The highly variable and synchronized production of large seed crops by plant populations, called masting or mast seeding, has been reported across a broad group of plant species. While there is consensus on the evolutionary drivers of masting, the proximate mechanisms are more controversial. Several hypotheses make predictions about the possible drivers of synchrony in masting species, including density-dependent pollination success (pollen coupling), environmental veto, or correlated environmental variation. The pollen coupling hypothesis predicts that weather and plant resources drive the flowering effort of trees, which directly translates into the size of seed crops through efficient pollination. In contrast, the veto hypothesis predicts that weather affects flower to seed transition, leading to occasional bumper crops. The recently formulated phenology synchrony hypothesis predicts that veto can arise because of weather effects on flowering synchrony, which, in turn, drives pollination efficiency.
I will first present the relationships between weather, airborne pollen, and seed production in common European trees, two oak species (Quercus petraea and Q. robur) and beech (Fagus sylvatica) with a 19‐yr data set from three sites in Poland. Results show that warm summers preceding flowering correlated with high pollen abundance and warm springs resulted in short pollen seasons (i.e., high flowering synchrony) for all three species. Pollen abundance was the best predictor for seed crops in beech, as predicted under pollen coupling. In oaks, short pollen seasons, rather than pollen abundance, correlated with large seed crops, providing support for the veto effect and phenology synchrony hypotheses.
Next, I will show extended the resource budget model of masting with correlated reproductive failure (veto). We parametrized the model for five forest tree species (two oaks, two pines, and mountain-ash), and run the model for 500 yrs. In four out of five species, among-year variation in resource gain and correlated reproductive failure were necessary and sufficient to produce masting. In the fifth (Quercus ilex) veto and pollen coupling were also required. Mechanisms of masting clearly differ among species, but environmental variation may drive large-scale synchronization without any other synchronizing mechanisms in a significant pool of species.