Comparison of native and non-native Impatiens species across experimental light and nutrient gradients

Background and aims – The success of invasive species over resident species is often assigned to their trait attributes and their ability to respond plastically towards fluctuating environmental conditions. To elucidate the role of differentiated trait responses towards environmental conditions promoting invasion success, we conducted a congeneric study with three Impatiens species, one of which is native to Central Europe (I. noli-tangere), while the other two are introduced and considered invasive (I. parviflora and I. glandulifera). We hypothesised that plasticity in plant traits increases from high resource environments to low resource environments, and we expected the two invasive species to exhibit higher trait plasticity than the native species.

Methods – Monocultures of the three species were grown in pots with nine combinations of light and nutrient availability (ambient, medium and high light plus N-limited, balanced and P-limited). We then measured species responses with respect to biomass allocation traits, growth rates and reproduction. We conducted multivariate analysis of trait responses via path analysis and structural equation modelling, and determined phenotypic plasticity indices (PIs) for plant traits across the nine treatments.

Key results – The performance of invasive Impatiens species is more prone to unfavourable nutrient conditions than to adverse light conditions. Following our initial model, we expected similar trait responses and trait-trait relationships for the three species, regardless of absolute biomass allocation patterns or whether they were native or non-native. Instead, we found that the three congeneric Impatiens species exhibited contrasting responses to light and nutrient treatments, leading to different trait-trait relationships. Specifically, I. noli-tangere and I. parviflora exhibited similar responses and trait-trait relationships, whereas those of I. glandulifera differed. Treatment effects on plant traits showed that the non-native I. parviflora was the most plastic species; however, this result was not consistent across all traits.

Conclusions – The success of invasive species over their native congeners is based on a combination of similar trait responses to environmental site conditions, but the invasive species exhibit higher trait plasticity, facilitating establishment.