Ecological requirements, short-term dynamics and competition of native and invasive balsam (Impatiens) species: A field study

Background/Question/Methods

Comparison of invasive, naturalised non-invasive, and native plants can provides useful insights into factors conferring invasiveness if carried out on congeners because such approach eliminates phylogenetic biases. We studied ecological requirements, short-term dynamics and competition of native and invasive species of Impatiens (Balsaminaceae) widespread in the Czech Republic in two field experiments. Impatiens noli-tangere is native in the study region, I. glandulifera and I. parviflora were introduced from Asia; the former species spread faster rapidly in Central Europe especially in the last 50 years, while the spread of the latter declined in the last decades. We established two sets of permanent plots in five localities. In the first set, site characteristics such as tree cover and soil humidity were directly measured, and nutrients, light, temperature, continentality, humidity and soil reaction estimated using Ellenberg indicator values (ranking species according to their demands for ecological factors on a semi-quantitative scale). In the second experiment, one of the species of native-invasive congeneric pair was removed and the growth compared to the intact control in terms of cover and abundance.

Results/Conclusions

The occurrence of Impatiens species was better predicted by directly measured variables than by Ellenberg indicator values. Although the species often occurred in the same locality, the patterns observed suggested micro-site differentiation. The occurrence of I. noli-tangere was strongly correlated with a high soil moisture; I. glandulifera exhibited an unimodal response to tree canopy cover, while I. parviflora responded positively to low soil moisture and a high tree canopy cover. These results indicate possible but limited coexistence of the three species in the field. In the second experiment, abundance and cover of the target species in all treatments increased if the congener was removed. The strongest increase in abundance was found in I. noli-tangere in response to removal of I. parviflora, and the weakest in I. parviflora in response to that of I. noli-tangere. The highest increase in cover was recorded for I. noli-tangere in response to the removal of I. glandulifera and the weakest in I. noli-tangere in response to the removal of I. parviflora. The results point to rather poor competitive abilities of the native I. noli-tangere, and its different response to the invasive congeners.