CONSEQUENCES OF THE PHYSICAL MANAGEMENT OF AN INVASIVE ALIEN PLANT FOR RIPARIAN PLANT SPECIES RICHNESS AND DIVERSITY

Management methods to control or eradicate invasive alien species should be guided by research into their efficacy in relation to the target species as well as their wider effects on the community. Impatiens glandulifera Royle (Balsaminaceae) is an invasive alien species, which has spread rapidly across Europe and North America, particularly colonizing riverbanks, wet woodlands, and wastelands. The effectiveness of two mechanical control measures and their impacts on community plant species richness and diversity within the riparian zone of the urban river Brent, London, England, was assessed. Over 2years, an experiment was performed across three sites that exhibited contrasting levels of invasion of I. glandulifera: high, moderate and low. The experiment involved three treatments on replicated plots at all three sites: pruning at 6-week intervals, weeding at 6-week intervals, and a control. The effects of these control measures were most clearly seen at the site with the highest level of invasion and, to a smaller extent, at the moderately invaded site. Weeding was more successful in controlling the species than pruning, and the greatest impact of these treatments on community plant species richness and diversity was only apparent after 2years of the experiment. Although seedlings of 10 other alien species were found, there was no evidence that their presence on the plots was detrimental to native species during the experimental period. Three common native species each approached 10% cover on the weeded plots at the heavily invaded site by the end of the experiment: Poa annua, Rumex obtusifolius, and Urtica dioica. These assessments provide guidance for implementing mechanical control of I. glandulifera in riparian zones. © 2012 John Wiley & Sons, Ltd.