Witnesses to industrial history: the recent return of wool alien plants on the banks of the River Meuse (the Netherlands, Belgium)

After the drastic redevelopment of the banks of the River Meuse in recent years, on both the Dutch and Belgian sides of the river, numerous exceptional exotic plants unexpectedly appeared. Although their precise introduction vector is difficult to determine with certainty, most of these species have most likely germinated from the old, long-buried seed bank that was uncovered by the works. The seeds were introduced many decades ago (at least 50 but perhaps more like 100 years ago, or even longer) via the wool processing industry, about 50 km further upstream, along the River Vesdre in Belgium. Here, we present a commented and illustrated overview of the 59 taxa observed in recent years. They belong to a very limited number of families that are known to have hard, long-lived seeds (mainly Fabaceae and Malvaceae, which together represent 48 taxa). Most taxa are native to the Mediterranean area, but are often widely naturalized elsewhere in the world, including in wool farming areas, while some are endemic to Australia, South Africa, or South America. The following species had not been recorded before in the Netherlands: Cullen cinereum, Dichondra micrantha, Erodium carolinianum, Lotus ornithopodioides, Malva olbia, Malvastrum americanum, Sida acuta, S. rohlenae, S. sivarajanii, Sphaeralcea purpurata, Tarasa tenella, Trifolium hirtum, T. spumosum, and T. squarrosum. Nicotiana glauca was reported for the first time in Belgium. The most remarkable species are depicted. Perhaps these records are of limited relevance for the Dutch and Belgian flora, because – as in the past – probably none of these wool aliens will be able to establish permanently (although this cannot be ruled out entirely, given the changing climate). Our findings highlight the exceptional longevity of seeds of certain plant families.