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

Version 1: 5 April 2023
Version 2: 29 January 2024
Version 3: 7 May 2024

The large grove-forming tropical tree Microberlinia bisulcata (Fabaceae subfamily Detarioideae) at Korup, SW Cameroon, shows strong mast fruiting with a ~3-year cycle. To test the nutrient resource limitation hypothesis, phenological recordings (1989 and 2017) were matched with climate variables and analyzed using logistic time-series regression. A strong predictor was mean daily rainfall in the dry season: low in the current year of masting and high in the year prior. Less strongly predictive was the increase in dry season radiation between prior and mast years. Masting events showed no relationship to annual stem increment. Masting became less strong after two caterpillar attacks. Fallen leaf phosphorus concentrations increased markedly in the inter-mast interval and potassium peaked mid-interval. These key elements were likely accumulating and being stored, and masting was triggered when internal thresholds were crossed. The drier season prior to masting enabled a rise in tree carbon, and the wetter dry season before that enabled acquisition and uptake of nutrients by roots and mycorrhizas. The storage of phosphorus may be mainly in bark of branches, that for potassium on soil organic colloids. A rooting-fruiting trade-off in carbon is indicated. Further hypothesized is that synchrony among masting trees is enhanced by equilibration of phosphorus across a mycorrhizal network. The long-term driver appears to be year-to-year stochasticity of dry-season rainfall, which leads to an important refinement of the hypothesis. Life history strategy linked to nutrient resource dynamics provides a plausible best explanation and more advanced model for the masting events observed.