Acorn production of introduced Quercus rubra is more strongly impacted by the weather than by the forest site (a case study from Poland)

Abstract

Recognition of the seed crop size and the periodicity of abundant seed production is essential for the management and control of introduced tree species. Here we studied acorn production of the North American northern red oak, Quercus rubra – the most common commercially important and invasive alien tree in European forests. A four-year (2017-2020) study conducted in even-aged Q. rubra stands, planted 55-60 years ago in sites of coniferous, mixed, and deciduous forests in central Poland, revealed a great variation in size of premature and mature acorn yields both among stands within the same year and among years for the same stand. However, each year numerous acorns of the introduced oak entered existing ecosystems, and the supply of mature propagules was more stable in the studied forests than in the Q. rubra native range. The total acorn crop was correlated significantly with the forest site, however, the forest site had a weaker effect on Q. rubra masting than the weather. Increases in abortion of acorns and decreases in mature acorn crops were preceded by reproduction-inhibiting weather events, and lower crops of mature acorns were correlated with abundant premature acorn abscission. In reaction to the weather, the phenology of premature and mature acorn shedding was very variable in subsequent years, but it was synchronized in time among all forest sites studied. The size of seed crop at the stand level was shaped by acorn crops of the highly-productive trees, and their identification within stands can be crucial for both effective management and control of Q. rubra in European forests.