To understand the role of leaf-level plasticity and variability in species invasiveness, foliar characteristics were studied in relation to seasonal average integrated quantum flux density (Qint) in the understorey evergreen species Rhododendron ponticum and Ilex aquifolium at two sites. A native relict population of R. ponticum was sampled in southern Spain (Mediterranean climate), while an invasive alien population was investigated in Belgium (temperate maritime climate). Ilex aquifolium was native at both sites. Both species exhibited a significant plastic response to Qint in leaf dry mass per unit area, thickness, photosynthetic potentials, and chlorophyll contents at the two sites. However, R. ponticum exhibited a higher photosynthetic nitrogen use efficiency and larger investment of nitrogen in chlorophyll than I. aquifolium. Since leaf nitrogen (N) contents per unit dry mass were lower in R. ponticum, this species formed a larger foliar area with equal photosynthetic potential and light-harvesting efficiency compared with I. aquifolium. The foliage of R. ponticum was mechanically more resistant with larger density in the Belgian site than in the Spanish site. Mean leaf-level phenotypic plasticity was larger in the Belgian population of R. ponticum than in the Spanish population of this species and the two populations of I. aquifolium. We suggest that large fractional investments of foliar N in photosynthetic function coupled with a relatively large mean, leaf-level phenotypic plasticity may provide the primary explanation for the invasive nature and superior performance of R. ponticum at the Belgian site. With alleviation of water limitations from Mediterranean to temperate maritime climates, the invasiveness of R. ponticum may also be enhanced by the increased foliage mechanical resistance observed in the alien populations.