Abstract/Summary

The UK native species Quercus robur and Quercus petraea provide a range of ecosystem services, support biodiversity, and provide high quality timber, but natural regeneration is poor whilst acorn supplies for planting are unreliable due to masting, i.e. large interannual variability in acorn production that is geographically synchronous. Three UK field studies were carried out with Q. robur. A four-year study of 39 trees found large individual variation in acorn crops among years and trees. There was synchrony in that all trees produced acorns in 2020’s mast year and none in 2021, but 51% of all acorn production over the study was provided by only 18% of the trees studied. Hand pollination on eight trees with out-of-stand pollen produced more acorns than within-stand pollen and/or natural pollination. Raising carbon dioxide concentration by 150 μL L−1 in a FACE study increased the numbers of immature acorns and all acorn evidence (empty cups + immature acorns + mature acorns) but did not consistently affect the numbers of mature acorns. Relatively consistent flowering and high abortion of developing acorns suggests Q. robur at these sites is a fruit-maturation masting species. Further, a modelling study of acorn production by Q. petraea and Q. robur over 41 years at 35 sites across Northwest Europe showed inter-annual variation in acorn production; mast years occurred at mean intervals of 2 years in NE France, 3 years in S France, 3.6 years in the UK, and 6.5 years in NW France. Masting in neighbouring sites was synchronised, but this declined the further apart the sites were. Models of inter-annual variation in acorn production based on weather cues were developed which quantified this data set well but did not predict independent data satisfactorily. Approaches to increase acorn production from oak seed stands are suggested.