The influence of the groundlayer structure on the invasion of small balsam (Impatiens parviflora DC.) to natural and degraded forests

Abstract

Impatiens parviflora DC. was brought to European botanic gardens in 1837, from which it escaped and spread over almost the entire continent colonising predominantly synanthropic habitats. However, in the second half of the 20th century it also became permanently naturalised in natural deciduous forests. The causes of small balsam's success in colonisation of new habitats are not known. They may include either the continuous degradation of forest phytocoenoses or those properties of the alien species that allow it to colonise degraded habitats more effectively as compared to the native species. The aim of this paper was to verify the hypothesis on the relationship between the floristic and structural degradation of the herbaceous or groundlayer and its resistance to the invasion of I. parviflora. The investigations were carried out in the Forest Reserve "Las Bielański" in Warsaw using 30 transects that crossed natural floodplain forests and natural and degraded lime-hornbeam forests. The study has shown that: (1)there is a significant negative correlation between the species richness and cover of the groundlayer and the frequency, cover, and density of the population of I. parviflora; (2) small balsam easily penetrates into degraded and floristically impoverished communities; (3) dense groundlayer of the natural communities provides an effective barrier to the expansion of I. parviflora. This expansion is promoted by such disturbances of the groundlayer that induce formation of gaps providing safe microsites for seed germination and further growth of seedlings.