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

The large grove-forming tropical tree Microberlinia bisulcata (Fabaceae subfamily Detarioideae) at Korup, Cameroon, shows strong mast fruiting. Reproductive allocation is considerable. The site has very nutrient-poor soil. To test the nutrient resource limitation hypothesis, phenological recordings between 1989 and 2017 were matched with climate variables and analyzed using logistic time-series regression. Masting happened mostly on 2- or 3-year cycles. 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, nor with local plantation yields. Later, the normally heavy mastings became moderate after two attacks by caterpillars. Collated studies of fallen leaf nutrient concentrations showed that P increased markedly, K rose and fell, but N and Mg changed little, in the inter-mast interval. P and K were likely being accumulated and stored and then triggered masting events when internal thresholds were crossed. The drier season prior to masting enabled a rise in C, and the wetter season the year before, with higher soil moisture, enabled better acquisition and uptake of nutrients by roots and mycorrhizas. The main storage of P may be in bark and branches, that for K on soil organic-colloids. A rooting-fruiting trade-off in C allocated over a minimal 2-year cycle is implied. The hypothesis is that synchrony among masting trees may be achieved, in part, by equilibration of P across the mycorrhizal network. The long-term driver appears to be the inherent year-to-year stochasticity of dry-season rainfall, the realization of which leads to an important refinement of the hypothesis. Life history strategy linked to nutrient resource dynamics provides a plausible explanation and more advanced hypothesis for the masting events observed.

Methods
The files in this archive are from different parts or aspects of a long-term study over 29 years of the fruiting phenology of the tree species Microberlinia bisulcata at Korup National Park, SW Cameroon. The time series of recorded mast fruiting events between 1989 and 2004 (reported and analyzed in Newbery et al. 2006) has been extended here to 2017. A second period of detailed leaf fall and flushing, flower, and fruit phenology between 2009 and 2014 is included, complementing the earlier similar one between 1995 and 2000 (Newbery et al. 2006). The basic methodology was one of scoring samples of trees for phenology over time within specific periods in the large permanent P-plot at Korup (Newbery et al. 2013), and outside of these periods, annual assessments were made. Masting occurrence over the full period is matched with climate data variables (mean daily dry-season rainfall and radiation; start and duration of the dry season). The raw daily climate data for 1984 to 2017 are found in Etta et al. 2022: see 'Related Works' below), Data are also presented for soil moisture contents from 2003 to 2005, leaf and fruit nutrient concentrations from two studies at the site in 1998 and 2011, and annual stem growth increment data derived from tree cores from 1989 to 2003 (see Newbery et al. 2013). For comparison with forest fruiting, data on oil palm yield in estates outside the Park are presented for 2003 to 2016. The methods are detailed in the main paper.

Newbery, D. M., G. B. Chuyong, and L. Zimmermann. 2006. "Mast fruiting of large ectomycorrhizal African rain forest trees: importance of dry season intensity, and the resource-limitation hypothesis."  New Phytologist 170 (3):561-579. doi: 10.1111/j.1469-8137.2006.01691.x.
Newbery, D. M., X. M. van der Burgt, M. Worbes, and G. B. Chuyong. 2013. "Transient dominance in a central African rain forest."  Ecological Monographs 83 (3):339-382. doi: 10.1890/12-1699.1.