypothesis

Abstract

The large grove-forming ectomycorrhizal tropical tree Microberlinia bisulcata displays strong mast fruiting on poor soils. Evidence is presented that masting is probably determined primarily by mineral nutrients and secondarily by radiation. To test the nutrient resource limitation hypothesis, phenological recordings were matched with climate variables and modelled using logistic time-series regression. A strong predictor was average daily rainfall in the dry season, low in the current year of masting and high in the year prior. Less strong was the increase in dry season radiation between prior and mast years. Masting events showed no relationship to annual stem increment. Masting intensity was reduced after two caterpillar attacks. Fallen-leaf phosphorus concentration increased in the inter-mast interval and that for potassium peaked mid-interval. A drier dry season prior to masting may have increased tree carbon, a wetter one increased nutrient uptake by roots and mycorrhizas. A rooting-fruiting trade-off in carbon is suggested. The long-term stochastic driver appears to be dry-season rainfall. Synchrony across masting trees may be enhanced by equilibration of nutrients across the mycorrhizal-root network. Life history strategy determined by mineral nutrient resource dynamics provides the best explanation and advanced model of the observed pattern of events.