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 Scheldt estuary, 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. Here, it now occurs in 90% of the permanent plots, dominating the herb layer of willow shrubs and woodlands. It also colonises reed beds which deteriorate within a few years after invasion. Only Urtica dioica seems able to compete. Nevertheless, within the permanent plots there is an apparent fluctuation through time in the cover of Himalayan balsam.
Accordingly, the associated Natura 2000 habitats are assessed to be in a bad ecological status, which calls for a better comprehension of the species’ niche in view of management options.
We will showcase how the distribution of Himalayan balsam along the Scheldt links with abiotic conditions including salinity and inundation regime and try to find out what causes the observed fluctuation in the species’ cover. Also, we will have a look at the species’ distribution within the freshwater tidal habitats along the Meuse (in casu the Biesbosch) where similar dynamics have been observed.