Modelling fructification intensity at the tree level reveals species-specific effects of tree age, social status and crown defoliation across major European tree species

Abstract Many tree species show pronounced masting patterns, i.e. high inter-annual variability in seed production, which is strongly synchronised across large areas. Yet, when observing fructification intensity at the individual level, substantial intra-annual variability between trees becomes apparent. The drivers of this variability have so far been only investigated in studies of few species, individuals and/or over short time periods. In this study, we thus analysed potential predictors of individual fructification intensity for eight tree species common in central Europe ( Fagus sylvatica , Quercus petraea / robur , Acer pseudoplatanus , Fraxinus excelsior, Picea abies, Abies alba , Pinus sylvestris, Pseudotsuga menziesii) across 15 years (2006–2020) and the area of Southern Germany. We utilised a comprehensive forest monitoring dataset, for which fructification intensity and crown condition are assessed visually each year for thousands of trees at fixed positions. Employing generalised additive models, fructification at the tree level was modelled best when considering predictors related to weather as well as tree age, crown condition and social position. The probability of (strong) fructification was higher in dominant trees, and did also increase as trees became older until a certain species-specific age had been reached, with the effect ultimately turning negative in very old trees. High crown defoliation had a negative effect on the probability of (strong) fructification in almost all species. Yet, in some species (e.g. Fagus sylvatica ), weak crown defoliation had a positive effect on fructification intensity, potentially indicating different life strategies between species.