Competitive dominance of the invasive plant Impatiens glandulifera: using competitive effect and response with a vigorous neighbour

Plant invasiveness was commonly attributed to the invader's competitive superiority over the native community, but a general pattern supporting this prediction is still lacking. This is particularly enhanced by the fact that competitive dominance and its role in plant invasiveness require the use of scarcely-practiced experimental elements. Here, we used a comprehensive experimental approach to evaluate the competitive superiority of the highly invasive annual Impatiens glandulifera. We used two competition-setting treatments, which independently examine competitive effect versus response of both native and invasive genotypes. As a neighbour species we used Urtica dioica, a vigorous perennial co-occurring with I. glandulifera at both its native and introduced ranges. By examining both components of competitive ability we were able to show that although invasive genotypes exert a weaker competitive pressure compared to their native conspecifics, they are still competitively superior to U. dioica. Our results also suggest that the high competitive ability of I. glandulifera could be attributed it to allelopathic effects on co-occurring native species. We suggest that an appropriate experimental setup, which examines competitive effect and response independently, can provide a more factual evaluation of the competitive ability of invasive plants. Furthermore, the use of a dominant species as a target plant rather than a random species or one with poor competitive ability, renders our results more general, implying that I. glandulifera might exert greater competitive effect on the less robust co-occurring species. We conclude that our approach is highly useful and advocate its application in future tests of invasion success.