High acorn diversity of the introduced Quercus rubra indicates its ability to spread efficiently in the new range

The morphological traits of seeds have a fundamental meaning for spread and regeneration of non-native zoochorous plants, as they determine the level of establishment success and the ability of native fauna to disperse the alien plant. Here we studied the size, mass and viability of acorns of the North-American northern red oak Quercus rubra in its introduced range (in central Poland). The study revealed that acorns sampled in 2018 and 2019, in Q. rubra stands which had just reached sexual maturity, achieved optimum age for seed crop production, or were close to the rotation age, and were planted in sites suitable to European coniferous, mixed, and deciduous forests, differed in their dimensions and weight. Acorn size, mass and viability were affected both by stand age and forest site type, but more strongly by the first factor, i.e. the older the stand, the larger the acorns. At the initial stage of reproduction larger acorns were produced in coniferous than in other forest sites, but this advantage decreased with the age of stand in favour of more fertile mixed and deciduous forest sites. Acorn dimensions and viability also differed in sequential years: larger acorns with a high proportion of viability were produced in 2018, while in 2019 the proportion of viable acorns was significantly lower. The moisture of (viable) acorns was well above the lethal level for most acorns in all stands studied, however, it was impacted by the autumn weather. The high intraspecific variability in size and weight makes Q. rubra acorns attractive for different species of native seed hoarders. It conditions effective acorn spread and regeneration of this introduced (invasive) oak in European forests. Therefore, leaving single oak trees over 90 years old – which produce the best quality seeds – on plots cleared during forestry operations can favour Q. rubra regeneration.