Rapid morphological change in UK populations of Impatiens glandulifera

Abstract The highly invasive Impatiens glandulifera (Himalayan balsam) is one of the most prolific and widespread invasive plants in the British isles. Introduced in the early 19th century, it has now been reported in almost every vice county across the U.K and is a fierce competitor that has adverse effects on the local community structure. Despite the negative impacts that invaders like I. glandulifera have on local communities, there have been very few studies which address the morphological changes that invasive plant populations have undergone since their initial introduction. This is the first study of its kind to investigate the morphological changes that have occurred in I. glandulifera since 50 years after its introduction. 319 herbarium specimens dating from 1890–2017 were used to measure changes in morphological traits such as leaf size, flower size and stomatal characteristics. We found that since 1890, there has been a significant reduction in overall leaf size, a significant reduction in stomatal density and a significant increase in the overall flower size. These results highlight the importance of monitoring the evolutionary change in prolific alien species over the course of their invasion, providing useful insights into changes in competitive ability which may prove useful in managing dispersal and providing options for potential management.