Norghauer, J. M. | bioRxiv Science

bioRxiv2023

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

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.

ecology↗

bioRxiv2022

Indications of an Achaea sp. caterpillar outbreak disrupting fruiting of an ectomycorrhizal tropical tree in Central African rainforest

Background and aimsWhere one or several tree species come to dominate patches of tropical forest, as many masting ectomycorrhizal legumes do in central Africa, ecological theory predicts they may be prone to herbivory which might alter their reproductive output. This was indirectly investigated in lowland rainforest in Cameroon for Tetraberlinia korupensis, whose crowns were attacked in 2008 by an outbreaking black caterpillar--identified as an Achaea sp., probably A. catocaloides--in Korup National Park. Material and methodsField-collected data on tree-level seed and fruit (pod) production of T. korupensis in its 2008 masting event were compared with that of its two co-dominant neighbours (T. bifoliolata, Microberlinia bisulcata) whose populations masted in 2007 (and 2010). To do this, bivariate regression models (linear, polynomial, ZiG [zero-inflated gamma model]), contingency table analysis, and non-parametric measures of dispersion were used. Key resultsAssuming T. korupensis is prone to Achaea caterpillar attacks, empirical data support the hypothesized lower proportion of adults participating in its masting (54% in 2008) than for either masting population of M. bisulcata (98% in 2007, 89% in 2010) or T. bifoliolata (96% in 2007, 78% in 2010). These fruiting T. korupensis trees were about one-third larger in stem diameter than conspecific non-fruiters and produced as many pods and seeds per capita as T. bifoliolata. But regressions only modestly support the hypothesis that the positive tree size-fecundity relationship for T. korupensis was weaker (i.e., lower adj. R2) than for M. bisulcata (whose leaves are morphologically similar) or T. bifoliolata, with mostly similar dispersion about the median among these species. ConclusionAltogether, the findings suggest a role for tolerance in nutrient-poor forests. It is postulated that instead of conferring resistance to herbivores, the ectomycorrhizas associated with these trees may enable them to more quickly recover from potential yet unpredictable insect outbreaks.

ecology↗