Both oaks and beech mast, i.e. show extremely high variability in seed production. In the new paper, now accepted in Ecology, we show that the mechanisms that lead to this variation are totally different between these species. The study was successful thanks to a work of a number of great researchers, with the great contribution from Jakub Szymkowiak, Mario Pesendorfer, Idalia Kasprzyk, and Łukasz Grewling.
Oaks flower with similar intensity every year. What determines the seed crop size is the transitions of these flowers to fruits. The factor that is responsible for the success of this transition is spring weather. When springs are warm, oaks release pollen in synchrony with other individuals, which translates into high pollination efficiency (most flowers get fertilized), and big seed crop. In contrast, when spring is cold, oaks get de-synchronized, which leads to low pollination efficiency and low crops. This supports the phenology synchrony hypothesis formulated recently by Walt Koenig.
In contrast, beech shows extreme inter-annual variation in flowering intensity. So, despite spring weather also affects the synchrony of flowering, it does not matter for crop size. What matters is the amount of pollen in the air – in some years its released in huge numbers which leads to mast years. In other, almost no pollen is produced, and thus, almost no seeds.
What determines pollen production? In both species, it was temperature in the previous year summer, when flowers get initiated. However, the amount of flowers did not really matter in case of oaks, so the last summer weather does not matter for crop size. In contrast, in beech pollen is the key. Therefore, mast years followed hot summers. These was attenuated by the plant resource state – if over the last years beech had no crop, the response to the weather cue was strong. Contrary, if hot summer happened after a mast year, the response to the weather cue was weak – simply because trees had no energy to produce lots of flowers after such high reproductive event. These results support the resource-limited floral induction model, recently proposed by Monks et al.
One cool thing is that the models are not contrary, but rather apply to different species!
Below, the abstract of the work, the paper can be accessed here:
Masting, the highly variable production of synchronized large seed crops, is a common reproductive strategy in plant populations. In wind-pollinated trees, flowering and pollination dynamics are hypothesized to provide the mechanistic link for the well-known relationship between weather and population-level seed production. Several hypotheses make predictions about the effect of weather on annual pollination success. 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 pollination Moran effect hypothesis predicts that weather affects pollination efficiency, leading to occasional bumper crops. Furthermore, the recently formulated phenology synchrony hypothesis predicts that Moran effects can arise because of weather effects on flowering synchrony, which, in turn, drives pollination efficiency. We investigated the relationship 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-year data set from three sites in Poland. Our 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 pollination Moran effect and phenology synchrony hypotheses. Fundamentally different mechanisms may therefore drive masting in species of the family Fagacae.