×Sorbaronia ′Mitschurinii′—an overlooked alien shrub rapidly expands in temperate Scots pine forests

Biological invasions are one of the most important threats to biodiversity. However, spread mechanisms have been described in detail only for some widespread taxa. Here, we aim to characterize the unique phenomenon of ×Sorbaronia ′Mitschurinii′ (Rosaceae) spread into temperate Scots pine forests. We assessed the surroundings of an abandoned ×Sorbaronia ′Mitschurinii′ plantation, established in 1986 near Ośno Lubuskie (W Poland). Around the plantation we established 66 study plots in 30 m intervals along eight transects, in a systematic design. Using generalized linear mixed-effects models we analysed how the distance from the propagule source (i.e. abandoned plantation), maximum age of ×Sorbaronia shrubs assessed using dendrochronological techniques, and light availability determined the density and biomass of the studied plant. We found that its mean aboveground biomass in the plantation was 7.22 ± 0.66 Mg ha−1, and decreased with distance from the propagule source: 6.83 Mg ha−1 30 m from the abandoned plantation, 1.38 Mg ha−1 at a distance of 90 m, and 0.11 Mg ha−1 at a distance of 180 m, almost disappearing at further distances (but present at the maximum distance of 450 m). The density and aboveground biomass of ×Sorbaronia ′Mitschurinii′ increased with increasing maximal age of the studied plant and its light availability. The ×Sorbaronia ′Mitschurinii′ plantation acted as a strong propagule source, as it reached a density of 100 ind. 100 m−2 after five years, forming a dense shrub layer, capable of reproducing. We conclude that alien species planted at high density and left with no management control might transform into invasion hotspots under favourable circumstances. The study highlights the importance of scientific awareness of similar cases of overlooked invasion sources, as management to limit their spread is still possible.