The spatio-temporal dynamics of Himalayan Balsam (Impatiens glandulifera) within the Scheldt estuary

Himalayan balsam (Impatiens glandulifera) is an annual plant species that is native to East Asia. It was introduced in Europe as a garden ornamental, but appears to have easily escaped cultivation. Though the first records for Belgium date back as early as the 19th century, it is only since the mid-20th century that the species started to become widely established.
We here report on the occurrence of Himalayan balsam along the lower Scheldt, based on time series of vegetation mapping and relevé data of permanent plots from the past two decades. During this period, the species has become ever more ubiquitous, though only in the freshwater zone. It now is the single most recorded species, occurring in about 90% of the permanent plots (figure 1). Himalayan balsam is found most abundantly in roughened vegetation characteristic for later-successional stages that formed through sediment accretion and related hydrological change (figure 2).
When testing for the importance of hydrological variables (by means of fuzzy ordination), the vegetation composition of plots appeared to be best explained by the frequency of inundation. However, the data suggested that Himalayan balsam in particular performs best in soils that drain relatively rapidly following inundation. This corroborates the observation that Himalayan balsam is relatively under-represented in newly established inundation areas that have a controlled reduced tide, and thus less extreme drainage conditions (cf. Sigma plan). Such restoration projects may thus prove particularly successful in breaking the species’ local dominance.