Factors limiting and promoting invasion of alien Impatiens balfourii in Alpine foothills

Impatiens balfourii is an Asian ornamental plant introduced to Europe in the 20th century from the Himalayas. It is far less invasive than two congeneric species from the same area, I. glandulifera and I. parviflora. The factors responsible for its limited success are poorly known, though they may be related to low frost tolerance and low popularity as an ornamental plant. Maladaptive habitat preferences, a factor not studied to date, may also play an important role. The aim of this study was to determine those responsible factors and to examine the consequences of habitat selection by I. balfourii on the Swiss-Italian border, where the species is assumed to be invasive. After exhaustive mapping of its distribution and measurement of the plants' performance parameters, we checked for signs of disease and pest attack on individual plants growing in different habitats, and analysed the local abiotic characteristics. The distribution pattern differed significantly between the two studied countries. There were four times as many Italian localities of I. balfourii and they were clearly concentrated along roadsides, whereas the few Swiss localities were scattered among other habitats. The level of leaf damage along roadsides was lower, suggesting higher release from natural enemies there, which, however, did not translate into investment in plant size or fecundity. Patch area along roadsides correlated positively with the presence of buildings and shading; the plant's ability to spread decreased with elevation. These results confirm that invasion by this shade-tolerant species is driven by propagule pressure and that its dispersal ability is low; that may change with climate warming. Although roads provide suitable conditions and invasion corridors, and despite possibly higher propagule pressure on the Swiss side, in Switzerland the preference for this habitat is maladaptive for I. balfourii, due to intensive mowing, which seems to create an ecological trap.