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“Flying under the radar” – How misleading distributional data led to wrong appreciation of knotweeds invasion (Reynoutria spp.) in Croatia

Knotweeds were introduced into Europe in the 19th century and soon after became aggressive invaders. While R. japonica and R. sachalinensis arrived from Asia, their hybrid Reynoutria × bohemica is believed to originate from the introduced range. Revised material shows that R. × bohemica has occurred in Europe since at least 1872, but was frequently misidentified as one of the parents. For that reason, the hybrid was detected relatively late in many European countries, meaning that it was silently spreading throughout the region for a long time. The present study was conducted in Croatia, where the parental species have been recorded since the 19th century, the hybrid, however, not being recognized until very recently, although it has probably been present for decades. As a result, the current knowledge of the distribution of knotweeds in Croatia is mostly erroneous. We revised specimens from Croatian herbaria and performed an extensive field survey across the whole of the territory of Croatia to collect new data. In this paper, we present accurate distribution maps of the three taxa, produced after a thorough revision, showing a result remarkably different to that previously known. In contrast to previously recorded distributions, we found that R. × bohemica is by far the most widespread, R. japonica is occasional, and R. sachalinensis is extremely rare. R. × bohemica, unlike its parents, occurs in Mediterranean Croatia as well. Compared to R. × bohemica, R. japonica occurs more frequently along watercourses. Taking into account that the studied taxa have different behaviour, ecological preferences, spreading ability and consequently different invasiveness, they should be analysed separately in any attempt to explain, manage and/or limit their spread. Therefore, the accuracy of their past and future identification is of major importance.

distribution maps, invasive plants, Reynoutria × bohemica, Reynoutria japonica, Reynoutria sachalinensis

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