Predicting native plant responses to Japanese knotweed invasion

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Japanese knotweed (Reynoutria japonica) is one of the world’s most problematic invasive species. Native to Asia, the species was introduced into European gardens in the 19th century as it looks like bamboo yet grows fast and can thrive in a variety of environments. After its prompt escape from gardens, the weed travelled fast and has had devastating impacts on natural ecosystems across the northern hemisphere. In the UK alone, measures to control knotweed cost £1.25 billion in 2014.

Reynoutria japonica is an ecologically plastic, clonal, fast-growing, herbaceous, perennial geophyte that dramatically reduces the occurrence of native species, causing biotic homogenization. It easily outcompetes and displaces most species from plant communities it invades. However, there are some plant species that, due to their functional traits, can coexist with R. japonica and can even benefit from its invasion.

In their new study published in AoBP, Woch et al. aimed to comprehensively analyse plant communities invaded by R. japonica, taking into account species traits, habitat conditions and seasonal variability, and to determine the ecological profile of species that withstand the invader’s pressure. The study was performed in fallow and riparian areas in southern Poland. Pairs of adjacent plots were established at 25 sites with no obvious signs of recent human disturbance. One plot contained R. japonica, and the other contained only resident vegetation.

In the study, Woch et al. showed that R. japonica, for the most part, had a strong negative effect on resident vegetation by either completely displacing or drastically reducing the abundance of many plant species. However, they also found several species that can coexist with or benefit from R. japonica invasion. Two groups of species managed so by simply avoiding competition with the invasive weed. Spring ephemerals were found to undergo a full vegetation cycle before the development of R. japonica canopy, whilst lianas treated shoots of R. japonica as support for climbing and grew over them. On the other hand, a third group of highly competitive native plants were simply able to outcompete R. japonica at its own game.