Mast fruiting in a large tropical African legume tree provides evidence for the nutrient resource limitation hypothesis

Background and aimsThe large grove-forming tropical tree Microberlinia bisulcata (Fabaceae subfamily Detarioideae) in southern Korup, SW Cameroon, displays strong mast fruiting. The soil is poor in phosphorus and potassium: the climate is strongly seasonal. Evidence is presented that masting is probably determined primarily by mineral nutrients and secondarily by radiation. Method and materialsField data on phenology and climate variation (1989 to 2017), and soil moisture, nutrients in litter and fruits, insect herbivory and tree growth are integrated. Some data come from previous studies at Korup, and they are combined with new data to enable an extended analysis. Key resultsTo test the nutrient resource limitation hypothesis, phenological recordings were matched with climate variables and modelled using logistic and time-series regressions. 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 was reduced after two caterpillar attacks. Fallen-leaf phosphorus concentration increased in the inter-mast interval and that for potassium peaked mid-interval. These elements were likely accumulating and being stored, triggering masting when internal thresholds were crossed. A drier dry season prior to masting may have increased tree carbon, the wetter one increased nutrient acquisition and uptake via roots and mycorrhizas. A rooting-fruiting trade-off in carbon is proposed. Storage of phosphorus may be mostly in bark of branches; that for potassium on soil organic colloids. ConclusionThe long-term driver appears to be stochastic dry-season rainfall. Synchrony across masting trees might 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 an advanced model for the observed pattern of masting events.